Main Groups of
Microorganisms
Microorganisms show a great diversity in their cell
structure and function. Some broad groups of Microorganisms are as follows.
[I] Prions
Prions are small proteinaceous
infectious particles that do not contain nucleic acid. They were first
identified by American neurobiologist Stanley Prusiner in 1982 in sheep affected by neurobiological
disease. These are the only disease causing organisms which do not have nucleic
acids. The major portion of this infectious agent is a protein called PrP (prion protein). The
prion protein exists in two
configuration. One is a normal cellular from (PrPc) that is found in
the brains of all adult mammals and does not cause disease and is found only in
infected cells. The infected form is given a symbol consisting of PrP with a
superscript that indicates the source of prion. For example, prions from sheep
infected with priom disease scrapie are PrPsc. Prions are
know to cause several disease in human and animals.
[II] Viroids
Viroids are subviral pathogenic particles consisting
of short stand of naked RNA.As there is no protective protein coat (capsid)
around the nucleic acid, the viroids lack a definite shape. The RNA strand a
circular configuration Absence of protein coat is probably due to intercom type
of base sequence in viroids which do not code for polypeptides. Many plant disease
(e.g., Chrysanthemum stunt, Chrysanthemum chlorotic mottle, cucumber pale
fruit) and a few animal disease are of viroid origin.
[III] Viruses
Viruses are
sub-microscopic obligate interacellular parasites, i.e.,they essentially
require living host cells in order to multiply. A simple virus particle, called
virion, has a nucleic acid core of genetic material enclosed within a protein
coat. They are smaller than bacteria, measuring 20-14,000 nm in length. Viruses
contain only a single type of nucleic acid, i.e., either RNA (ribovirus) or DNA
(deoxyvirus). They use the machinery of the host cell for their multiplication.
Viruses are host specific and infect vertebrates, invertebrates, algae, fungi,
protists and bacteria. Many disease of human beigns like hepatitis
b, AIDS, cancer, herpes, mumps, polio, rabies, European encephalitis and
common cold (influenza) are caused by viruses. In plants viruses cause mosaic
diseases, leaf curls, etc.
[IV] Rickettsias
Rickettsias are
smallest prokaryotes. 0.3-0.7μm wide and 1-2μm long. They are usually rod
shaped (commonly called coccobacilli) but can exist in many alternate forms.
Unlike most bacteria, it is difficult to stain rickettsias with ordinary
aniline dyes. They can be stained with Giemsa,s stain. They have a
typical prokaryotic structure. Mucopolysaccharide is the main constituent of
their cell wall. Like viruses, rickettsias are also obligate intracellular
parasites and grow only within the living host cells. Most of the rickettsias
are transmitted to human beings by insects and ticks. In humans rickettsias
cause diseases of spotted fever group (e.g., rocky spotted fever, Endemic
typhus, epidemic typhus, scrub typhus. Q fever,etc.).
[v] Mycoplasmas
Mycoplasmas can be
defined as prokaryotes without a cell wall, hence highly pleomorphic. Most of
them are aerobes or facultative anaerobes.
Mycoplasmas have been
cultured in the laboratory in organic media containing sterols. They contain
both RNA and DNA but it is usually less than half that normally occurs in other
prokaryotes. Mycoplasmas are known to cause diseases in animals and plants.
[VI] Bacteria
Bacteria from the largest
group of prokaryotes. They vary greatly in their cell shapes, cell
arrangements, motility, oxygen requirements, nutritional and metabolic
properties and reactions to gram,s stain. Their varied physiological
activities enable them to develop on a very wide range of organic and inorganic
substrates.
Modern techniques in
molecular biology and biochemistry have provided sufficient evidences to
support the division of bacteria into two groups, viz., Archaebacteria and Eubacteria.
Archae-bacteria are characterized by (i) the absence of muramic acid and
D-amino acid in their cell wall, (ii) absence of fatty acid in their membranes,
and (iii) presence of several subunits in the RNA polymerase. Besides these in
many archaebacteria the membrane is monolayered.
They are resistant to antibiotic chloramphenicol. They often live in extreme
environments such as high salt concentration or hot acidic stremes.
Eubacteria are
characterized by the presence of N-acetylmuramic acid and N-acetylglucosamine
in their cell wall. Lipids present in their cell membrane are straight chain
molecules connected with ester linkages forming bilayered structure. In
eubacteria the DNA polymerase is made of four subunits. They are sensitive to
chloramphenicol.
[VII] Cyanobacteria
Cyanobacteria are prokaryotes.
They resemble algae in the presence of chlorophyll a and oxygenic
photosynthesis (photosynthesis in eubacteria is anoxygenic). Many bacteria have
specialized cells called heterocysts which contain nitrogen fixing enzymes.
Metabolic
Diversity Among
Microorganisms
Microorganisms
basically differ from one another in the substrate that they can utilize as
food and in their mechanisms for gaining energy. There are microorganisms in
nature that can utilize, as a nutrient source, any carbon containing
constituent that is a component of living cells. Besides this, many other
compounds unrelated to living cells can also be utilized as substrate for
growth.
Among these compounds
that microorganisms can actually utilize as carbon/energy source are carbon
monoxide, cyanide and methane. Thus it is evident that microorganisms can
sustain themselves on inorganic substances by using pathways that are
unavailable to either plants or animals. These marked nutritional differences
are referred to collectively as representing the microbes, metabolic
diversity,.
On the basis
of sources of energy, organisms are either
phototrophs (use light as primary source of energy) or chemotrophs (derive energy by the oxidation-reduction of inorganic
or organic compounds). On the basis of their principal carbon source, organisms
can be classified into autotrophs
(use CO2 as carbon source) and heterotrophs
(take carbon from an organic source). Combining energy and carbon source,
organisms have been grouped into photoautotrophs,
photoheterophs, chemoauto-trophs and chemoheterotrophs .
[I] Photoautotrophs
Photoautotrophs use
light as source of energy and CO2 as their chief source of carbon. Photosynthetic bacteria (e.g.,
cyanobacteria) are good examples of photoautotrophs. Like higher plants
cyanobacteria use hydrogen atoms of water to reduce carbon dioxde and produce
oxygen.
Several other
photosynthetic bacteria, such as green
sulphur bacteria (e.g., chlorobium) use sulphur compounds (such as H2S) to
reduce carbon dioxide, instead of water. As such photosynthetic process in
these bacteria does not produce oxygen (anoxygenic photosynthesis).
[II] Photoheterotrophs
Photoheterotrophs use
light as source of energy but use organic compounds (such as alcohol, fatty
acids or carbohydrates) as carbon source instead of carbon dioxide. Green nonsulphur bacteria (e.g.,
chloroflexus) and purple sulphur
bacteria (e.g., Rhodopseudomonas) are common examples of photoheterotrophs.
[III] Chemoautotrophs
Chemoautotrophs use
electrons from reduced inorganic compounds, such as hydrogen sulphide,
elemental sulphur, ammonia, hydrogen gas, etc., as a source of energy. Like
photoautotrophs these organisms also use carbon dioxide as their principal
source of carbon. Bacteria such as Beggiatoa, Thiobacillus sp., Nitrosomonas
and Nitrobacter are some common chemoautotrophs.
[IV] Chemoheterotrophs
Chemoheterotrophs
usuall use the same organic compounds as the the energy source and carbon
source. For example, if a chemoheterotrophic microorganism uses glucose, then
carbon in glucose molecule serves as source of carbon and hydrogen atom of
glucose molecule is used as source of energy, Most of the microbes belong to
this category.
Environmental Diversity
Among Microorganisms
A high rate of growth
and adaptability make the microbes ubiquitous. They occur wherever life is
possible and form a significant percentage of total biomass on the earth.
Microorganisms have
the ability to live in the extreme conditions of temperature, acidity and
alkalinity. This may be attributed either to their less sensitive cellular
mechanisms or to their capacity of controlling the cell wall composition in the
presence of extreme environments.
As microorganisms have high
metabolic rate, they metabolize common nutrients more rapidly or use nutrients
that competing organisms can not metabolize. This makes the microorganisms
successful even in a very competitive environment. On the other hand,
microorganisms adapted to extreme environments face no competition. For
example, a hot spring will exclusively have thermophiles and salt lakes will
have only halophiles.
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