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Introduction
The purpose of this chapter is to describe the structural organisation of the prokaryotic cell
i.e. Bacteria. The bacterial cell represents the simplest of all cellular organisms when seen
under the microscope. Bacteria (plural word) is a prokaryotic structure. The singular for this
word is “bacterium” . They do not have true nucleus. They have one chromosome of doublestranded DNA in a circular form. They reproduce by binary fission. They do not have any
intracellular organelles. They belongs to kingdom Monera which is a very diverse group.
There are some photosynthetic bacteria which can do photosynthesis--they don’t have
chloroplasts, but their photosynthetic pigments are their cell membranes. These organisms are
called blue green algae or cyanobacteria. They are the most common and omni present
microorganisms on the earth.
Statement of cell theory
Cell is the structural and functional unit of life.
All organisms are made up of cells.
New cells arise from the pre existing cells.
Structure of bacterial cell
A variety of structures are found in prokaryotic cells. Following image exhibits the typical
structure of a cell.
All this structures are not found in all genus. Gram positive and Gram negative bacteria
differs in their cell wall content. All bacterial cells were surrounded by a cell wall except
mycoplasma. After cell wall there is a periplasmic space beneath which lies a plasma
membrane. Bacteria does not contain the intracellular organelles so their internal structure is
very simple. Genetic material consist of the circular chromosome lying in the nucleoid
region. Ribosome and inclusion bodies are scattered in the cytoplasm. Many cells have
flagella for locomotion. Pilli is also observed for sexual reproduction. Some bacteria also
contain capsules surrounding their cell wall.
Morphology of bacteria
Shape:
Generally four representative shapes of bacteria are observed.
Coccus (singular)/ Cocci (plural): These bacteria are spherical in shapes.
Bacillus (singular) / Bacilli (plural): They are rod-shaped
Spirillum (singular.)/Spirilla (plural): They are spiral shaped.
Comma shaped cells Vibrio
Apart from the above sited shapes, some typical shapes of bacteria are also observed. But
these shapes are restricted to few species only.
Square shaped bacteria ( A.E.Waksby, 1980. )
Star shaped ( Stella gnus)
Actinomycetes charecteristically form long multinucleate filaments structure.
Pleomorphism: Some bacteria are variable in shape and lack a single, characteristic form
and they are called as pleomorphic.
Pattern of cell arrangement
Bacterial cells may be categorised according to arrangement or cell grouping. Every bacterial
species have their characteristic arrangement pattern. Variety of arrangement pattern is
observed in cocci. Most common arrangement pattrns are

Single

Diplococcus: A pair of cells

Streptococcus: Chains of four or more cells.

Tetrads: Four cocci in boxlike or square arrangements

Sarcinae: Cuboidal Packet

Staphylococci: cluster of cocci.
Size
Bacteria range in size from about 0.2 to 5 micrometer. Smallest bacteria mycoplasma are
about the same size as the largest viruses. (Pox virus). Cocci measures anywhere from 0.5 to
3.0 micrometer in diameter. Bacilli range from 0.2 to 2.0 micrometer in diameter and from
0.5 to 250 micrometer in length. Escherichia coli has the size of 1.1 to 1.5 micrometer wide
by 2.0 to 6.0 micrometer long.
Habitats of bacteria
They are found in the bottom of the deepest oceans , at the peak of mount , inside the body
of animals, and even in the frozen rocks and ice of Antarctica. One feature that has enabled
them to spread so far, and last so long is their ability to go dormant for an extended period.
They are found in the hot springs, frozen soils, acid drained mines, saline lakes.
Reproduction in Bacteria
Bacteria reproduce very rapidly. E.coli can reproduce in after every 20 minutes. Bacteria
reproduce by two basic methods
1) Asexual reproduction
Asexual reproduction involves only one parent.
The offspring are exact copies of the
parent.Bacteria splits into two cells using binary fission. Each daughter cell receives the
same gentic material like parent cell.
2) Sexual reproduction.
It requre two parent cell. It involves the physical contact between the two cell. It occurs with
the help of the conjugation tube. Here progeny cell have a mixture of the parent cells' traits.
It occurs by the process of Conjugation.
Structures external to the cell wall
Flagella
Majority of motile bacteria moves with the help flagella. It is a thread like appendages
extending outward from the plasma membrane and cell wall. Size of the flagella is 15-20
micrometer long. The detailed structure of a flagellum can only be seen in the electron
microscope.
Flagellation (Flagellar arrangement): There are different arrangement of flagella found on
bacterial cell. Different pattern of flagellation are
Monotrichous: One flagella, located at one end. E.g. Pseudomonas
Amphitrichous: Two flagella located at each pole. E.g. Spirillu
Lophotrichous: Cluster of flagella at one end. E.g. Aquaspirillum
Peritrichous: Many flagella spread evenly on whole surface. E.g. Proteus
Ultra structure of flagella:It consist of three parts.

Filament: longest portion. Extend from the cell surface

Basal body: This part is embedded in cell body.

Hook: curved part which connect the filaments to basal body.
Basal body is most complex structure of the flagella. In E.coli , basal body consist of four
rings i.e. L,P,S,M rings. Gram positive bacteria have only two basal rings.
Chemical composition of Bacteria: It is made up of protein fibers. These protein fibers are
made up of flagellin protein. Molecular weight of the flagellin protein is in the range of 20 kd
to 60 kd. Flagellin protein of different bacterial species is not identical.
Mechanism of Flagellar movement: Bacteria moves when this flagellar helix rotates.
Direction of flagellar rotation determines the nature of bacterial movement.Monotrichous
polar flagella rotates counterclockwise during normal forward movement.Monotrichous
bacteria stop and tumble randomly by reversing the direction of flagellar rotation.
Peritrichous flagella forms bundle of the flagella and moves in the similar way. It is believed
that flagella moves due to interaction of the rings present in the basal body.
Swimming motility: Spirochetes moves due to the axial filament present in between the
periplasmic space. It is also called endoflagellum.
Gliding Motility: This movement occurs without flagella. Gliding bacteria are able to move
on solid surface.
Fimbriae and Pili: Fimbriae is used to denote the nonflagellar appendages other than those
involved in sexual conjugation. Pili refers to appendages that are involved in the transfer of
nucelic acid during a sexual process between called conjugation. They are found mostly on
Gram negative bacteria particularly on Enterobacteriaceae family.
Prosthecae: It is an extension of cell wall and cytoplasmic membrane.. They are observed in
bacteria which are found in the freshwater and marine envrionment. E.g. Caulobacter, Stella
etc. It increases the surface are of the cells for nutrient adsorption.
Stalks: Stalks are non living tubular extension that are excreted by the member of Genus
Gallionella and Planctomyces. It aid to attachment of the cell to the surface.
Spines: Some marine Gram negative bacteria exhibits this structure. It is a rigid protein
appendages. It protect bacteria against ingestion by larger microorganisms.
Spirae: Nitrogen fixing bacteria Azospirillum lipoferum produces sevaral spiral structures
called spirae. It is single coiled thread like structure. Its function is not known.
Capsule and slime layers: Bacterial cells exude extracellular substance in the form of a
capsule or a slime layer. A slime layer is loosely connected with the bacterial cell and can be
simply removed, whereas a capsule is attached firmly to the bacterial cell. Capsule has
specific margins. Capsules can be seen under a light microscope using capsule staining india
ink. Capsule does not stain by the stain and appear as a clear halos surrounding the bacterial
cells. Capsules are made up of simple sugars (polysaccharides).Most capsules are hydrophilic
and may help the bacterium evade dehydration by preventing water loss. Capsules can protect
bacteria from phagocytosis. It may be due more slippery nature of the capsule, which helps
the bacterium to from engulfment by WBC. Capsules are also related to the pathogenicity of
the bacteria eg. Streptococcus pneumoniae. S. pneumoniae mutants which are not able to
synthesize capsule do not cause disease. Such association of pathogenicity and capsule is also
found in some other bacteria. It is also associated with biofilm formation.
Sheaths: Some species of bacteria, particularly those from fresh water and marine
environments, form chains or trichomes that are enclosed by a hollow tube called sheaths. It
may be sometimes become impregnated with ferric or maganese hydroxides; which stregthen
them.