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Chapter 4
VIRUS, BACTERIA AND CYANOBACTERIA
The most abundant organisms on earth are so small that they cannot be seen
with the naked eye. They are most successful hence so abundant to be found
every where in large numbers. Though structurally simple viruses and bacteria
play an important role in the biosphere.
Learning objectives:
Micro-organisms, a heterogenous group of living organisms which can only
be seen with the help of microscope.
Discovery of virus, characteristics, structure, types and the diseases caused
by viruses in humans, other animals and plants.
Characters of bacteria.
Shapes, nutrition and importance of
bacteria. Characters of cyanobacteria.
4.1
MICRO-ORGANISMS — AS A HETEROGENOUS GROUP
OF ORGANISMS
The minute living organisms which cannot be seen without the help of
microscope are called micro organisms. They form a heterogenous group which
includes viruses, bacteria, cyanobacteria, protozoa, unicellular algae and
some fungi.
Bacteria and cyanobacteria are prokaryotes and studied under kingdom
prokaryota (Monera). Rest of the micro-organisms are eukaryotic and studied
under protoctista (Protista). Bacteria are heterotroph as well as autotroph, but
all members of cyanobacteria and algae are autotroph. Protozoa are heterotroph.
Viruses lie somewhere between living and nonliving organisms.
4.1.1 Viruses:
Viruses (Latin word Viron = Poison) are the smallest, the simplest and perhaps
the most primitive living things.
By 1800's many biologists had demonstrated that many diseases of man and
other organisms were caused by bacteria. Some diseases puzzled them. One
such disease was tobacco mosaic disease occurring in tobacco plant leaves. In
1892, Russian biologist, Iwanowsky showed that this disease was due to some
thing smaller than bacteria. He named them as viruses. No one had seen them
because they were too small to be seen even with the compound microscope.
The year 1935 was important in solving the story of what viruses really are and
how they behave. A new kind of microscope the electron microscope had been
constructed and any object smaller than mµ (millimicron 1,000,000,000th part of
a metre) can be observed. Wendell Stanley crystallized the infectious particles,
now known as tobacco mosaic virus (TMV). This TMV and many other viruses
were actually seen with the help of the electron microscope.
4.1.2 Characteristics:
Viruses are non-cellular obligate parasites that always have a protein coat and a
nucleic acid core. They cannot live and reproduce outside of living cells since
they lack the ability to do so by themselves. They range in size from 20 nm to
250 nm (One nm =10-9 metre). They are sub-microscopic. There is no sexual or
asexual reproduction. They reproduce by replication.
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The simple viruses use the enzymes of the host cell for both their protein
synthesis and gene replication; the more complex ones contain upto 200 genes
and are capable of synthesizing, through their host many structural proteins and
enzymes themselves.
Viruses are either virulent, destroying the cell in which they occur or temperate,
becoming integrated into their host genomes (total hereditary materials) and
remaining stable there for long periods of time.
4.1.3 Structure:
They appear like small rods, tadpoles 6r may be polyhedrals or like little spheres.
Viruses may consist of nucleic acid, capsids, envelopes and tail-fibers. Their
nucleic acid may consist of a single or several molecules of DNA or RNA. The
smallest viruses have only four genes while the largest have upto two hundreds.
The protein coat that encloses the nucleic acid is called a capsid. It may be of
different shapes. Capsid is made up of protein subunits called capsomeres. The
number of capsomeres is characteristics of a particular virus.
Some viruses have accessory structures called viral-envelopes that help them
infect their hosts. They are membranes covering their capsids.
The simplest viruses consist of a single molecule of a nucleic acid (DNA or RNA)
surrounded by a capsid, which is male up of different protein molecules.
Some bacterial viruses or bacteriophages, are among the most complex viruses.
Each of them is made up of at least five separate proteins: these make up the
head; the tail core, the molecules of the capsid, the base plate of the tail, and
the tail fibers. A long DNA molecule is coiled within the head.
They may be:
Plant-viruses
Animal-viruses
Bacteriophages
infecting plants,
infecting animals and
which infect bacteria.
4.1.4 Viruses living or non-living:
Viruses are considered as living organism because of the following characteristics;
1.
2.
3.
4.
i)
ii)
iii)
Viruses with their core of DNA or RNA surrounded by a protein coat
somewhat resemble the chromosomes of other living organisms.
They have the ability to reproduce (Property of replication-reproduction).
Many kinds of viruses are known to undergo mutations.
Viruses show genetic recombination.
However, they show following characteristics not exhibited by living
organisms.
Non-cellular structure.
Undergo crystallization.
Completely inactive outside host's cell.
4.1.5 Viral diseases (Transmission/spread and control)
1.
Animal diseases:
Several of the animal viruses cause many important diseases. Poliomyelitis
caused by polio virus was a wide spread, crippling disease. Although poliomyelitis
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is now largely under control by vaccination in the developed countries, it remains
a serious and common disease in the tropics and elsewhere in the less developed
parts of the world.
Colds are viral infections of the upper respiratory tract. More than 200 of viruses
that cause colds have been identified.
Measles and Mumps are the common viral diseases of children. However, proper
vaccination of MMR vaccine develops immunity against these infections.
Human Immune Virus (HIV) was discovered in 1985 to be the cause of
Acquired Immune Deficiency Syndrome (AIDS).
Rabies, in humans is also caused by a virus but flu is the most common disease
caused by the influenza virus.
The majority of human viral diseases are spread through respiratory tract by air
borne droplet spread by sneezing and coughing of infectious people or by the
contaminated water and food or by blood or other body fluids. Hygienic living
and vaccinations are the two best controls of viral diseases.
2.
Plant diseases:
One of the best known plant disease is caused by tobacco mosaic virus. This
virus affects the leaves of tobacco plants and light green and yellow patches
appear on the leaves of diseased plant.
Agriculturists have not yet succeeded to device cure for most of viral diseases of
plants. Therefore, their efforts are focused largely on reducing the incidence and
transmission of such diseases and also on developing varieties of crop that are
relatively resistant to certain viruses.
4.2
BACTERIA
Bacteria grow in every habitat of earth under all possible environmental
conditions. They are the largest number of creatures and are found everywhere.
They can survive in freezing temperature and also in hot sulphur springs.
Bacteria, first observed by Leeuwenhoek are considered as the smallest, oldest
but the simplest living organisms. Bacterial cell ranges in size from 0.2 µ.
(micron) to 2 µ in width and 2 to 10 µ (microns) in length. Bacteria are strictly
unicellular prokaryotes. Bacterial cell consists of cell-wall, plasma-membrane,
cytoplasm and nuclear material. Cell-wall is chemically complex and totally
different from ordinary plant cell-wall as cellulose is not present. It is thick and
rigid and made up of amino-acids, sugar and sometimes chitin.
Next to cell-wall is thin outer most layer of plasma membrane or cell membrane
which is attached to cell-wall at few places. It has many pores. Chemically, it is
made up of lipids and proteins. Cell-membrane performs the function of
respiration as mitochondria are absent in them. It also acts as selective
membrane.
Cytoplasm is granular, present in between cell-membrane and nuclear region. It
has many but small vacuoles, ribosomes and glycogen-particles. It has no
endoplasmic reticulum and no other membranous organelles like golgi apparatus,
plastid and mitochondria.
Bacteria do not possess well organized nucleus as they have no nuclearmembrane, no nucleolus, no nucleoplasm and no structures like typical
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chromosomes. However, DNA is found as concentrated structures called
chromatin-bodies. Shortly before a bacterium divides, these DNA bodies replicate
(double) and are equally distributed to the daughter cells. Thus, there is no
typical mitosis instead they divided by binary fission.
4.2.1 Form (Shapes):
There are different shapes of bacteria.
i)
Cocci: (Singular-Coccus = Gr. Kokkos = Berry, rounded). They are
spherical and according to cell-arrangement they are: Solitary (single)
(Monococcus), in pair (Diplococci), in chain (Streptococci), in cluster
(Staphylococci) etc.They are non-flagellated.
ii)
Bacilli: (Singular-Bacillus = L, Bakulus = A rod) .They are rod shaped.
They may be found in pairs (Diplobacillus) or in chains (Streptobacillus)
etc. They may be flagellated.
iii)
Spirilla: (Singular-Spirillum = Gr. Sperira = A coil. They are spiral or
cork screw shaped (spirillum).
iv)
Vibrio or comma: They are slightly curved or comma (,) shaped e.g
Vibrio cholerae. They may be flagellated.
4.2.2 Nutrition:
Most bacteria are heterotrophic with few autotrophic. Heterotrophic bacteria are
those which cannot synthesize their organic compounds from simple inorganic
substances. According to their mode of feeding, heterotrophic bacteria may be
saprophytic, symbiotic or parasitic.
i)
Saprophytic bacteria:
They get their food from dead organic matter. The soil is full of organic
compounds in the form of humus. Bacteria living in the soil have large number of
enzymes that break down the complex substances of humus to simpler
compounds. These bacteria absorb and utilize these simple compounds as a
source of energy. Many other saprophytic bacteria cause decay of dead animal
and plant material as they convert complex organic compounds to simpler ones.
ii)
Symbiotic bacteria:
They are found associated with other living organisms. They obtain food from
the host without harming it, e.g. nitrogen fixing bacteria in the roots of
leguminous plants.
iii)
Parasitic bacteria:
They grow inside the tissues of other living organisms and obtain food at the
expense of host. These bacteria lack certain complex systems of enzymes.
iv)
Autotrophic bacteria:
They can synthesize organic compounds from simple inorganic substances.
Autotrophic bacteria may be photosynthetic or chemosynthetic.
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a) Photosynthetic bacteria: They have pigments very similar to the
chlorophyll and named as bacterio chlorphyll. These pigments are bounded by
invaginated plasma membrane in the cytoplasm and not in the chloroplast. They
carry out photosynthesis.
The majority of bacteria are free-living heterotrophs (saprophytic decomposers)
that contribute significantly to recycling matter through ecosystems. Many are
also symbiotic heterotrophs, including those that cause disease.
b) Chemosynthetic bacteria: They obtain their energy from oxidation of some
inorganic substances like iron, hydrogen, nitrogen and sulphur compounds to
synthesize their organic compounds.
4.2.3 Respiration:
Another aspect of metabolism which can be used in the classification of bacteria
is their need for oxygen in respiration.
Aerobes require oxygen for respiration.
Anaerobes respire without oxygen.
Some bacteria are killed in the presence of oxygen, they are called obligate
anaerobes. Others use oxygen but can respire without it, they are called
facultative anaerobes. Bacteria which can only survive with oxygen present are
obligate aerobes.
4.2.4 Importance of bacteria:
1.
Useful bacteria:
i)
Agricultural bacteria: Bacteria decompose dead plants and animals bodies
and convert various organic compounds into simple forms such as nitrates, sulphates,
phosphates etc for utilization by green plants again. Nitrifying bacteria convert the
proteins of these dead bodies into nitrates. Then these nitrates are absorbed and
utilized by the green plants. Soil bacteria increase the fertility of the soil by bringing
about physical and chemical changes in the soil.
ii)
Alimentary canal bacteria: They help herbivores in the digestion of
cellulose by producing an enzyme Cellulose. Similarly some are present in
human beings and make vitamins.
iii)
Industrial bacteria: They help in curing and ripening of tobacco leaves,
fermentation of sugar into alcohol, ripening of cheese, retting (softening) of
fibres, curdling of milk, conversion of hides into leather etc.
iv)
Medicinal bacteria: Valuable antibiotic drugs have been obtained from
bacteria, e.g Thyrothycin, Subtilin. Riboflavin is a vitamin produced by
Clostridium.
2.
Harmful bacteria:
i) Pathogenic bacteria: They are responsible for most varied kind of diseases
in human beings, animals and plants. They may act as invisible enemies for
man. Some of the diseases found in man due to bacteria are typhoid, tetanus,
cholera, diphtheria, and tuberculosis (T. B) etc. Plant diseases caused by
bacteria are black rots of cabbage, citrus canker, fine-blight of pear and apple,
ring rot of potato etc.
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Food spoilage: Bacteria spoil food by fermentation and decomposition.
4.3) CYANOBACTERIA (Blue green algae)
Salient features of this group are:
1.
2.
3.
4.
5.
6.
7.
These blue-green algae are Prokaryotic.
They may occur alone or in the form of colony.
Cell wall is double layered.
Protoplasm is differentiated into an outer coloured region—chromoplasm
and an inner colourless region—centroplasm.
Found in fresh water, few are marine.
Total absence of sexual reproduction.
Only asexual reproduction takes place e.g. Nostoc, Anabena.
Cyanobacteria are photosynthesizers that sometimes can also fix atmospheric
nitrogen. They were probably the first organisms who introduced oxygen into the
primitive atmosphere and probably first to colonize land.
NOSTOC:
Taxonomic position: According to new classification, Nostoc belongs to
kingdom prokaryotae(Monera).
Structure:
The Nostoc is filamentous. The filaments are intermixed in a gelatinous mass
forming a ball like structure. It floats on water. A single filament look like a chain
of beads. Each filament is unbranched and has a single row of rounded or oval
cells. Each cell has double layered wall. The outer thicker layer is made up of
cellulose mixed up with pectin. The inner thin layer is made up purely of
cellulose. The protoplasm is differentiated into an outer coloured region chromoplasm and an inner colourless region centroplasm.
There is total absence of endoplasmic reticulum, mitochondria, golgi bodies and
vacuole. However ribosomes, pseudovacuoles and reserve food are present.
The centroplasm also called central body have hereditary material (DNA) in the
form of long coiled structure called chromatin body.
At interval there are found slightly larger oblong, colourless cells with slightly
thicker walls. These are called heterocyst. Each heterocyst is the center of
nitrogen fixation. But they are specially concerned with the multiplication of
filaments during unfavorable conditions.
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SUMMARY
Living organisms which cannot be seen without the help of microscope are
called micro organisms. They form a heterogenous group which includes
viruses, bacteria, cyanobacteria, protozoa, unicellular algae and some fungi.
Viruses are the smallest, the simplest and perhaps the most primitive living
organisms.
Viruses are non-cellular parasites. They cannot live and reproduce outside
the living cells.
Some bacterial viruses or bacteriophages are among the most complex
viruses.
There are three types of viruses. Plant-viruses infecting plants. Animalviruses infecting animals. Bacteriophages are those which infect bacteria.
Bacteria are the most abundant prokayrotes with variety of shapes.
Most bacteria are heterotrophic a few are autotrophic. According to their mode of
feeding, heterotrophic bacteria may be saprophytic, symbiotic or parasitic.
Some bacteria are killed in the presence of oxygen, they are called obligate
anaerobes. Others use oxygen but can respire without it, they are called
facultative anaerobes. Bacteria which need oxygen for survival are obligate
aerobes.
Bacteria are responsible for most varied kind of diseases in human beings,
animals and plants. They may act as invisible enemies of man.
The Nostoc is filamentous. The filaments are intermixed in a gelatinous mass
forming a ball like structure. It floats on water. A single filament look like a
chain of beads.
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EXERCISE
1.
Fill in the blanks with appropriate terms:
i)
Previously, living organisms were classified into two
kingdoms namely Plant Kingdom and ________ kingdom.
ii)
Whittaker proposed ___________ kingdoms.
iii)
Kingdom Protista has been renamed as ________ .
iv)
Kingdom Monera contains all ________ organisms.
v)
A single filament of Nostoc look like a _______ of beads.
2.
Write whether the statements are false or true:
3.
i)
Kingdom Protoctista includes all the eukaryotic organisms,
which are no longer classified as animals, plants or fungi.
ii)
iii)
iv)
v)
Bacteria, cyanobacteria, protozoa are all micro-organisms.
Electron microscope was constructed in. the year 1945.
Virus mainly consist of capsid and Nucleic acid.
AIDS is a disease caused by bacteria.
Encircle the appropriate answer:
i)
Viruses are
(a) Cellular
(b) Noncellular
(c) Multicellular
(d) Unicellular
ii)
Characteristic of viruses due to which they are considered as
living.
(a) Noncellular structure
(b) Total parasite
(c) Undergo crystalization (d) Undergo reproduction
iii)
Bacilli bacteria are
(a) Rounded
(b) Rod shaped
(c) Spiral shaped
(d) Curved
iv)
Streptococci are the bacteria which occur
(a) Two in number
(b) Four in number
(c) In chain
(d) In bunch
v)
Nostoc belongs to
(a) Cocci bacteria
(b) Eubacteria
(c) Cyanobacteria
(d) None of them
Write detailed answers of the following questions:
i)
Discuss in detail the discovery, characteristics, structure
and types of viruses.
ii)
Give the general characters, shapes, nutrition and
economic importance of bacteria.
Write short answers of the following questions:
i)
What are the four types of bacteria according to shapes?
ii)
What do you mean by Industrial bacteria?
iii)
Name the five Kingdoms proposed by Whittaker.
iv)
Explain the different autotrophic modes of nutrition
in Bacteria.
v)
Describe salient features of Cyanobacteria.
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