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Diversity of Microbes and Cryptogames
Bacteria
Dr. A. K. Paul
August, 2006
English
Contents
Introduction
Morphology
Nutrition and Growth Of Bacteria
Bacterial Reproduction
Economic Importance of Bacteria
A General Account of Cyanobacteria
Significant Keywords:
Bacteria, Peptidoglycan, Capsule, Flagella, Binary fission, Endospore, Cyanobacteria,
Antibiotics, Bioremediation
Introduction
Bacteria are a heterogenous group of single celled prokaryotic microscopic organisms
characterized by the lack of a membrane-bound nucleus and membrane-bound cell
organelles, like mitochondria, plastids etc. They represent the first living inhabitants of the
earth and evidence indicates that they existed more than 3.5 billion years ago i.e. almost 3
billion years before the appearance of plants and animals. Earlier they were considered as
part of the plant kingdom; eventually they were separated into a kingdom called Protista by
Ernst Haeckel (1866), which included microorganisms sharing characteristics of both plants
and animals. Later, Robert H. Whittaker (1969) in his five-kingdom classification of
organisms placed them in the kingdom Monera. Carl Woese and his co-workers (1977) in
their three-domain system of classification divided the living organisms into three domains
the Archaea (archaebacteria), the Bacteria (eubacteria), and the Eukarya (eukaryotes). Both
Archaebacteria (the primitive bacteria) and Eubacteria (true bacteria) have prokaryotic cells.
Therefore, it is apparent that all organisms with prokaryotic cells are bacteria and conversely,
all bacteria are prokaryotes.
The archaebacteria are a group of bacteria that live in unusually harsh environments where no
other living organisms can survive and probably represent the first forms of life. They are
found in extreme environments such as acidic hot springs, deep-sea hydrothermal vents, in
arctic ice and glaciers and highly salty water. Archaea are structurally and chemically distinct
from other bacteria. The cell walls, cell membranes, and ribosomal RNA are different from
those of other bacteria.
The eubacteria are ubiquitious (omni presence) in nature, mostly occur as free-living
organisms in the air, water and soil including quite a variety of natural and manmade
environments. They are of great importance both in nature and in industry being involved in
recycling of wastes and production of industrial products. Eubacteria are associated with
human body as normal microflora, many of which are beneficial, while others are pathogenic
causing diseases of different types. Bacteria have also associated with plants as symbionts
and plant pathogens. This chapter discusses the gross morphology, ultrastructure, growth and
economic importance of bacteria including cyanobacteria.
Morphology
Size:
Bacterial cells are extremely small in size and could be visualized with the help of
microscope at a magnification of 1000 times. They vary in size depending on the species and
range from 0.5 – 1.0 µm in diameter (micrometer, µm = 10-6 m) in spherical bacteria like
Streptococcus and Stapylococcus while, the rod shaped bacteria are usually 0.5-1.0 µm wide
and 1-10 µm long. For example the rod shaped cells ranges from long Bacillus anthracis (1.0
to 1.3 µm X 3 to 10 µm) to very small cells such as Pasteurella tularensis (0.2 X 0.2 to 0.7
µm). Mycoplasmas or pleuropneumonia like organisms (atypical pneumonia group) are very
thin measuring 0.1 to 0.2 µm in diameter. These bacteria could pass through the bacterial
filters but rests are retained on such filters. In addition, there are few exceptionally large
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bacterial species such as Epulopiscium fishelsoni (80 µm X 600 µm ) which grows in the
intestine of Surgeonfish from Red Sea and Thiomargarita namibiensis (100 – 750 µm in
diameter ) isolated from Namibia coast because of the small size, the ratio of surface area to
volume for bacteria is very high compared to eukaryotic organisms with larger cells. This
high surface/volume ratio facilitates absorption of nutrients and gases from the environment
leading to high rate of metabolism and growth of bacteria. Moreover, the small size also
helps them to spread rapidly in the environment.
Shape: Bacteria have three characteristic shapes: coccus (pl. cocci), bacillus (pl. bacilli), and
spirillum (pl. spirilla). The arrangement of cells is also typical of various species or groups of
bacteria (Fig. 1). The spherical or ovoid cocci have several arrangements depending on the
planes of division. Division in one plane produces either a diplococcus (a pair of cocci, e.g.
Neisseria) or streptococcus (a chain of cocci, e.g. Streptococcus) arrangement. Division in
two planes produces a tetrad (square of 4 cocci) arrangement, while division in three planes
produces a sarcina (cube of 8 cocci). Division in random planes produces a staphylococcus
arrangement where cocci are arranged in irregular groups or grape-like clusters (e.g.
Staphylococcus).
Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus (a single
bacillus), streptobacillus (a chain of bacillus), or coccobacillus (oval, almost similar to
coccus). The curved rods or comma-shaped cells are called vibrios (e.g. Vibrio cholerae).
A
B
C
D
Plate 1. Photomicrographs showing morphological forms of bacteria. (A) Staphylococcus,
cocci in groups, (B) Ralstonia, short rods, (C) Bacillus, typical rod shaped bacteria, (D)
Streptomyces, filamentous bacteria with chain of spores (indicated by arrow).
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Figure 1 . Characteristic shapes and arrangements of bacterial cells
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Spirilla, the spiral or corkscrew shaped bacteria come in two forms, a spirillum with rigid
wall (e.g. Spirillum), or may be thin, flexible as in a spirochete. Many species of bacteria
characteristically show the tendency of cell branching as in Mycobacterium, while in
streptomycetes (e.g. Streptomyces, Actinomyces) there is formation of branched mycelia
similar to those in fungi. In addition, bacteria of unusual shapes like square, star-shaped,
spindle-shaped and lobed structure have also been discovered recently. The characteristic
shapes of bacterial species are determined by the genetic makeup of the organism and most
bacteria maintain a single shape and are called monomorphic, while pleomorphic bacteria
alter their shapes and occur in different forms. Cyanobacteria or blue-green algae also show
wide diversity in their morphological forms, which has been discussed in details under
section Cyanobacteria.
Surface Structures
Bacteria have number of surface structures that are broadly distinguished as (1) surface
appendages and (2) surface layers. Two types of surface appendages have been recognized in
bacterial cells. These include flagella (singular flagellum), the organs of locomotion and pili
(singular pilus) or fimbriae (singular fimbria). The surface layers include glycocalyx
(capsules and slime layers) and S-layers.
Flagella: Structurally, bacterial flagella are thin, hairlike, filamentous surface appendages with
helical shape that imparts motility to the bacterial cell. They are extremely thin and cannot be
seen directly under the ordinary light microscope unless they are suitably stained (Leifson’s
flagella stain) by layering a dye precipitate on its surface. However, they are easily detected
under electron microscope. Bacterial flagella are about 15 to 20 µm long and about 12 to 20
nm (nm = 10-9 m) in diameter.
The number and distribution of flagella on the bacterial surface are characteristic for a given
species and hence are useful in identifying and classifying bacteria. Cocci rarely have
flagella, but in rod-shaped bacteria, flagella may be polar or lateral. Polar flagellation may be
monopolar or bipolar. In monopolar flagellation, some bacteria like Vibrio and Caulobacter
have only a single flagellum called monotrichous, while others have a cluster of flagella at
one pole and are called polytrichous also referred to as lophotrichous (e.g. Pseudomonas and
Chromatium). Bipolar polytrichous flagellation is called amphitrichous (e.g. Spirillum). In
lateral flagellation the flagella may be inserted on lateral walls of the bacterial cell as in
Selonomonas ruminatum or over the entire cell surface and is called peritrichous (e.g.
Bacillus, Proteus etc.) (Figure 2). As an exception, flagella in members of Spirochaetales are
internal i.e. they remain beneath the outer membrane and wrap around the cell body. They are
commonly termed as endoflagella or axial filament.
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Figure 2 . Flagellation pattern in of bacteria (a) Monotrichous (b) Polytrichous (c)
Amphitrichous (d) Peritrichous
A
B
Plate 2. Electronmicrographs showing bacterial flagella. (A) Peritrichous flagella in Bacillus
(B) Monotrichous flagella in Rhodopseudomonas.
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Fine structure analysis following electron microscopic studies has subdivided the bacterial
flagellum into three morphologically and chemically distinguishable parts designated as (i)
filament, (ii) hook and (iii) basal body (Figure 3).
Figure 3 . Ultrastructure of bacterial flagellum
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Filament: The section of the flagellum between hook and distal end which lies external to the
cell surface is known as the filament. The filament is around 20 nm in diameter, 10-20 µm in
length and wavy in nature with constant amplitude. In some bacteria like Bdellovibrio and
Vibrio cholerae, a sheath surrounds the flagellar filament. Chemically, flagella are
constructed of a class of low molecular weight proteins called flagellins, which are
characteristic of a given species of bacterium and are arranged in helical order around an
axial cylinder. During synthesis of the flagellar filament, flagellin molecules synthesized in
the cytoplasm are added to the growing tip of the filament increasing its length.
Hook: The section of flagellum between filament and basal structure is known as the hook. It
is short, slightly curved, and has a diameter somewhat greater than the filament. The hook
ranges from 70-90 nm in length and consists principally of single polypeptide.
Basal body: The most complex part of the flagella, which anchors the flagellum in the
cytoplasmic membrane and the cell wall. It consists of a small central rod and a series of
discs or rings. The basal body acts as a motor system, enabling the flagellum to rotate and
propel the bacterium in the liquid environment.
Function: Flagella, the organelles of locomotion impart motility by rotating in clockwise and
anticlockwise manner controlled by the basal body. The movement of the basal body is
driven by a proton motive force rather than by ATP directly. Bacteria swim through liquid by
means of the propeller-like action of the flagella in response to environmental stimulus. The
stimulus may be due to chemicals (chemotaxis), light (phototaxis), osmotic pressure
(osmotaxis), oxygen (aerotaxis), and temperature (thermotaxis). Chemotaxis, referred to as
movement in response to attractant and repellent substances in the environment help bacterial
pathogens to move through the mucous layer and colonize the mucous membranes and
thereby facilitate bacterial pathogenesis.
Pili (Fimbriae): Pili (singular, pilus) are thin, hairlike, hollow proteinaceous appendages on the
surface of many Gram-negative bacteria. They originate from the cytoplasmic membrane and
occur profusely on the cell surface. Structurally they are very simple, 3-10 nm in diameter
and can only be seen by electron microscope. The pilus has a shaft composed of a protein
called pilin and at the end of the shaft there is an adhesive tip structure.
Based on their morphology and physiological function the pili are distinguished into two
distinct types: i) the common or attachment pili, also known as fimbrae are short and
abundant and ii) Sex pili or F pili, also know as conjugation pili, are long and very few in
number.
The common pili or fimbriae confer adhesive properties on the bacterial cells and allow the
bacteria to adhere and colonize environmental surfaces or cells and are referred as colonizing
factors. The sex pili are involved in sexual reproduction. They attach male to female bacteria
during conjugation and help in the transfer of genetic material from the donor to the recipient
cell.
Surface Layers
Glycocalyx: Some bacteria are surrounded by layers of viscous or amorphous substances
called glycocalyx. These surface layers can be detected by staining and light microscopy;
electron microscopy of thin-sectioned cells, freeze-fractured, and negatively stained (with
indian ink) cells. They are distinguished into two types: i) capsule and ii) slime layer.
In some bacteria like Streptococcus pneumonia, Leuconostoc mesenteroides and
Xanthomonas campestries the glycocalyx appear as thick outermost gelatinous covering
tightly bound to the cell wall and is called a capsule. Capsules may be up to 10 µm thick and
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are distinctly visible under light microscope. The glycocalyx when remain loosely adhered to
the cell wall in an unorganized manner it is commonly referred to as a slime layer. Slime
layers commonly diffuse into the medium when the organisms are grown in liquid medium.
The glycocalyx is not essential for viability since viability of cells is not affected when
capsular polysaccharides are removed enzymatically from the cell surface. The exact
functions of capsules are not fully understood. However, they have been found to protect the
cells against drying or desiccation, help in traping of nutrients and enable some bacteria to
adhere to environmental surfaces like, rocks, root hairs, teeth, etc. and colonize to form a
biofilm consisting of layers of bacterial populations adhering to host cells and embedded in a
common capsular mass. As an example, Streptococcus mutans, a bacterium responsible for
initiating dental caries adhere to the enamel of the tooth and form plaque. The glycocalyx
enables bacteria to confer resistance to phagocytosis and hence provide the bacterial cell with
protection against host defenses to invasion.
The composition of glycocalyx varies with bacterial species and the conditions under which it
is grown (Table 1). Most capsules consist of polysaccharide – either homopolysaccharide
(e.g. dextran and levan) or hetropolysaccharide (e.g. alginate). These capsular
polysaccharides are composed of different sugars (glucose, galactose, mannose, rhamnose
etc.) and sugar acids (glucuronic acid, mannuronic acid etc.). Polypeptide capsules consisting
of D- and L-amino acids are found in some bacteria like Bacillus anthracis and Xanthomonas
sp.
Plate 3. Electronmicrograph of an unidentified rod shaped bacteria showing distinct capsule
surrounding the cells.
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Table 1 . Nature of capsular substances produced by different bacteria
Bacterial species
Capsular substances
Streptococcus mutans
Homopolysaccharide (polymer of glucose)
Xanthomonas campestris
Heteropolysaccharide
(contain
glucose,
mannose,
(contain
glucose,
rhamnose,
glucuronic acid)
Streptococcus pneumoniae Type II
Heteropolysaccharide
glucuronic acid)
Pseudomonas aeruginosa
Polysaccharide (contain mannuronic acid, glucuronic acid)
Bacillus anthracis
Polypeptide (polymer of glutamic acid)
Bacillus megaterium
Polypeptide (polymer of glutamic acid)
The S-layer: Many Gram-nergative and Gram-positive bacteria, as well a many Archaea possess a regularly
structured layer called an S-layer attached to the outermost portion of their cell wall. It is composed of protein or
glycoprotein and performs a number of possible functions like protection of bacteria from harmful enzymes,
changes in pH, and from the predatory bacterium Bdellovibrio. The S-layer also enables the bacterium to adhere
to host cells and environmental surfaces and protect them from phagocytosis.
Figure 4. Diagrammatic representation of the internal structures of the bacterial cell
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Cell wall
All bacterial cells with the exception of mycoplasmas are enveloped by a semi-rigid cell wall.
In bacteria (eubacteria) it is composed of peptodoglycan, while in Archaea (archaebacteria) it
is composed of protein or pseudomurein, which is chemically distinct from peptidoglycan.
Peptidoglycan, also called murein, mucopeptide or mucopolysaccharide is a vast polymer
consisting of interlocking chains of identical peptidoglycan monomers and forms a sac-like
three dimensional covering around the cell. The primary chemical structures of
peptidoglycans have been established. It consists of a glycan part that constitutes the
backbone of the peptidoglycan and is made of long chains of two amino sugars, Nacetylglucosamine (NAG) and its lactyl ether, N-acetylmuramic acid (NAM) joined together
by β-1,4-glycosidic bond. These glycan chains remain parallel to each other. Tetrapeptides of
L-alanine-D-glutamic acid-L-lysine (or diaminopimelic acid)-D-alanine are linked through
the carboxyl group by amide linkage of muramic acid residues of the glycan chains. The
terminal D-alanine residues of one tetrapeptide are directly cross-linked to the e-amino group
of lysine or diaminopimelic acid on a neighboring tetrapeptide, or a peptide bridge links
them. In Staphylococcus aureus peptidoglycan, a glycine pentapeptide bridge links the two
adjacent peptide structures. The extent of direct or peptide-bridge cross-linking varies from
one peptidoglycan to another and provide tremendous strength to the cell wall. The
staphylococcal peptidoglycan is highly cross-linked, whereas that of Eschericia coli is
comparatively much less.
Most bacteria can be placed into either Gram-positive or Gram-negative groups based on
their color after specific staining procedures commonly called Gram staining named after
Hans Christian Gram (1835-1938) who developed this staining procedure. The procedure
involves treating a heat-fixed bacterial smear with crystal violet followed by a dilute iodine
solution as mordant. This is followed by treatment of cells with ethyl alcohol. This treatment
results two different responses depending on the nature of the cell wall. Some bacteria retain
the initial violet colour of the crystal violet and appear purple when observed under the
microscope and are commonly called Gram-positive (example Bacillus subtilis,
Streptococcus pyogenes, Staphylococcus aureus). The other group of bacteria, which failed to
retain the violet colour become colourless and pick up the counterstain of safranin. They
appear pink when observed through microscope and are called Gram-negative (Escherichia
coli, Pseudomonas aeruginosa, Haemophilus influenzae). The steps of Gram staining
procedure and the appearance of cells at each step are shown in Figure 6. In addition, some
Gram-positive bacteria resist decolorization of the initial dye carbol fuchsin with an acidalcohol mixture during the acid-fast stain procedure. They appear red when observed through
the microscope. Common acid-fast bacteria include Mycobacterium tuberculosis and
Mycobacterium leprae.
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Table 2. Steps of Gram staining procedure and the appearance of cells at each step
Appearance of cells under microscope
Step
Gram-positive bacteria
Gram-negative bacteria
Cells colourless
Cells colourless
Cells stain violet
Cells stain violet
Cells remain violet
Cells remain violet
Cells remain violet
Cells become colourless
Cells remain violet
Cells stain red
Cells smeared and fixed on slide
Primary stain: Crystal violet
Mordant: Gram’s iodine
Differentiation: Ethyl alcohol/
acetone
Counterstain: Safranin
The Gram-positive cell wall: In electron micrographs of ultrathin sections of gram-positive
bacteria the cell walls appear as a broad dense covering of 20-80 nm thick and consisting of
numerous interconnecting layers of peptidoglycan . The peptidoglycan represents 60 to 90%
of the gram-positive cell wall. Teichoic acids, the polyphosphate polymers are associated
with the cell wall of Gram-positive bacteria as secondary wall component and represent
nearly 10-50% of the wall material. Teichoic acids are of two different types: i) polymers of
glycerol phosphate called glycerol teichoic acid (found in Bacillus subtilis) and ii) polymers
of ribitol phosphate called ribitol teichoic acid (found in Staphylococcus aureus).
Teichouronic acid consisting of long chains of alternating glucouronic acid and Nacetylgalactosamine linked to each other by 1-3 glycosidic bond is found in Bacillus
licheniformis. Some teichoic acids have lipids attached to them and are called lipoteichoic
acids.
The peptidoglycan in the Gram-positive cell wall prevents osmotic lysis. The techoic acids
probably help make the cell wall stronger. The surface proteins in the bacterial
peptidoglycan, depending on the strain and species, function as enzymes, help the bacteria to
adhere and colonize.
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Figure 5. Diagrammatic representation of Gram-positive and Gram-negative cell wall
of bacteria
The Gram-negative cell wall: In Gram-negative bacteria cell wall appears multilayered. It
consists of (i) a thin layer (2-3 nm thick) of peptidoglycan which represents only 10-20% of
the cell wall component and (ii) an outer membrane, a lipid bilayer of about 7 nm thick. The
space between the cytoplasmic membrane and the outer membrane is known as periplasmic
space, which remain filled with gelatinous material called periplasm. It contains a variety of
enzymes for nutrient breakdown as well as binding proteins to facilitate the transfer of
nutrients across the cytoplasmic membrane. The outer membrane is composed of
phospholipids, lipoproteins, lipopolysaccharides (LPS), and proteins. Phospholipids are
located mainly in the inner layer of the outer membrane, as are the lipoproteins that connect
the outer membrane to the peptidoglycan. The lipopolysaccharides, located in the outer layer
of the outer membrane, consist of a lipid portion called lipid A embedded in the membrane
and a polysaccharide portion extending outward from the bacterial surface called and is
differentiated into R core region (core polysaccharide) and O side chain (O-antigen). The
LPS portion of the outer membrane is also known as endotoxin. The outer membrane also
contains many cross membrane channels that allow diffusion of small molecules. These pores
are composed of proteins called porins.
The outer membrane because of its semipermeable nature helps retain certain enzymes and
prevents entry of some toxic substances, e.g., penicillin G and lysozyme etc. The LPS portion
of the outer membrane, when released, functions as a harmful endotoxin.
The acid - fast cell wall: In addition to peptidoglycan, the acid-fast cell wall of
Mycobacterium contains a large amount of glycolipids, especially mycolic acids that make up
approximately 60% of the acid-fast cell wall. The mycolic acids along with other glycolipids
prevent the entry of variety of substances causing the organisms to be more resistant to
chemical agents and lysosomal components of phagocytes that normally kill the bacteria.
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The archaebacterial cell wall: The archaebacteria are characteristically different from
eubacteria. Like bacteria most archaebacteria do contain cell wall with the exception of
Thermoplasma species. Cell wall-containing archaebacteria can stain either Gram-positive or
Gram-negative. Unlike bacteria all archaebacteria, however lack murein.
Gram-positive archaea have rigid cell wall sacculi similar to the ultrastructure of Grampositive bacteria and are composed of polymers of diverse chemical nature. In
Methanobacterium and Methanothermus the cell wall contain pseudomurein or
pseudopeptidoglycan which contain N-acetyl talosaminuronic acid instead of muramic acid
and N-acetyl glucosamin which are linked to each other by a β- 1,3 glycosidic bond and
alternate to form the cell wall backbone. In addition, cell walls of many archaebacteria are
compsed of polysaccharides of different types. Archaea that stain Gram-negative (e.g.
Methanolobus, Sulfolobus, Thermoproteus, Desulfurococcus and Pyrodictum) possess only
proteinaceous or glycoproteinaceous cell envelopes (S layers). They lack the outer membrane
and complex lipopolysaccharide found in Gram-negative bacteria.
Cytoplasmic Membrane
The cytoplasmic membrane, also called a cell membrane or plasma membrane lies internal to
the cell wall and encloses the cytoplasm of the bacterium. It is about 7 nm thick and appears
as 2 dark bands separated by a light band when observed under electron microscope.
Chemically it is composed of phospholipid (20-30%) and protein (60-70%). Structurally the
cytoplasmic membrane conforms to the fluid mosaic model. According to this model, the
phospholipids form a bilayer in which the hydrophilic or polar ends of the molecules form the
outermost and innermost surface of the membrane while the non-polar or hydrophobic ends
form the center of the membrane. The protein molecules are partly or wholly embedded in the
lipid layer and are differentiated as integral proteins and peripheral proteins. Integral or
intrinsic proteins remain embedded into the lipid bilayer, while the peripheral or extrinsic
proteins remain adhered to the hydrophilic layer of the membrane. With the exception of the
mycoplasmas, the prokaryotic cell membranes lack sterols. Sterols, such as cholesterol are
found only in eukaryotic cell membrane.
The cytoplasmic membranes of eubacteria and archaebacteria are structurally very similar.
However, the chemical composition of eubacterial cell membranes is significantly different
from the arcahebacteria and this is one of the most important characteristics that distinguish
eubacteria from archaebacteria. The eubacterial cytoplasmic membranes contain straight
chain fatty acids that are linked to glycerol by ester linkage, while the archaebacteria have
membranes composed of branched hydrocarbon chains attached to glycerol by ether linkages.
The cell membrane is a selectively permeable membrane and controls the movement of
various biochemicals into and out of the cells. In simple diffusion, solute molecules move
from higher concentration to lower concentration across the membrane. Osmosis is the
process of diffusion in which water molecules move through the selectively permeable
membrane from a region of low concentration of solute to a region where the concentration
of solute molecules is high. The pressure that encourages the movement of water is called
osmotic pressure. Cytoplasmic membranes also help in the transport of substances across the
membrane by transport (carrier) proteins. In addition, it acts as the site of energy production
through the membrane bound electron transport system, peptidoglycan synthesis,
phospholipid synthesis, division of the nucleoid and formation of endospores.
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The cytoplasm and cytoplasmic structures
The cellular components that are often found in bacterial cytoplasm include the nucleoid or
the nuclear material, plasmids, the ribosomes, endospores and various inclusion bodies, and
organelles used for photosynthesis.
Cytoplasm: In bacteria, the cytoplasm refers to everything enclosed by the cytoplasmic
membrane. The cytoplasm of bacteria is composed of 80% water and contains nucleic acids
(DNA and RNA), enzymes, amino acids, carbohydrates, lipids, inorganic ions, and many low
molecular weight compounds. Some bacteria produce cytoplasmic inclusion bodies of
various types that carry out specialized cellular functions.
The cytoplasm contains a large number of enzymes and is the main site of bacterial
metabolism, which includes both catabolic, and anabolic reactions. In catabolic reactions
molecules are broken down in order to obtain building block molecules for more complex
molecules and macromolecules, while anabolic reactions are concerned with synthesis of
other molecules and macromolecules. Apart from these intracellular enzymes, bacteria also
produce and secrete extracellular enzymes, which are transported across the membrane and
hydrolyze macromolecules into smaller molecules
Ribosomes: Ribosomes are nucleoproteinaceous particles and acts as the site of protein
synthesis. It contains 60% ribosomal RNA (rRNA) and 40% protein. Complete ribosome is a
70S particle ("S" refers to Svedberg unit.) and is about 25nm in diameter. It is composed of
two subunits with densities of 50S and 30S. The small subunit (30S subunit) contains about
21 proteins and a 16S rRNA molecule, while the large subunit (50S subunit) is composed of
approximately 34 proteins and one molecule each of a 23S and 5S rRNA. These two subunits
combine during protein synthesis to form a functional 70S ribosome.
The function of ribosomes is to carry out protein synthesis. During protein synthesis, the
message in mRNA molecules is translated to amino acid sequence in a protein. This process
is known as translation and involves, apart from ribosomes, amino acid-carrying tRNAs, a
number of enzymes and energy in form of ATP.
The Nucleoid: The bacterial cell, as a prokaryotic structure, lacks a distinct membrane-bound
nucleus. Electron micrograph of ultra-thin sections of cells reveals that the nuclear material
occupies a position near the centre of the cell as a light fibrilar area and is referred to as the
nucleoid. The nucleoid consists of a long, single, circular chromosome composed of double
stranded deoxyribonucleic acid or DNA that remain in a supercoiled form. Super coiling of
this DNA macromolecule is accomplished by a group of enzymes called DNA
topoisomerases to fit it into the bacterium. In addition to topoisimerases, a number of proteins
involved in DNA replication (DNA polymerase) and transcription (RNA polymerase) are
also found to be associated with bacterial chromosome. Although bacteria generally lack the
basic histone proteins, histone-like proteins have been reported in some arcahebacteria and
eubacteria.
The bacterial nucleoid does not divide by mitosis and since it contains a single chromosome
(haploid) and only reproduce asexually, there is also no meiosis in bacteria. The chromosome
is generally around 1000 µm long and frequently contains as many as 3500 genes. Cells of
Escherichia coli, which are 2-3 µm in length, have a chromosome approximately 1400 µm
long.
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Since the nucleoid contains most of the genetic information of the bacterium it determines the
synthesis of proteins and enzymes of an organism and there by regulates the overall
biochemical reactions of the bacterial cell.
Plasmids: In addition to the nucleoid, many bacteria often contain one or more, small,
circular, nonchromosomal DNA molecules called plasmids. Plasmids contain only a limited
amount of genetic information and are not essential for normal bacterial growth and
survivability. They can, however, provide an advantage under certain environmental
conditions like resistance to antibiotics, heavy metals, degradation of toxic compounds etc.
Moreover, plasmids impart pathogenicity in many bacteria where the production of toxins are
encoded by plasmids.
Endospore: Endospores are specialized resting bodies formed within the body of a small
group of bacteria and remain in a dormant or metabolically inert form. But they are highly
resistant to the lethal effect of heat, drying, freezing, deleterious chemicals and radiation.
Members of some selected bacterial genera like, Bacillus, Clostridium, Desulfotomaculum,
Sporosarcina, Sporolactobacillus, and Thermoactinomyces produce endospores. They are
unusually dehydrated and appear highly refracticle under microscope. They could also be
stained selectively with specific dye called malachite green.
Figure 6 . Drawings showing the location of endospores in spore-forming bacteria.
(a) Terminal spore without swelling of mother cell. (b) Central spore without swelling of
mother cell. (c) Terminal spherical spore, mother cell distended. (d) Sub-terminal spherical
spore mother cell distended. (e)Lateral spore, mother cell distended.Endospores may be
spherical, ellipsoidal or cylindrical in shape and in the cell there position may be central,
subterminal or terminal. Each bacterial cell forms a single endospore and the mother cell in
which the spore is produced is called a sporangium. A mature spore may have a diameter
same as, or greater than that of the vegetative cell. The latter causes a bulging of the cell, if it
is central it is called clostridium and if terminal a plectridium. As a rule each bacterial species
has its own characteristic size, shape and position of the spore but this is subject to variation
under different environmental conditions.
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Figure 7 . Drawings showing the ultrastucture of bacterial endospore
Endospores are formed under conditions of nutrient limitation, especially the lack of carbon and
nitrogen sources. The process of spore formation is called sporogenesis. The process of
sporogenesis is depicted in Figure . It gives rise to a single endospore within a single cell,
which is novel in structure and composition from the mother cell that produces it. During
endospore formatio a unique compound called dipicolinic acid is synthesized and Ca++ is
accumulated in the spore. The dipicolinic acid form chelate with Ca++ to form calcium
dipicolonate, the concentration of this compound has been implicated with the heat resistance
of the bacterial endospore.
The mature endospore as revealed by the electron microscope consists of a very thick
envelope, which is distinguished as spore coat, cortex and spore wall. Spore coat is the
outermost layer of the spore and consists of two layers: the outer coat and the inner coat. In
some species the spores are surrounded by a exosporium outside the coat. Beneath the spore
coat is the cortex, which is followed by a very thin spore wall surrounding the interior or core
of the spore. The core is equivalent to cytoplasm of the vegetative cell and contains a
nucleoid, some ribosomes, RNA molecules, and enzymes.
The endospores are liberated upon autolysis of the vegetative cell. The mature spores are
metabolically inert but exhibit high degree of resistance to heat, radiation and chemicals. The
resistance of endospores to physical and chemical agents is due to a variety of factors which
include: (i) low water content or dehydrated condition of the spores, (ii) abundance of
calcium-dipicolinate that stabilizes and protect the endospore's DNA, (iii) presence of
specialized DNA-binding proteins saturate the endospore's DNA and protect it from heat,
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drying, chemicals, and radiation, and (iv) DNA repair enzymes contained within the
endospore are able to repair damaged DNA during germination.
In suitable nutrient media most of the spores germinate to give rise vegetative cells. Though
some bacterial spores germinate spontaneously in a favourable medium, others remain
dormant unless heat or chemical substances first activate them. During germination water is
rapidly imbibed and the physio-biochemical activities of the spores increases rapidly and the
vegetative cell eventually bursts out of the spore coat.
Inclusion bodies: Bacteria when grown under different environmental conditions are found to
synthesize and accumulate variety of chemical substances as insoluble deposits in their
cytoplasm commonly called inclusion bodies.
Some bacteria produce inorganic inclusion bodies in their cytoplasm including polyphosphate
or metachromatic granules that store phosphate, and sulfur granules that store sulfur. These
may serve as energy reserve for the bacteria.
Many bacteria also accumulate either polyhydroxybutyrate granules (Ralstonia eutropha) or
glycogen granules as carbon and energy reserve. Polyhydroxybutyrate granules are
accumulated when the bacteria are grown under nutrient limiting condition in presence of
excess of carbon sources.
Some motile aquatic bacteria (e.g. Aquaspirillum magnetotacticum) have been found to
possess magnetosomes. Magnetosomes are membrane-bound crystals of magnetite or other
iron-containing substances that function as tiny magnets.
Autotrophic bacteria (Thiobacillus, Nitrosomonas etc.) that reduce CO2 in order to produce
carbohydrates, possess carboxysomes containing ribulose bis posphate carboxylase, an
enzyme used for CO2 fixation.
The green bacteria (e.g. Chlorobium) carry out anoxygenic photosynthesis. Their
photosynthetic system is located in ellipoidal vesicles called chlorosomes that are
independent of the cytoplasmic membrane. The purple bacteria (e.g. Rhodopseudomonas,
Rhodospirillum) also carry out anoxygenic photosynthesis but their photosynthetic system is
located in spherical or lamellar membrane systems that are continuous with the cytoplasmic
membrane.
Nutrition and Growth of Bacteria
Nutrition
Bacteria in order to grow require drawing from its environment all the necessary substances
for synthesis of their cellular components and generation of energy. The chemicals and
elements of this environment that are utilized for bacterial growth are referred to as nutrients.
Different bacteria have very different nutritional requirements that remain dissolved in water.
Like all other living systems, water is the main component for growth of the bacteria. The
nutritional requirements of a bacterium are evident from the elemental composition of the
bacterial cells, which are broadly categorized as macroelements or major elements and
microelements or trace elements.
Macroelements or major elements are required by the bacteria in high concentration. Ten
macroelements such as carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, potassium,
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calcium, magnesium and iron are found in the form of water, inorganic ions, small molecules,
and macromolecules, which serve either a structural or functional role in the cells.
Microelements or trace elements are metal ions required by cells in small amounts. These
trace elements usually act as cofactors for essential enzymatic reactions in the cell. Trace
elements in bacterial nutrition include manganese, zinc, copper, chlorine, sodium, cobalt,
nickel, molybdenum etc.
Carbon and energy sources: Every bacterial cell for its growth in nature or in the laboratory
must have a supply of carbon source and a source of energy. Carbon, the main constituent of
the cell represents nearly 50% of bacterial cell dry weight. The carbon requirements of
bacteria are fulfilled either by CO2 or organic carbon. Organisms that use CO2 as a sole
source of carbon for growth are called autotrophs and that use organic carbon are known as
heterotrophs. The organic carbon sources include simple sugars, amino acids, proteins,
complex carbohydrates and lipids. As source of energy many bacteria use the radiant energy
(light) and are called phototrophs, while organisms that use (oxidize) an organic form of
carbon are called heterotrophs or chemo(hetero)trophs. Organisms that oxidize inorganic
compounds are called lithotrophs (Thiobacillus, Nitrobacter).
Therefore, based on the type of carbon and energy sources for growth all bacteria are
distinguished into four major nutritional categories.
Photoautotrophs: They use light as the energy source and CO2 as the sole source of carbon.
Some purple and green sulfur bacteria belong to this category.
Photoheterotrophs or Photoorganotrophs: They use light as the energy source and an organic
compound as the principal carbon source. This category includes some purple and green
sulfur bacteria.
Chemoautotrophs or Lithotrophs (Lithoautotrophs): They use inorganic compounds like H2,
NH3, NO2, H2S as the source of energy and CO2 as the principal carbon source. These
bacteria obtain their energy by the oxidation of H2, NH3, NO2, H2S. A few bacteria and many
archaea belong to this category.
Chemoheterotrophs or Heterotrophs: They derive their energy as well as carbon source from
the metabolism of a single organic compound. Most bacteria belong to this group.
Nitrogen: Nitrogen represents nearly 14% of the dry weight of bacterial cells. Majority of
bacteria thrive on inorganic nitrogen compounds like ammonium salts, nitrates and nitrites,
while others obtain nitrogen from organic nitrogenous compounds like amino acids. A few
bacteria are able to use atmospheric nitrogen.
Phosphorus and sulfur: Phosphorus and sulfur represent 3 and 1% of cell dry weight
respectively. Phosphorus is supplied to bacterial cells as inorganic phosphates and serves as
essential component of nucleotides, nucleic acids, phospholipids etc. Sulfur is required for
synthesis of some amino acids. Some bacteria get their supply of sulfur from inorganic
sulfates, some from organic sulfur compounds and some also utilize elemental sulfur.
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In addition to these non-metallic elements, bacteria also require metal ions, such as K+, Mg++,
Ca++, Fe++ as cofactors for many enzymes and are essential for normal growth. Other metal
ions such as Zn++, Cu++, Mn++, Mo++, Ni++, Co++ etc. are often required in trace amount and
serve as the cofactor for many enzymes.
Growth Factors: Apart from the requirement of carbon and energy source, macro- and
microelements, many of the bacteria may require small amounts of certain organic
compounds, which they cannot synthesize from available nutrients. Such compounds are
required in small amounts and are called growth factors. The need for growth factors in
bacteria varies from species to species and are broadly categorized into (i) purines and
pyrimidines: required for synthesis of nucleic acids (DNA and RNA); (ii) amino acids:
required for the synthesis of proteins, and (iii) vitamins: needed as coenzymes and functional
groups of certain enzymes.
Some bacteria like E. coli do not require any growth factors. They can synthesize all essential
purines, pyrimidines, amino acids and vitamins, starting with their carbon source, as part of
their own intermediary metabolism. Certain other bacteria (for example Lactobacillus)
require purines, pyrimidines, vitamins and several amino acids in order to grow. These
compounds are to be added to the culture media, which are used for their growth. The growth
factors are not metabolized directly as sources of carbon or energy, rather they are
assimilated by cells to fulfill their specific role in metabolism.
Bacterial Growth
The term growth as it is applied in biology refers to an irreversible increase in cellular mass
due to synthesis of all its essential constituents and is accompanied by an increase in cell
numbers. In multicellular organisms, growth is reflected by an increase size while in
unicellular organisms like bacteria growth refers to an increase in the number of cells in a
population. Therefore, growth of bacteria is usually measured by measuring the increase in
cell numbers, increase in dry weight of cell mass or by monitoring the uptake and synthesis
of cellular components like DNA, RNA, protein etc. and is expressed in the form of a growth
curve.
When a population of viable bacterial cells (inoculum) is introduced into a fresh liquid
growth medium contained in a flask and is incubated under proper environmental conditions,
the population will pass through distinct phases of growth. The changes in number of cells or
the biomass are plotted against time to obtain a sigmoid growth curve. The typical growth
curve as illustrated in Figure 14 shows four distinct phases: the lag phase, the exponential or
logarithmic phase, the stationary phase and the death or decline phase. In between each of
these phases there is a transitional phase.
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Figure 8 . Typical bacterial growth curve showing phases of growth
The first phase, the lag phase encompasses several hours and represents the time between the
addition of inoculation and the beginning of the second phase, the exponential phase. During
this period the cells do not divide immediately, but the individual cells grow in size and
synthesize all essential organic constituents like protein, nucleic acid and carbohydrate
through active metabolism. In other words, during this phase the cell division lags behind
cellular metabolism. However, the bacterial cells start dividing showing a slow increase in
population at the end of this phase. This phase is also considered as the phase of adjustment
during which bacterial cells adapt themselves with the new physical and chemical conditions
of the growth environment.
During the next phase, the logarithmic phase (log phase), the bacterial cells undergo rapid
cell division at a constant rate resulting in logarithmic or exponential increase of the
population and the cell mass. The generation time of bacteria i.e. the time required for the
population to double can be determined from this phase, which is characteristic for individual
bacterial species under optimum growth conditions. For example, in suitable medium and
temperature the generation time of Eschericia coli is 20-30 min., while for slow growing
organisms like Mycobacterium tuberculosis it is 344 – 461 min. During this phase, the cells
are most nearly uniform in terms of their chemical composition, metabolic activity and
physiological characteristics. Cells from logarithmic phase exhibit their highest metabolic
and physiological activities.
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During the third phase, the stationary phase, the rate of cell division decreases and the older
cells begin to die. This might be due to accumulation of toxic metabolic products and
exhaustion of essential nutrients from the growth medium. As a result the population appears
constant with no net increase in cell number. During this phase bacteria used to accumulate a
number of secondary metabolites like antibiotics.
The stationary phase is followed by the death phase during which death rate exceeds the
growth rate. The number of viable cells decreases exponentially and there may be very few
viable cells at the end of this phase. The increased death rate is due to further accumulation of
inhibitory metabolic products and almost total deletion of nutrients.
Physical and Environmental Requirements for Bacterial Growth
Bacteria in order to grow require a wide range of physical and environmental conditions. The
optimum condition for the bacterial growth varies depending on the nature of the organism
and the habitat in which it grow.
Effect of Oxygen: Oxygen is a universal component of cells. Depending on the requirement
of oxygen bacteria are classified as: i) Obligate aerobes, which require O2 for growth. They
use O2 as a final electron acceptor in aerobic respiration; ii) Obligate anaerobes do not use O2
as a nutrient. In fact, O2 is a toxic substance, which either kills or inhibits their growth; iii)
Facultative anaerobes (or facultative aerobes) are organisms that can switch between aerobic
and anaerobic types of metabolism. Under anaerobic conditions (without O2) they grow by
fermentation or anaerobic respiration, but in the presence of O2 they switch to aerobic
respiration; iv) Aerotolerant anaerobes are bacteria with an exclusively anaerobic
(fermentative) type of metabolism but they are insensitive to the presence of O2. They live by
fermentation alone whether or not O2 is present in their environment.
Effect of pH: The pH, or hydrogen ion concentration, [H+], of the environment is an
important requirement for growth of the bacteria. The range of pH over which an organism
grows is differentiated as the minimum pH, below which the organism cannot grow, the
maximum pH, above which the organism cannot grow, and the optimum pH, at which the
organism grows best. Microorganisms, which grow at an optimum pH well below neutrality
(7.0) are called acidophiles (Thiobacillus, Acidithiobacillus, Sulfolobus species). Those which
grow best at neutral pH are called neutrophiles (Staphylococcus aureus) and those that grow
best under alkaline conditions are called alkaliphiles.
Effect of Temperature: A particular microorganism usually exhibits a range of temperature
over which it can grow. Considering the total span of temperature required for growth,
bacteria are classsified (Figure 10) as: i) mesophiles, organisms growing best within a
temperature range of 25-40°C; ii) thermophiles, organisms with an optimum temperature
between about 45°C and 70°C; iii) extreme thermophiles or hyperthermophiles, organisms
with an optimum temperature of 80°C or higher and a maximum temperature as high as
115°C; and iv) psychrophiles, the cold-loving organisms that are able to grow at 0°C or lower
but have an optimum temperature of 15-20°C.
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Figure 9 . Classification of bacteria based on temperature requirement
Figure 9 . Classification of bacteria based on temperature requirement
Water Availability: Water is the solvent in which the molecules of life are dissolved, and the
availability of water is, therefore, a critical factor that affects the growth of all bacteria. The
availability of water for a cell depends upon its presence in the atmosphere (relative
humidity) or its presence in solution or a substance (water activity). The water activity (Aw)
of pure H2O is 1.0 (100% water). Water activity is affected by the presence of solutes such as
salts or sugars that are dissolved in the water. The higher the solute concentration of a
substance, the lower is the water activity and vice-versa. Microorganisms live over a range of
Aw from 1.0 to 0.7.
Sodium chloride: The only common solute in nature that occurs over a wide concentration
range is salt [NaCl], and some microorganisms are named based on their growth response to
salt. Microorganisms that require some NaCl for growth are halophiles. Mild halophiles
require 1-6% salt, moderate halophiles require 6-15% salt; extreme halophiles that require
15-30% NaCl for growth are found among the archaea. Bacteria that are able to grow at
moderate salt concentrations, even though they grow best in the absence of NaCl, are called
halotolerant.
Bacterial Reproduction
In case of bacteria, growth is equivalent to reproduction as the growth of individual cell is
accomplished by the reproduction of the entire organisms in which the genetic information is
transmitted to the next generation.
Binary fission: Binary fission is the most common mode of reproduction in bacteria. In
binary fission a cell divides to produce two equal-sized daughter cells (Figure 10). The
process of cell division requires the doubling of bacterial chromosome prior to cell division
so that each daughter cell receives a complete genome. The process of binary fission starts
with the inward growth or invagination of the cytoplasmic membrane and formation of new
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cell wall material, which ultimately separates the two daughter cells. This is commonly called
septa formation. Formation of septa or cross wall physically separates the two complete
bacterial chromosomes in two daughter cells. This ensures the equal distribution of complete
genome between the cells. As result of cross wall formation two equal sized cells are formed.
Figure 10 . Diagram showing bacterial multiplication by binary fission.
Figure 10 . Diagram showing bacterial multiplication by binary fission.(a) Mother cell,
(b) Cell enlargement, (d) Initiation of septum formation, (d) Transverse septum formation and
equal distribution of cell components, (e) Separation of daughter cells
Budding: Budding is a process in which small protrusion expands outward from a mother cell
forming a daughter cell. The daughter cell increases in size until it breaks off from the mother
cell. In budding the cell wall extends from one point instead of growing evenly throughout
the cell, which allows polar division. During the process a copy of the chromosome from the
mother cell must pass into the daughter cell before the bud breaks off. This type of division is
found in Culobacter, Rhodomicrobium etc.
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Fragmentation: Filamentous bacteria such as Nocardia reproduce by fragmentation of the
filaments into rod-shaped or coccoid cells. Each of these bacillary or coccoid cells can give
rise to a new organism.
Formation of spores: Bacteria belonging to the group Actinomycetes form characteristic
branched filamentous hyphae similar to eukaryotic fungi. These filamentous bacteria
reproduce by the formation of asexual spores called conidiospores or simply conidia
developed singly or in chains from the tips of the filamentous hyphae (e.g. Streptomyces).
During the formation of spores the hyphal tip undergoes septation by crosswall formation to
form chain of spores. In some members like Streptosporangium, the spores are enclosed
within specialized sac called sporangium and the spores are called sporangiospores. The
spores are produced in large numbers and each of them on germination gives rise to a new
organism.
Economic Importance of Bacteria
Microorganisms are widely distributed in our environment and are involved in every aspects
of human life. They have long been exploited in manufacturing of industrial products and
medicines, in bioremediation of polluted ecosystems and also in increasing soil fertility.
Biotechnological advancement has led to exploring the versatile nature of microorganisms
towards the benefit of mankind. Some of the major applications of microorganisms are
enumerated below:
Food and Fermented beverages
Bacteria play a major role in the production of food and fermented beverages including dairy
products, alcoholic beverages, fish and meat products along with a variety of fermented plant
products.
Dairy Products: Milk from cow, buffalo, sheep, goat and horse are used as raw materials for
making of cheese, yogurt, butter, sour cream, etc. Traditionally, they are produced by lactic
acid bacteria, like Lactococcus, Lactobacillus, Leuconostoc, Streptococcus, etc. Table 3
illustrates examples of some fermented dairy products and the producer organisms.
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Table 3. Examples of fermented dairy products and microorganisms involved in their production.
Products
Raw ingredients
Fermenting microorganisms
Acidophilus milk
Milk
Lactobacillus acidophilus
Milk curd
Lactococcus lactis, Leuconostoc citrovorum
Soft Cheese (unripened)
Cottage
Soft Cheese (Ripened, 1-5 months)
Camembert
Milk curd
Lactococcus lactis, Lactococcus cremoris
Semi-soft Cheese (Ripened, 1-12 months)
Roquefort
Milk curd
Lactococcus lactis, Lactococcus cremoris
Hard Cheese (Ripened, 3-12 moths)
Cheddar
Milk curd
Lactococcus lactis, Lactobacillus casei,
Lactococcus cremoris, Strptococcus durans
Yogurt
Milk,
Streptococcus
milk solids
delbrueckii ssp. bulgaricus
thermophilus,
Lactobacillus
Manufacturing of these fermented products starts with inoculation of pasteurized milk with a
specific bacterium. Fermentation generates lactic acid, which modifies milk proteins and
forms flavor and aroma compounds. Cheese manufacture involves formation of solid curd
from milk, removal of the liquid whey and ripening of curd. Hundreds of varieties of cheese
are available throughout the world depending upon the type of milk and microorganism used,
ripening period and methods of processing. Sour milk or yogurt is also a major dairy product
usually prepared from whole milk using strains of Streptococcus and Lactobacillus. The
production of butter involves ripening of pasteurized cream with Lactococcus lactis and
Leuconostoc citrovorum for 24 to 48 hours prior to churning.
Alcoholic beverages: Around the world different types of alcoholic beverages like wine, beer
and distilled spirits are manufactured through microbial fermentation of fruits juices,
hydrolysed grain and root starch. Though bacteria such as Zymomonas species are involved
in the production of these beverages, yeasts like Kluveromyces and Saccharomyces
cerevisiae are primarily used. These organisms ferment sugar and produces ethanol along
with compounds imparting aroma and flavor and carbondioxide. Most of the beverages are
aged to modify the flavor and increase alcohol concentration. Wine is produced mainly from
grape vines because it contains high level of fermenting sugar, pleasant flavor and inhibits
growth of spoilage organisms. The immense range of wine depends on variety of grapes,
cultivation conditions, fermentation processes and post fermentation treatments. Cider is the
alcoholic beverage prepared from apple juice and contains 2-8% ethanol. Beer is the nondistilled alcoholic beverage containing 3-8% ethanol and made from partially germinated
cereal grains termed as malt. Distillation of the intermediate products during production of
wine, beer, etc. results in formation of whisky (from beer) or brandy (from wine and cider).
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Fermented food: The fermented meat products like sausages, pepperoni, salami, etc. are
produced by the activity of bacteria Pediococcus cerevisiae, Lactobacillus plantarum and
Staphylococcus carnosus. Meat is allowed to ripen after adding salt and bacteria for several
days and a change in texture, color and flavor is noted. Fermented fish products like sauces
and paste are used as flavoring agents and produced by fermentative activity of
Staphylococcus carnosus and S. piscifermentans. Several locally available fruits, vegetables,
cereal grains, legumes, oilseeds are used for preparation of fermented food. Fermented soya
beans are a major dietary source in South-east Asia. Soy sauces, mainly used as condiments
and flavoring agents is prepared using Pediococcus acidophilus. Sauerkraut or fermented
cabbage, is prepared by salting shredded cabbage and fermenting with natural microflora like
Lactobacillus plantarum, L. brevis, etc. along with Leuconostoc mesenteroides for
acidification.
Health-care products
Antibiotics are the most important group of compounds produced by industrially important
bacteria and they have been used for the last 60 years in improving human health. The other
major health-care products are vaccines, vitamins, amino acids, proteins, etc. Antibiotics are
secondary metabolites produced by variety of microorganisms and are used as antimicrobial
chemotherapeutic agents. The first antibiotic, penicillin, was discovered by A. Flemming in
1928 from the fungus Penicillium notatum. Later Waksman discovered streptomycin (1946)
from filamentous bacteria Streptomyces griseus. Since then a wide variety of antibiotics have
been discovered from bacteria, particularly the filamentous group called actinomycetes. Some
of the common antibiotics and their producer organisms are shown in table 4. An ideal or
broad spectrum antibiotic should be active against Gram-positive as well as Gram-negative
bacteria. Commercial production of antibiotic involves growth of the suitable organism in a
large submerged tank called biofermentor for 7-8 days and desired compound is recovered
from the culture filtrate by filtration, adsorption, precipitation and purification.
Table 4 . Some common antibiotics and their producer organisms
Antibiotic
Bacteria
Bacitracin
Bacillus licheniformis
Polymyxin B
Paenibacillus polymyxa
Streptomycin
Streptomyces griseus
Neomycin
S. fradiae
Chloramphenicol
S. venezuelae
Tetracyclin
S. rimosus
Erythromycin
S. erythreus
Nystatin
S. noursei
Amphotericin B
S. nodosus
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Vitamins: Cyanocobalamine or Vitamin B12 is produced commercially by fermentation of
Propionibacterium freudenreichii and P. shermani. It is also recovered as a by product of
antibiotic fermentation from Streptomyces olivaceous. Another vitamin, pantothenic acid is
produced from immobilized cells of Escherichia coli. Precursor of vitamin C or ascorbic acid
is also produced by Gluconobacter.
Vaccines: Live attenuated (weakly virulent) strains of bacteria or its inactivated forms are
used for preparation of vaccines (Table 5). They are the most important tool for fighting of
infectious bacterial diseases. Commercial production requires growth of the desired organism
in large quantities followed by subsequent treatment and testing for safe use. Bacterial protein
toxins can also serve as vaccines following their inactivation with formaldehyde or heat to
form toxoids. In addition metabolic by products of bacteria produced by recombinant DNA
technology are also being used as vaccines.
Table 5. Examples of some bacterial vaccines applied for medical use
Vaccine
Disease
Live attenuated vaccine
Bacillus anthracis
Anthrax
Salmonella typhi
Typhoid
Inactivated vaccine
Meningites
Neisseria meningitides
Vibrio cholerae
Cholera
Clostridium tetani
Tetanus
Toxoids
Diphtheria
Corynebacterium diphtheria
Proteins: Recombinant DNA technology has allowed production of many therapeutic protein
and peptide like insulin, human growth hormone, etc. Therapeutic proteins have also been
developed for treatment of cancer and viral diseases, cardiovascular diseases, neurological
disorders, etc. Apart from Escherichia coli, Bacillus subtilis and Pseudomonas aeruginosa
have been used for preparation of these cost effective recombinant drugs.
Industrial Chemicals and Fuel
Bacteria have been employed for generation of a huge variety of industrial agents including
enzymes, organic acids, microbial polymers and polysaccharides. Biofuel production also
involves microbial fermentation.
Microbial Enzymes: Many bacterial strains are employed for commercial production of a
large number of enzymes which find application in laundry, paper, textile and leather
industries and various pharmaceutical preparations. These enzymes include amylase,
protease, glucose isomerase, etc. (Table 6). Commercially produced bacterial enzymes are
nowadays replacing chemical catalysts because they are easy to produce, cheaper and
environment friendly.
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Table 6. Examples of some bacterial enzymes and their industrial application
Enzyme
Source
Applications
Amylase
Bacillus subtilis
Starch processing, baking, textile manufacture, etc.
B. licheniformis
Pullulanase
Klebsiella aerogenes
Starch processing
Glucose isomerase
B. coagulans
Production of syrup
Streptomyces galbus
Protease
Biological detergents, meat tenderization and cheese
B. subtilis
manufacture
Alkaline protease
Laundry detergents
B. licheniformis
Amino acids: Microbial production of important amino acids is preferred because they are
biologically active. They can be used as a supplement for animal as well as vegetable
proteins, as food additives to improve taste and for pharmaceutical preparations. Several
bacteria like Corynebacterium, Arthrobacter, Brevibacterium, etc. produce large amount of
amino acids in the culture medium from where it is isolated and later purified. Commercially
produced amino acids include lysine, glutamic acid, methionine, etc.
Organic acids: Acetic acid, citric acid, lactic acid, gluconic acid, etc. are produced from
microbial fermentation and used in the food industry as an acidulant and flavoring agent.
They are also used in electroplating, detergent industry and pharmaceutical processes.
Vinegar is a condiment containing 4% acetic acid. It is produced from maple syrup, molasses,
honey, cereals, root starch, etc. or alcoholic beverages like wine, cider, spirit alcohol, etc.
Acetic acid fermentation occurs under aerobic condition using strains of Acetobacter and
Gluconobacter. Lactic acid is good solvent and finds application in polymer industry.
Besides, it is also a good preservative. In commercial plants, lactic acid bacteria like
Leuconostoc delbrukii, L. bulgaricus, L. pentosus, Streptococcus lactis, etc. are grown in a
medium of semi-refined sugars and the lactic acid thus produced is recovered in crystalline
form as calcium lactate by addition of CaCO3.
Microbial polymers: Bacteria synthesizes a host of biopolymers mostly polysaccharides like
xanthan, dextran, curdlan, etc. Xanthan gum produced by Xanthomonas campestris is used in
preparation of food like sauces, syrup, etc., in paint and textile industries, making of
explosives, deodorants, etc. Dextran, a polymer of α-glucose, is obtained from Acetobacter,
Klebsiella and Leuconostoc and finds major application in pharmaceutical industry. Curdlan,
from Alcaligenes faecalis is also used in pharmaceutical preparations. Bacteria like
Alcaligenes, Pseudomonas, Bacillus, Azotobacter, etc. under suitable growth conditions
produce large amount of polymers inside the cell as reserve substances. Chemically they are
poly-β-hydroxybutyrate (PHB). It is biodegradable in nature, offering a desirable alternative
to synthetic plastics. The bioplastics obtained from microbial sources are nowadays being
used in medical and packaging industries.
Biodetergents: Microbial products like glycolipids from Pseudomonas aeruginosa,
Rhodococcus erythropolis and Bacillus subtilis have been found to possess characteristics
similar to detergents and are, therefore, used for preparation of environment-friendly
biodetergents and biosurfactants. They are used for emulsification, wetting, dispersion,
solubilization, etc.
Biofuel: The depletion of fossil fuel supplies within the next few years has led to the search
for alternative energy sources. Apart from geothermal, nuclear, wind, water and solar energy,
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biological fuel is interesting because it is renewable, available in both liquid and gaseous
state and environment friendly. Biofuel includes methane, ethanol, butanol, propanol,
methanol and hydrogen. They are produced from renewable resources like plant biomass,
natural vegetation, domestic and industrial wastes. Methanogenic bacteria produces methane
while, butanol fermentation involves species of Clostridium. Although yeast is exclusively
used for ethanol production, commercial potential of bacteria like Zymomonas mobilis,
Clostridium thermocellum, Thermoanaerobacter, etc. are not inferior. The purple nonsulphur bacteria are mostly responsible for production of hydrogen.
Environmental application
Biomining: The environment-friendly method of extracting minerals utilizing bacteria that
leach or solubilize metals from its ore is referred to as biomining. Acidophilic sulphur
oxidizing bacterium, Thiobacillus ferroxidans plays a major role. It is an aerobic
chemolithotropic organism capable of oxidizing elemental sulfur or sulphide ore to sulfuric
acid. In comparison to traditional smelting process requiring high energy, costly chemicals,
etc., bioleaching offers a cost effective solution without causing environmental
contamination. Moreover, this process is efficient in removal of metals from low grade ore
and thus useful for recovery of costly metals like uranium, gold and silver.
Bioremediation: The use of microorganisms for detoxification of areas polluted by heavy
metals, nuclear wastes, petrochemical products, pesticides, etc. is termed as bioremediation.
Microorganisms growing in contaminated areas develop mechanisms to avoid toxicity and
they either transform the compounds to less toxic state or immobilize them by binding to cell
surfaces. Metal-resistant bacteria like Pseudomonas, Bacillus, Alcaligenes, etc. helps in
cleaning up of soil and water contaminated by heavy metals. Nowadays, bacterial biofilms or
mixed consortium of bacterial species is being used for bioremediation process development.
Bacterial cells immobilized on a cheap support system also find potential application in
bioremediation plants.
A General Account Of Cyanobacteria
Introduction
Cyanobacteria (Greek Cyanos Meaning Blue) Belongs To A Phylum Of Bacteria That Obtain
Their Energy Through Photosynthesis. They Are Often Referred To As “Blue Grren Algae”
(BGA) Due To Their Similarity In Their Appearance And Ecological Role. The
Cyanobacteria Are Photoautotrophic Aquatic Prokaryotes, Being Devoid Of Proper Nucleus,
Mitochondria, Golgi Apparatus And Endoplasmic Reticulum. The DNA Is Not MembraneBound And Thylakoids Are Also Free In The Cytoplasm. Hence, They Are More Akin To
Bacteria Having Similar Biochemical And Structural Characteristics. Fossil Records Suggest
That Cyanobacteria Appeared On Earth Nearly 2.8 Bilion Years Ago. They Were The First
Dominant Organisms To Use Oxygenic Photosynthesis Via Photosystem II Where Water Is
Broken Down To Release Electrons And Protons, Which Drive Photosynthesis. The
Organisms Changed The Composition Of The Earth’s Atmosphere By Releasing About 20%
Of Oxygen That Permitted Evolution Of Modern day plants and animals. The change marked
the end of Archean Era of the Precambrian time and the started the Proterozoic Era or the age
of Cyanobacteria.
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Occurrence and Habit
Cyanobacterial members are widely distributed in all kinds of habit. They are mostly aquatic,
inhabiting fresh water lakes, pool, ponds, etc. Several species of blue green algae like
Microcystis, Anabaena, Oscillatoria, etc sometimes grow in sufficient quantities in free
floating condition and are called phytoplanckton. They colour the entire water body and often
consist of a single species. Such growth is referred to as “water bloom”. This abnormal
proliferation of the cyanobacteria consumes oxygen and produce toxic substances rendering
the water obscure and inducing mortality of fish and other animals. Other fresh water species
of cyanobacteria like Scytonema, Porphyrosiphon, etc. are subaerial, growing upon moist
rocky edges, damp cliffs and soil. Nostoc commune is often conspicuous on soil surface. In
rainy season they form an extensive coating on the soil.
Cyanobacteria also inhabit limnic and marine environments. They flourish in salty, brakish or
fresh water, in cold or hot springs, etc. Marine algae grow along the shore as benthic
vegetation in the zone between the high and low tide marks. Many cyanobacteria isolated
from coastal environments tolerate saline environment. Halotolerant marine cyanoforms
include species of Microcoleus, Apanothece, Synecococcus, Synechocystis, Anabaena, etc.
However, many fresh water cyanoforms are also able to withstand high concentrations of
sodium chloride. Cyanoforms inhabiting hot springs of different countries like North
America, Japan, New Zealand, Italy, etc. have been reported. The best example of
thermophilic cyanobacterium is the single celled Synechococcus, found in the microbial mat,
covering the hot spring in Yellowstone National Park. In contrast, Cyanoforms tolerating
extremely freezing temperatures have been reported from Antarctic and Arctic lakes.
Fresh water localities with diverse trophic zones are inhabited characteristically by numerous
species of cyanobacteria both near-surface epilimnic and deep, euphotic, hypolimnic zones.
Many species colonize surfaces by attaching to rocks or surfaces forming microbial mats.
These members also have an impressive ability to colonize infertile substrates like volcanic
ash, desert sand and rock. Species of Calothrix and Pleurocapsa inhabit calcareous substrata
like shell and corals, both as epilithic and endolithic forms. Cyanobacteria have also been
reported to live in symbiotic associations with animals, plants and endophytically on other
algae. Symbiotic associations of Nostoc and Anabaena have been reported from Bryophytes
(Anthoceros, Blasia, Clavicularia), Pteridophytes (Azolla), Gymnosperms (Cycas, Zamia)
and Angiosperms (Trifolium alexandrium – root nodules). The symbiotic partnership of
Nostoc, Gloeocapsa, Scytonema, Stigonema, etc and fungi is very well known in many
lichens. Cyanoflora like Anabaeniolum and Oscillospira have been found to occur as
parasities on intestines of man and other animals.
STRUCTURAL RANGE
The blue green algae exhibit the simplest form of thallus. The basic morphology comprises
unicellular, colonial and multicellular filamentous forms. Unicellular cyanoforms have
spherical, ovoid or cylindrical cells. The daughter cells formed as a result of binary fission
separate and occur singly or may aggregate in irregular colonies being held together by slimy
matrix secreted during growth (Eg. Chroococcales). However, a regular series of cell division
combined with sheath secretion results in members of Chamaesiphonales and Pleurocapsales.
Filamentous cyanoforms result from repeated cell division occurring in a single plane at right
angle to the main axis of the filament. This multicellular structure consisting of a chain of
cells is termed as trichome which may be straight or coiled and is usually surrounded by a
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mucilaginous sheath. A filament may contain a single or many trichomes. Variability in cell
shape and size occurs in filamentous cyanobacteria.
A
B
C
D
Plate 4. Photomicrographs showing vegetative and reproductive structures in some
cyanoforms. (A) Nostoc filament, intercalary heterocyst indicated by arrow (B) Filament of
Lyngbya indicating (arrow) hormogonia (C) Rivularia with terminal heterocyst (D)
Gloeotrichia with terminal heterocyst and sheath.
Members of the order Oscillatoriales consists of identical cells arranged in uniseriate and
unbranched trichome, while trichomes composed of heterogenous population of cells are
observed in Nostocales and Stigonematales. In unbranched filamentous type, either there is
no differentiation into base and apex or the trichomes are whip-like with broad bases attached
to a substratum tapering terminally into a colourless hair. Cyanobacteria can develop two
kinds of branching. False branching is the result of extrusion of the filament throughout the
sheath producing either a Y shaped branch (Eg. Tolypothrix) or may germinate a false branch
(Eg. Scytonema). True branching is produced by lateral division of cells in a main axis as in
Fischerella.
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Cellular Ultrastructure
Cell wall: Cyanobacteria has four-layered cell wall. The soft innermost layer faces the cell
membrane. The second layer is rigid composed of murein or peptidoglycan, made of
alternating N-acetylglucosamine and N-acetylmuramic acid residues. The softer two outer
layers are made up of lipopolysaccharides. The cell wall is susceptible to penicillin as well as
lysozyme as in many eubacteria. The cell wall is perforated with small pores of 70nm in
diameter which allows secretion of mucilaginous sheath composed mainly of complex
polysaccharides.
Cell membrane: Cyanobacterial cell membrane is similar to that of eukaryotic cells.
Electron microscopic studies reveal that it is a three-layered structure: the outer and inner
layer being composed of extrinsic and hydrophilic extension of intrinsic membrane proteins
while the middle layer is phospholipid or glycosylglyceride bilayer traversed by the
hydrophobic portions of intrinsic membrane proteins. In some points the cell membrane is
able to invaginate creating a space between the cell membrane and the cell wall. These areas
called mesosomes are rich in proteins and involved in transfer of electrons.
Figure 11 . Ultrastructure of a typical Cyanobacterial cell
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Cytoplasm: The cyanophycean protoplast is differenciated into two parts: the outer
peripheral coloured portion called chromatoplasm or chromoplasm and the inner colourless
portion called the central body or centroplasm or nucleoplasm.
Chromatoplasm: This constitutes the intensely pigmented part of the cell interior, which
mainly contains the photosynthetic lamelle or the thylakoids which are packed in layers. The
thylakoid membrane harbours the electron transport system needed for the light reaction of
photosynthesis including ‘chlorophyll a’ attached to membrane-bound proteins. This reaction
center harvests light of broad wavelength with the help of other accessory membrane-bound
pigments like zeaxanthin, B-carotene, echinenone, canthaxanthin, and myxoxanthophyll.
Phycobilisomes are found attached to the thylakoid and act as a light-energy antenna for
photosynthesis. They consist of pigments, phycocyanin (blue) and phycoerythrin (red).
Phycobilisomes preferentially funnel light energy into photosystem II for the splitting of
water and generation of oxygen. Polyhedral bodies containing crystalline reserves of
ribulose-bis-phosphate-carboxylase-oxygenase (RUBISCO, RuBP carboxylase), are found
near the chromatoplasm to assist in photosynthesis.
In the cytosol between the thylakoids a number of spherical or irregularly shaped granules are
embedded which act as reserve food materials. These are called Cyanophycean starch.
Chemically it is α-1,4-glucan. However, another proteinaceous reserve food is visible in the
chomatopalsm called cyanophycin granules. It is composed of arginine and asparagine and
meant for accumulating and sequestering nitrogen for future use. Lipid droplets also act as
storage inclusions inside cyanobacterial cells.
Nucleoplasm: The central region of the cyanobacterial cell is less colorful than the
chromatoplasm and is termed centroplasm or nucleoplasm. This region includes the nucleoid
or small, circular DNA molecules without nuclear envelope, nucleolus and histone proteins.
RNA is also present in cyanobacteria. Identical to other eubacterial members, cyanobacteria
also possess abundant 70S ribosomes which accomplish the task of protein synthesis.
Cyanobacterial cells accumulate highly polymerized phosphate called polyphosphate
granules which are produced by enzymes from ribosomes and represents a storage house for
phosphates.
In planktonic species of cyanobacteria, like Microcystis, Anabaena, Gloeotrichia,
Tolypothrix, Nostoc, etc. gas vacuoles are present. The structures are not membrane bound
but consist of laterally connected cylindrical tubes of protein that are permeable to gas but not
water. They are quite visible under light microscope as gray bodies. They contain metabolic
gasses and therefore serve as buoyancy regulator of the cells.
Movement
Cyanobacterial members do not possess flagella but shows gliding motion. The movement
can be forward and backward or a slow wavy movement of the terminal portion of a
trichome. This is enabled by secretion of gelatinous material through the minute pores on the
cell wall or on account of rhythmic waves of alternate expansion and contractions passing the
whole length of the trichome. Gliding motility requires contact with a solid surface and
occurs in a direction parallel to the long axis of the cell or filament. The mechanism of
gliding motion is variable amongst cyanobacterial members. In the unicellular cyanoform,
Synechocystis, movement occurs via type IV pili, while cells of Synechococcus can swim in
the absence of flagella. Filamentous cyanoforms move in one direction for 5 - 8 min before
they move in reverse direction in response to external stimulus. In members of
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Oscillatoriaceae translocation is accompanied by species-specific revolution along the long
axis of the filament, while, in Nostocaceae, rotation is not observed during gliding motility. A
number of external stimuli of which light is the most important one, is known to control
gliding movement of cyanobacteria.
Nutrition
Cyanobacterial members have a photosynthetic lifestyle and carry out oxygenic
photosynthesis using a diverse range of light harvesting complexes. While most of the
cyanoforms are aerobic, some species are capable of anaerobic metabolism and perform
anoxic photosynthesis using reduced electron donors. Although most species are obligate
phototrophs, a few cyanobacteria are capable of chemoheterotrophic growth in presence of an
exogenous carbon source.
Reproduction
Mode of reproduction in cyanobacterial members is extremely simple and is vegetative and
asexual in nature. There is complete absence of gametes or sex cells. In unicellular forms,
multiplication takes by binary fission and cell contents divide by constriction with
simultaneous splitting of the chromatin material into two equal masses. However, in
Chamaesiphonales and Pleurocapsales, budding and multiple fission are specific modes of
reproduction. Filamentous cyanoforms reproduce by trichome fragmentation or by formation
of hormogonia which are distinct reproductive segments of the trichome exhibiting gliding
movement and gradually develop into a new trichome. Breaking of filament may result from
animal feeding, due to death of certain cells in a trichome or by formation of double concave
discs of gelatinous material called separation discs. In some cases filaments break at places
where heterocysts are present. These are thick walled, yellow to dark brown coloured cells
containing reserve food which can survive under unavoidable conditions. Such modified
vegetative cell are developed at the onset of adverse environmental conditions. Heterocystous
cyanobacteria reproduce by trichome fragmentation, hormogonia and akinetes. Akinetes are
large reserves of carbohydrates and owing to their density and lack of gas vesicles, they
eventually settle at the bottom of the lake. They tolerate adverse conditions and grow into
juvenile filaments under favorable conditions.
Figure12 .Reproductive structures in cyanoforms
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Figure12 .Reproductive structures in cyanoforms
Heterocycts
Vegetative cells of the filaments often get differentiated possessing thick wall and hyaline
protoplast, which are capable of fixing nitrogen. Such cells are termed as heterocysts. They
may be either intercalary (situated in the middle of the filament) or may be basal (at the base
of the filament) and occur singly but rarely two or more in series. Intercalary heterocysts are
distinguished by the presence of two pores situated on either side while the basal one has a
single pore situated on the side attached to the trichome. Heterocysts are connected to the
neighboring cells by means of protoplasmic connections passing through these pores.
Heterocysts has two wall layers, an outer thick wall and an inner thin one or investment,
photosynthetic lamellae, and ribosomes. Heterocysts are devoid of phycocyanin but contain
glycolipid and acyl lipid as reserve food. Heterocysts have positive role in nitrogen fixation
and act as storehouses of reserve food material. They also serve simple mechanical functions
like providing suitable place for fragmentation or secrete substances, which stimulate growth
and cell division.
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Figure 13 . Cyanobacterial cells showing location of heterocyst
Economic Importance
Soil fertility: Blue-green algae, mainly those belonging to Nostocaceae, are able to convert
atmospheric nitrogen to ammonia. Species of Nostoc, Anabaena, Cylindrospermum,
Calothrix, Tolypothrix, etc. have shown capability to fix nitrogen during growth thus
contributing towards nitrogen cycle and is utilized to increase soil fertility.
Soil reclamation: Cyanobacteria helps in reclamation of barren or alkaline soils in several
parts of the world. Species of Nostoc, Scytonema, Anabaena form a thick stratum on the
surface of the saline usar soils and successive growth of the algal mat in water logged
condition helps in reclamation by decreasing the pH to near neutral range. This is followed by
an increase in total N2 content and organic matter. The treatment has allowed growth of crops
in formerly barren soils.
Water pollution: The formation of water blooms lead to a noxious condition in the
management of water reservoir. When a huge population of BGA (Anabaena flos-aquae,
Aphanizomenon flos-aquae and Microcystis aeruginosa) gets lodged on the water surface and
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exposed to solar radiation, O2 saturation, etc. massive cell lysis and disintegration occurs
leading to deoxygenation of water. This causes death of fishes and other animals of the lake
and creates an unpleasant smell due to large amount of suspended organic matter making the
water unfit for domestic consumption.
Use as food: Cyanoforms are a valuable source of food for human. Spirulina contains all
amino acids essential for human beings and is a rich proteinaceous diet. In China, Japan and
Taiwan, several BGA serve as side dish and a delicacy. Cyanobacteria are cultured and
commercially exploited for various food and medicinal products like vitamins, drug
compounds and growth factors.
Bioremediation: Large scale culture of cyanobacterial mats helps to sequester or precipitate
metals / radionuclides by surface absorption or by conditioning the surrounding chemical
environment. Organic contaminants are degraded and may be completely mineralized by
blue-green algae.
Bioenergy: Cyanobacteria possess the unique ability to simultaneously evolve oxygen and
hydrogen. This is the basis for the exploitation of BGA for development of a biophotolytic
system for solar energy conversion. Cyanobacterial mats have been tested for production of
biohydrogen and this holds immense potential for the future.
Toxicity: Poisonous toxin-producing cyanoforms occur in ponds and lakes throughout the
world and causes fatal effects on animals including humans. Humans affected by toxic strains
of Lyngbya majuscule, Schizothrix calcicola, Oscillatoria nigroviridis show symptoms which
include redness and itching of skin, eyes and throat, headache, diarrhea, vomiting and nausea.
Classification
Rippka et al. (1979) divides Cyanobacteria into five sections. The first two includes
“Unicellular cyanoforms: cells single or forming colonial aggregates held together by
additional outer cell wall layers” while the rest includes “Filamentous cyanoforms: a
trichome (chain of cells) which grows by intercalary cell division”.
Section I: Unicellular cyanobacteria that reproduce by binary fission or by budding. These
are of 3 types:
Unicellular, cylindrical to ovoid cells, reproduce by binary transverse fission. Examples:
Synechococcus, Gloeothece, Gloeobacter
Unicellular, spherical cells, divide in two or three successive planes at right angles to one
another. Examples: Synechocystis, Gloeocapsa
Unicellular, reproduce by forming successive spherical buds from one pole of the ovoid cell.
Example: Chamaesiphon
Section II: Unicellular cyanoforms that reproduce by multiple fission. They are of 2 types:
Reproduction only by multiple fission. Examples: Dermocarpa, Xenococcus
Reproduction by both binary fission and multiple fission. Examples: Dermocarpella,
Myxosarcina.
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Section III: Filamentous non-heterocystous cyanobacteria that divide in only one plane.
Members are of 2 types:
Trichomes helical. Example: Spirulina
Trichomes straight. Examples: Oscillatoria, Pseudoanabaena
Section IV: Filamentous heterocystous cyanobacteria that divide in only one plane. They are
of 2 types:
Reproduction by random trichome breakage, germinate from akinetes, trichomes
indistinguishable from mature vegetative trichomes. Examples: Anabaena, Nodularia.
Reproduction by random trichome breakage, some germinate from akinetes, form
hormogonia, distinguishable from mature trichomes by absence of heterocysts and one or
more of the following characters: rapid gliding motility, smaller cell size, cell shape and gas
vacuolation. Examples: Nostoc, Scytonema
Section V: Filamentous heterocystous cyanobacteria that divide in more than one plane.
Reproduction by random trichome breakage, by formation of hormogonia and by germination
of akinetes. Examples: Clorogloeopsis, Fischerella
Suggested Readings:
General Microbiology: H. G. Schlegel
Microbiology: concepts and applications: M. J.Pelczer, E.C.S. Chan and N. R. Krieg
General Microbiology: R. Y. Stanier, J. L. Ingharam, M. L. Wheelis and P. R. Painter
The Algae: V. J. Chapman, and D. J. Chapman
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