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GENERAL MICROBIOLOGY
Microbial World, History and Development of Microbiology, Scope of Microbiology
Introduction
Microbiology, the study of microscopic organisms, derived its name from three Greek words: mikros (“small”),
bios (“life”), and logos (“science”). Taken together they mean the study of microorganisms which are very small
and cannot be seen by unaided eye. If an object has a diameter 0.1 mm or less, eye cannot see it and very little
details can be seen in an object having diameter 1 mm. So roughly speaking organisms having diameter 1 mm or
less are called microorganisms and are studied in Microbiology.
Although microorganisms are ancient by many standards, microbiology itself is a comparatively new science. The
existence of microorganisms was unknown until the discovery of Microscope. Microscope is an optical instrument
which can magnify small objects which cannot be seen by naked eye. Microscopes were invented in the beginning
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of 17 century. Early Microscopes were of two types; Simple Microscope, with a single lens of very short focal
length and Compound Microscope, with two double convex lens system including ocular and objective lens with
higher magnifying power. Most of the epoch making original discoveries about microorganisms was all made using
simple and compound microscopes.
Characteristics of microorganisms
1. Their size is very small.
2. There is no cellular differentiation. They are unicellular and one cell is capable of performing all the functions.
Some microorganisms are multicellular with little or no cellular differentiation.
3. Microorganisms are present everywhere on the bodies of animals and humans, on plant surfaces, in the air, water,
dust, soil, and even inside the intestinal canal of all insects, birds, animals and human beings.
Taxonomic Groups
Microorganisms have wide taxonomic distribution and include organisms such as protozoa, algae, fungi, bacteria
and virus.The schematic illustrations of different microorganisms are shown in Figure S.1.
Protozoa are unicellular eukaryotic organisms, motile having cilia, flagella and pseudopodia, saprophytic or
parasitic. They are generally present in soil, water and marshy places and their size varies from 5-200 μm. They are
animal-like in that they ingest particulate food, lack a rigid cell wall, do not contain chlorophyll. The study of
protozoa is known as protozoalogy. They are differentiated on the basis of morphological, nutritional and
physiological characteristics. Their role in nature is varied, but the best known protozoa are the few that cause
disease in human beings and animals, such as malaria in humans. Some protozoa are beneficial, such as those found
in stomach of cattle, sheep and termites that help digest food.
Algae are relatively simple organisms, their size varies from 1 μm to several feet. They are considered plant-like
because they contain the green pigment chlorophyll, carry out photosynthesis, and have rigid cell walls. They are
unicellular to multicellular and either motile or nonmotile. The study of algae is known as Algology or Phycology.
These organisms are autotrophic and are found most commonly in aquatic environments or in damp soil. They
cause problems by clogging water pipes, releasing toxic chemicals into water bodies, or growing in swimming
pools. But extracts of some species have commercial uses: as emulsifiers for foods such as ice-creams; as a source
of agar used as solidifying agent in microbial medias and as anti-inflammatory drugs for ulcer treatment.
Fungi are either saprophytes or parasites. They have eukaryotic cell structure which, like algae, have rigid cell
walls. They form characteristic hyphae called mycelium which may be septate, nonseptate or coenocytic. They
form fruiting structures called conidia or exospores and endospores. Spores of fungi are always present in air, dust
and soil. Multicellular fungi are also called molds while yeast is an important unicellular fungus. Size range of
molds is 2.0-10 μm and yeast has size varying in the range of 5-10 μm. Molds have considerable value; they are
used to produce antibiotics- penicillin, cephalosporin etc, fermented products like soy sauce, tempeh, miso,
Roquefort and Camembert cheeses, and many other products. But they are also implicated in various human,
animal and plant diseases including athlete’s foot and the moldy spoilage of grains and peanuts. The unicellular
yeasts are widely used in Baking industry and for the production of all alcoholic beverages like wine, beer etc. On
the other hand, some yeasts cause food spoilage and diseases such as vaginitis and thrush (an oral infection).
Bacteria are unicellular microorganisms. Their size varies from 1-5 μm and have rod, coccus or spiral shape. They
have prokaryotic cellular organization and cell division is usually by binary fission. Some bacteria having mycelial
morphology are known as Actinomycetes and are very important in production of antibiotics. Bacteria are
important in agriculture and play important role in cycle of biological nitrogen fixation. With respect to food, they
are important in fermentations, food spoilage, food poisoning and food preservation. The wide range of industrial
products derived from bacteria affect the human society in numerous ways. Their activities are of enormous
importance and some are beneficial while others are harmful. The study of bacteria is known as Bacteriology.
Viruses are ultra-microscopic, noncellular obligate parasites of plants, animals and bacteria as well as other
protists. Their size varies from 0.015μm -0.2 μm and shapes from spherical, rod, flexuous to cozohedral. They can
be seen only under an electron microscope. Unlike cells, viruses contain only one type of nucleic acid, either DNA
or RNA, which is surrounded by a protein-coat. They lack the cellular components necessary for metabolism or
independent reproduction, viruses can mutiply only on living cells. The study of viruses is known as virology.
Viruses cause large number of diseases in humans (such as AIDS, common cold, poliomyelitis, SARS, genital
herpes, hepatitis etc), plants (tobacco mosaic disease, papaya ring spot disease etc) and foot-and-mouth disease of
animals. In addition, some retroviruses have also been implicated in the growth of some malignant tumors.
Historical Developments in Microbiology
Some momentous discoveries in science were made by amateurs, rather than by professional scientists. One of such
major stalwarts in the history of microbiology a natural scientist, owned his own dry goods store, and was also the
official wine taster for the city of Delft in Holland. Antony van Leeuwenhoek (1632 -1723; Figure S.2A) known as
the Father of Microbiology, was a pioneer in the field of Microscopy and used microscopes of his own design and
manufacture. He was a linen merchant who built microscopes as a hobby, had little formal education and knew only
Dutch language. He made about 500 optical lenses that could magnify objects 275 times and was an amazing feat.
He enjoyed using his microscopes to look at the various things including river water, pepper infusions, saliva feces
and more. He communicated his findings to the Royal Society of London in the form of long series of letters which
were translated and published in the Proceedings of Royal Society.
Leeuwenhoek constructed many microscopes with a single lens which consisted of a spherical lens mounted
between two small metal sheets of silver or brass (Figure S.2B). The specimen was placed on the point of a blunt
pin and brought into focus by manipulating two perpendicular and a linear screws.
Leeuwenhoek’s Findings
Leeuwenhoek had unusual degree of curiosity and observed every object that could be seen through his microscope.
He was one of the greatest Innovators driven by curiosity and infinite energy.
In his letter of September 17, 1683 with his drawings about ‘animals’ in the scrapings of teeth he described
different types of bacteria and called them ‘animalcules’ (Figure S.2C). He also made magnificent observations on
the microscopic structure of seeds and embryos of plants and some invertebrate animals. He discovered
Spermatozoa, RBC and is therefore known as Father of Animal Histology. He described characteristic microflora of
human mouth, curd, vinegar and in fact discovered all the different types of microorganisms known today including
protozoa, algae, yeast and bacteria. He also emphasized the abundance of these microorganisms besides their great
diversity.
He was elected a fellow of the Royal Society of London
(FRS) in 1680.The Royal Netherland Academy of Arts and
Science established a ‘Leeuwenhoek Medal’ in his honor in
1877. The ‘Antony van Leeuwenhoek chair’ established in
Delft places a strong emphasis on research and
appointments are made on the basis of achievements.
Origin of Leeuwenhoek’s microorganisms
Soon after the discovery of microorganisms by Leeuwenhoek, scientists began to study the origin of these small
organisms from the point of view of the two schools of thought; One believed in the Theory of Abiogenesis or the
concept of spontaneous generation i.e. living animalcules are formed spontaneously from nonliving matter, while
the other supported the theory of biogenesis i.e. they are formed from the ‘seeds’ or ‘germs’ of these animalcules
which are always present in the air. It was also believed at that time that many plants and animals can be generated
spontaneously under special conditions.
Biogenesis versus Abiogenesis
It took several clever experiments, which appear too simple today, and more than hundred years to resolve the
controversy. The abiogenesis for plants and animals was disapproved as a result of the experiments by Italian
Physician Fransesco Redi in 1665, who showed that maggots developing in putrefying meat are the larval stages of
flies and will never appear if the meat is well protected in a vessel with the fine gauge mesh so that flies cannot lay
their eggs on meat.
In 1745, John Needham took hot boiling mutton gravy (meat infusion) in a flask
and closed this flask with a cork. He found the spoilage of this infusion and
observed animalcules in it. He killed and destroyed the living matter by boiling
and thus concluded that animalcules arose spontaneously from the meat. In 1769,
Lazaro Spallanzani (1729-1799) an Italian naturalist performed a series of
experiments and showed that heating can prevent appearance of animalcules in
infusion although duration and level of heating required is variable. He was not
satisfied with using cork to plug the flask and sealed it hermetically to prevent
contact with the air completely. He found that sealed infusions remained barren for a long time. A tiny crack in the
flask can result in development of animalcules and they will not appear unless new air entered the flask to come in
contact with the infusion. Spallanzani infact took a series of flasks and gave heat treatments for different interval of
times. He could distinguish animalcules of different types, i.e., Superior or animalcules of higher class which were
destroyed by slight heating undoubtedly protozoa and animalcules of lower class which were very minute, and
much more heat resistant - the bacteria.
Although the experiments Spallanzani conducted were very good but faulty experiments continued to be performed
and evidence gathered in favor of abiogenesis. Moreover, Needham objected to the observations by Spallanzani that
there was no growth in the infusions because air which is essential for life had been excluded from his flasks. An
interesting practical application of Spallanzani’s observation was done by Francois Appert (1805) for preservation
of foods by enclosing them in airtight containers and then heating the containers called ‘Appertization’. He was
thereby able to preserve highly perishable food and the process named canning (called later) was widely used much
before its scientific principles were understood. He performed an experiment by first making the air free of
microorganisms by passing it through red hot tube. This air which still contained 19.4 % oxygen, with and without
heating was passed through a set of flasks containing boiled infusions, the former remained unaffected.
Proof of Biogenesis
During the period when these experiments were done, a new figure was emerging in science, in the name of Louis
Pasteur (1822-1895) in France (Figure S.3). A chemist by training, he emerged as one of the greatest biologists of
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the 19 century. His contributions are the most significant in the history of science and industry and his work with
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germs and microorganisms opened new areas of scientific studies. Pasteur was born on 22 Dec 1822 in the eastern
French town of Dole and later became the Dean of the new science faculty at Lille University in 1854. We pay
tribute to him as he was a great benefactor of humanity.
Pasteur first demonstrated through a series of definitive experiments that air contains microscopically observable
‘organized bodies’. He aspirated large quantities of air through a tube which contained a plug of cotton to serve as a
filter. He removed the cotton plug and suspended it in a solution of alcohol and ether. When he examined the
sediments microscopically he observed the presence of small oval shaped bodies. He later confirmed that when
heated air is passed through a boiled infusion no microbial development takes place but when cotton plug is
suspended in the heated infusion, microbial growth occurs. Pasteur repeated his experiment through swan necked
and gooseneck flasks (Figure S.4) so that the germs from air cannot ascend into it. He boiled the broth in it to kill
all microorganisms in the neck as well as in the flask. The infusion remained sterile in this flask until the neck of
the flask was broken resulting in the growth of microorganisms. Thus he established that development of
microorganisms in organic infusions bring about chemical changes. Schroeder and Von Dusch (1850) started the
technique of cotton plugging because cotton acts as a filter and traps microorganisms.
One of the traditional arguments against biogenesis was the claim that heat used to sterilize the air or specimens
was destroying an essential “vital force”. Those supporting abiogenesis said that, without this force,
microorganisms could not spontaneously appear. In response to this argument, an English Physicist John Tyndall
(1820-1883, Figure S.5) conducted experiments in a specially designed box called ‘Tyndall chamber’ to prove that
dust carries the germs. He demonstrated that if no dust was present, sterile broth remained sterile for indefinite
period. While doing these experiments, Tyndall (1877) also devised a process for complete sterilization by alternate
heating and cooling known as ‘Tyndallization.’ He found that in some cases even boiling the infusion for more than
5 hours was not sufficient to sterilize it and concluded that bacteria have both thermo stable and thermo labile
phases. These thermo stable resting bodies were also observed by Ferdinand Cohn in hay bacteria and were called
endospores. The experiments by Pasteur and Tyndall finally disapproved the theory of spontaneous generation and
promoted the general acceptance of theory of biogenesis.
Other contributions of Pasteur
Pasteur’s inventions were based on his work on fermentation (1857 -1876) and had a practical significance. He was
called by a winery in Lille in France where they were facing the problem of getting a poor product. Careful
investigation of the problem by Pasteur led to conclude that alcoholic fermentation was replaced by another type of
fermentation which converted sugar to lactic acid. Not only did he find the reason for the problem but also the
remedies. It was his ability to apply and relate discoveries to practical world that made his contributions very
significant. The important contributions made by Pasteur are summarized as ; a) Fermentation is a biological
process and is brought about by development and activities of microorganisms, b) A typical fermentation can be
defined by its principal end product, e.g; lactic, alcoholic and acetic acid fermentation, c) Fermentation is a specific
process meaning thereby every fermentation is accompanied by development of a specific type of microorganism
which shows physiological specificity with respect to fermentation e.g., alcoholic fermentation by yeast and lactic
acid fermentation by lactic acid bacteria.
Pasteur also discovered the process known as pasteurization after his name for preservation of wine by sudden
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heating to 60-70 C for few minutes and then cooling to destroy the harmful organisms. This not only saved the
wine industry but the process was also applied later to preserve milk and other liquid foods. Today pasteurization is
widely used in fermentation industries, but we are more familiar with it in the dairy industry.
Germ theory of Disease
Even before Pasteur proved by experiments the possibility of microorganisms as agents of disease, several careful
observers had made strong arguments for the germ theory of disease. A little earlier to this, John Bassi in1836 and
M.J. Berkeley in 1845 had shown that silkworm disease and the great Potato Blight of Ireland was caused by
fungus. Few years later, J.L. Schonlein showed that certain skin diseases in humans are caused by fungal infections.
In 1860 a disease called ‘Pebrine’ was killing large number of silkworms and destroying the silk industry. Pasteur
described that microbes were killing the silkworms and eliminating the worms will wipe out the disease. He also
demonstrated that by weakening the disease germs in lab and then infecting the weakend germs into animal or
person, the animal developed immunity against that disease.
Antiseptic surgery
In the 1860’s, an English surgeon named Joseph Lister was
searching for a way to prevent microorganisms infecting
wounds, as deaths from post-surgery infections were
frequent and accounted to about 45% of the total deaths.
Lister used dilute solution of phenol/carbolic acid to soak
surgical dressings and by performing surgery under a spray
of disinfectant to prevent airborne infections (Figure S.6).
The incidence of surgical sepsis was greatly reduced and
this became a common practice. This also provided
indirect evidence for germ theory of disease. His
experiments were the origin of the present-day aseptic techniques used to prevent infections.
Contributions of Robert Koch and Germ Theory of Disease
In Germany, Robert Koch (1843-1910, Figure S.7) confirmed Pasteur's germ theory
and took it several steps further. His investigations began with a study of Bacillus
anthracis, which causes a disease in cattle. He cultured the anthrax bacillus and later
used the same techniques in tracking and culturing the organism responsible for
tuberculosis and cholera. Koch won the Nobel Prize for his work on tuberculosis, but is
perhaps better remembered for his formulation of four basic principles or postulates of
bacteriology known as Koch’s postulates and they are:
1. Microorganism must be present in every case of the disease.
2. Microorganism must be isolated from the diseased host and grown in pure culture.
3. The specific disease must be reproduced when a pure culture of microorganism is injected into healthy
susceptible host.
4. Microorganism must be recovered once again from experimentally inoculated host.
He then carried out a series of experiments to demonstrate biological specificity of disease causing agent. In the
meantime Pasteur also undertook work on Anthrax and reached conclusions similar to findings by Koch. In 1880,
Pasteur used Koch’s technique to isolate and culture the bacteria that caused chicken cholera. To prove his
discovery, he arranged a public demonstration of the experiment that had been repeated successfully in the
laboratory. He injected healthy chickens with pure culture of cholera bacterium and waited for them to develop the
symptoms and die. But the chickens survived. Pasteur, on reviewing very carefully each step soon, found an
explanation. He discovered that bacteria, if allowed to grow old, could become avirulent. But these avirulent
bacteria could still stimulate something in the host to resist subsequent infection and immunize the host to that
disease. He then applied this principle of immunization for prevention of anthrax, and again succeeded. He called
these avirulent cultures as vaccines and the process of immunization with such cultures as ‘vaccination’. This
recognized the earlier work of Edward Jenner, who had successfully vaccinated a boy named James Phipps against
smallpox in 1798 (Figure S.8). Jenner had observed that milkmaids exposed to cowpox never developed the serious
smallpox, and thus hypothesized that exposure to cowpox somehow led to protection against smallpox. In order to
prove his point, he inoculated James Phipps first with cowpox material, and later with smallpox- causing material.
The boy did not get smallpox.
Pasteur by now was considered as miracle worker in France
and was asked to develop vaccine against rabies- a disease
transmitted to humans from the saliva of infected dogs, cats
and wolfs. Pasteur took the challenge and reproduced the
disease in rabbits by inoculating them with saliva from rabid
dogs. He then removed its brain and spinal cord, dried and
pulverized them, and mixed the powder into liquid and
successfully vaccinated dogs with a series of shots of this
mixture. In 1885, a boy named Joseph Miester, who was
bitten by a rabid wolf, was inoculated by Pasteur with his vaccine, which saved his life (Figure S.9). When he saved
most of the group of Russian farmers bitten by rabid wolves, the Czar sent him 100,000 francs, which along with
many other donations from around the world, founded the famous Pasteur Institute in Paris.
Rise of Medical Microbiology
The work on Anthrax and rabies started the golden age of medical bacteriology.
The Pasteur Institute in Paris and Institute in Berlin became the world centers of
science of Bacteriology. Robert Koch developed methods for isolation of pure
cultures of bacteria and concentrated his work more on isolation, cultivation and
characterization of disease causing agents of major diseases in man. On the other
hand, Pasteur focused his attention on seeking experimental evidence of how
infectious diseases occur in human body and how recovery and immunity
occurs. This was a great medical revolution and within 25 years, methods for
prevention by immunization or hygienic methods were developed against most
of the major bacteria causing human diseases (Table 1).
Discovery of Filterable virus
At Pasteur Institute, filters to retain bacterial cells were developed and were used to get filtrates free from bacterial
cells. In 1892, a Russian scientist Dmitri Iwanovski filtered infectious extract from tobacco plant infected with
mosaic disease. To his surprise he found that filtrate was still fully infectious. This specific discovery was
confirmed soon and within few years, many other plant and animal diseases were found to be caused by some
submicroscopic agents retained in the filtrates passed through bacterial filters. A new class of infectious agents was
discovered and these were called ‘viruses’ (from latin word virus, meaning a slimy liquid or a poison). It was later
found that these agents are different from cellular organisms already known in structure and development.
Frederick Loeffler and P Frosch (1898) found that Foot and Mouth disease (FMDV) is caused by a virus. Bacterial
viruses were independently discovered later by F.W. Twort and F. d’ Herelle (1917). Stanley (1935) crystallized
virus and found that it is made up of protein and nucleic acid.
General characteristics of Virus
1. They are obligate parasites and can grow and multiply in living plant, animal or bacterial cell as host. Such
viruses are known as plant, animal and bacterial virus respectively.
2. They are nucleoprotein particles made up of protein and nucleic acid.
3. They have only one type of nucleic acid either DNA or RNA. No virus contains both.
4. They are ultramicroscopic and can only be seen with the help of an electron microscope.
5. They do not have cellular structure and unit of structure is ‘virion’
Chemotherapy
The methods to control and prevent various diseases became very important and the use of chemicals to kill or
inhibit the disease causing organisms was termed chemotherapy. Chemotherapy has been practiced for hundreds of
years. Mercury was used toto treat syphilis as early as 1495, and cinchona bark was (Quinine) was used in South
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America since 17 century to treat malaria. Paul Ehrlich (1909, Figure S.10) discovered a synthetic arsenic
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compound (606 compound) named Salvarsan that proved effective against syphilis bacterium. The discovery of
antibiotic (penicillin) by Alexander Fleming (1929, Figure S.11) opened another era for chemotherapy to control
infectious diseases as they were found to be most effective chemotherapeutic agents against bacterial diseases. The
discovery of penicillin was an accidental one. One day Alexander Fleming noticed that bacteria were not growing
near the mold in some of the mold contaminated culture plates. This was identified and established to be
Penicillium notatum. He correctly guessed that the mold was producing some substance that was inhibiting
bacterial growth. His original report went unnoticed for almost ten years, when a group from Oxford University, led
by Howard. W. Florey and E. Chain, conducted clinical trials with penicillin to prove it as a ‘miracle drug’. Florey,
Chain and Fleming later were awarded Noble Prize in 1945 for their work.
Another major discovery in chemotherapy occurred in 1932, when, G. Domagk, a German physician, discovered a
group of chemicals called sulfonamides, or sulfa drugs, which were observed to be effective against various kinds
of bacterial infections. Domagk won the Noble Prize for his discovery which helped to launch a second wave of
research on chemotherapeutic agents. Now hundreds of new antibiotics have been discovered for the treatment of
several infectious diseases. Development of vaccines since then has become a continued process and an industry by
itself.
Development of pure culture methods
Around 1870 it was realized that pure cultures must be
used for proper understanding of form and function of
microorganisms. What is a pure culture? A pure
culture of an organism is the culture which contains
large population of only one type of microorganism
generally developed from a single cell. Brefeld
introduced the practice of single cell isolation and cultivation of fungi on solid medium containing gelatin as
solidifying agent. Joseph Lister developed serial dilution technique for pure culture isolation. He devised a small
syringe prototype of the modern micropipettes to dispense small aliquots of liquid in different tubes containing milk
so that final dilution contained one or none of the organism and isolated pure culture of bacteria which was
confirmed by microscopic examination. Robert Koch was experimenting with solid media and used sterile cut
surfaces of potato placed on sterile covered plates to grow bacteria. Since the surface was opaque it was difficult to
examine cultures of bacteria. Richard. J. Petri introduced Petri dish as a suitable medium container for the culture of
bacteria. Pour plate and streak methods for the isolation of pure cultures were also developed by Koch. The use of
gelatin as solidifying agent had following disadvantages; viz., It is a protein which is susceptible to microbial
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digestion and changes from gel to liquid at 28 C while the optimum temperature for growth of wide range of
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bacteria is between 30 – 37 C. Fran Hesse introduced agar- a complex polysaccharide extracted from red algae as
the solidifying agent, which was found to be a suitable solidifying agent because of following properties; a) It is not
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digested by bacteria easily and its melting point is 100 C and remains solidified below 44 C, besides producing a
transparent stiff gel and growth of bacterial/fungal colonies can be seen easily on their surface.
Microorganisms as Geochemical Agents
The pioneering work by Sergei Winogradsky and M. W. Beijerinck (1851-1931) showed that microorganisms
exhibit wide range of physiological diversity and carry out chemical transformations that can’t be performed at all
by plants and animals. They play important role in cycles of C, N and S etc and are responsible for turnover of
matter on earth. They discovered a unique class of bacteria called chemoautotrophic bacteria which can grow in
complete inorganic environments getting energy by oxidation of reduced inorganic compounds and carbon from
carbon dioxide. Important bacteria in this category are sulfur bacteria which oxidize inorganic S compounds and
nitrifying bacteria which oxidize inorganic N compounds. They also developed enrichment techniques to culture
specific bacteria and their isolation.
Nitrogen fixing Bacteria
Microorganisms play an important role in the fixation of atmospheric nitrogen which cannot be used directly as
source of N by most living organisms. Symbiotic N fixing bacteria (Rhizobium) and asymbiotic N fixing bacteria
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(Azotobacter) which use gaseous N for synthesis of their cell constituents help to maintain supply of combined N
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on which all other forms of life are dependent. These kind of bacteria were also discovered by Winogradsky and
Beijerinck.
Microbiology in the Twentieth Century
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In the 20 century, studies on microorganisms have contributed towards development of other disciplines such as
industrial microbiology, biochemistry and molecular genetics etc. The discovery of cell free alcoholic fermentation
by H. Buchner in 1897 laid down the foundation for the beginning of Biochemistry. The discovery that vitamins
used by animals are similar to growth factors required by bacteria led to the finding that there is similarity of
metabolism in all living systems and hence microorganisms were used as models to understand basic fundamental
metabolic processes. Escherichia coli has been extensively used in this category to understand biochemistry and
genetics of various cellular processes. George Beadle and Edward Tatum (1941, Figure S.12) obtained mutants of
bread mold Neurospora and studied the consequences of permanent genetic changes in biochemical terms. Oswald
Avery, Colin Macleod and Maclyn Mc Carty (1944) proved that DNA is the basic genetic material and a model was
proposed for the molecular structure of DNA by James Watson and Francis Crick (1953, Figure S.13). Collapse of
the boundaries between the subjects such as Microbiology, Genetics, and Biochemistry has lead to deeper
understanding of biology at molecular levels under new discipline called Molecular Biology and Genetic
Engineering. The application of Molecular biology has revolutionized the use of genetically engineered
microorganisms for technological purposes. Now microorganisms are being used to produce non-microbial
products at commercial scale for the welfare of human beings. Production of injectable insulin by genetically
engineered E. coli has opened the possibilities for search and development of other suitable organisms for
production of useful products on industrial scale. The exploitation of microorganisms, their systems or their
processes for technological purposes is studied under Microbial biotechnology or industrial microbiology.
Activities of microorganisms important in our daily life
Microorganisms play an important role in sustaining life on this planet and in
our daily life through the following activities:
1. Transformation of matter: Microorganisms degrade dead organic matter and return to the atmosphere in
inorganic form. They complete the cycle of matter and are responsible for transformation of C, N and S and other
important elements which are essential for life.
2. Biological nitrogen fixation: They fix nitrogen from atmosphere and make it available to the plants in usable
form. Important microorganisms under this category include, Rhizobium, Azotobacter, Azospirillum etc.
3. Mycorrhiza: Association of roots of many plants with fungi forms a composite structure called mycorrhiza.
Fungus helps in absorption of mineral salts from soil and plant in turn provides carbohydrates for the growth of
fungus.
4. Silage: This method is used to preserve feed with its characteristic flavor, taste and nutritive value. Leaves of
green plants are compacted in size and some molasses is added. Lactic acid bacteria develop and produce lactic acid
which helps to conserve the cattle feed.
5. Cellulose degradation in Rumen: Ruminants feed on straw and grass which contains about 50 % cellulose. There
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is symbiotic association of microorganisms with rumen for degradation of cellulose and about 10 – 10 cells/ml
of different bacteria are usually present in the rumen. Most important of these include Ruminococcus and
Clostridium.
6. Biogas: Animal waste products and cellulose containing waste is fermented by microorganisms (Methanogens).
N of the animal excreta is preserved in rotting sediment and methane gas so formed is used as a fuel.
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7. Composting: Decomposition of organic matter by microorganisms to convert it into nutrient rich manure is
known as composting. Bacillus, Aspergillus and Thermoactinomyces are important in this process.
8. Industrial uses: Different microorganisms are used for the production of wide range of products at industrial
scale. These include alcoholic beverages, antibiotics, enzymes, pharmaceuticals etc.
Scope of Microbiology
Depending on their applications in different fields, the major areas of applied microbiology are:
Agricultural Microbiology/ soil Microbiology: Microorganisms related to soil fertility, plant diseases,
transformation of matter, biological nitrogen fixations etc are studied.
Food Microbiology: Microorganisms important with respect to food viz., food fermentations, food spoilage, food
poisoning and food preservation are studied in this area.
Industrial Microbiology: Microbial production of useful products like antibiotics, fermented beverages, alcohols,
industrial chemicals, organic acids, enzymes, hormones etc are studied in this area.
Medical Microbiology: Besides their usefulness, microorganisms are casual agents of several diseases of plants,
animals and human beings. Many diseases are caused by viruses also. Medical Microbiology deals with studies on
causative agents of disease, diagnostic procedures, identification of disease causing organisms, development of
effective vaccines and preventive measures etc.
Exo-Microbiology: It deals with exploration of existence of biomolecules and microbial life in outer space.
Geochemical Microbiology: prospects for deposits of coal, mineral and gas, recovery of minerals from low grade
ores, sea water mining operations, coal, mineral and gas formation and exploration are studied in this area.
Molecular Biology: is the program of interpreting the specific structure and function of organisms in terms of their
molecular structures. Microorganisms have been used as a tool to explore fundamental life processes because of
many advantages; their fast rate of reproduction, their growth can be easily manipulated, and lysed cells can be
studied in terms of specific chemical reactions, specific products and specific structures involved.
Genetic engineering and Biotechnology: This is an important development in applied Molecular Biology which
refers to the human capability to alter the genetic make up of an organism. It has been possible because of the
detailed knowledge of structure and function of DNA and discovery of the restriction enzymes which can cleave or
cut the DNA at specific sites along the chain length. Use of genetically engineered microorganisms has opened
great potential for production of drugs, vaccines, improvement of agricultural crops etc.
Environmental Microbiology: deals with use of microorganisms to protect the environment from the toxic
pollutants, reduction of microbial load in the sewage and industrial wastes, pesticides, insecticides, heavy metals
etc. and to develop suitable methods for treatment of these wastes and their recycled use.
Some recent applications of bacteria
Biosensors: Bacteria and their components are being used to detect toxic pollutants and continuous monitoring of
nutrients/other parameters during food processing and fermentation processes. Technically a Biosensor is a
miniaturized analytical tool comprising of highly specific biological sensing element i.e. either integrated within or
associated with transducer which convert physiochemical interaction into a discrete or continuous digital electronic
signals, which are proportional to single or related groups of analytes. The detection of toxic pollutants in soil,
water and environment is important for the protection of human and animals. Biosensors using components from
bacteria have been constructed which can detect biologically active toxic pollutants. Such bacterial sensor requires
both a receptor which is activated in the presence of pollutant and a reporter which will make this change apparent.
Biosensors use lux operon from bacteria Vibrio or Photobacterium. This operon contains inducer and structural
gene for enzyme luciferase. In the presence of coenzyme FMNH , luciferase reacts with molecule to form enzyme
2
substrate complex which emits blue green light changing FMNH to FMN. Hence a bacterium containing lux gene
2
will emit light when the receptor is activated. This operon has been transferred to bacteria like:
Xanthomonas…to monitor progression of infection in plants.
Bradyrhizobium...to monitor root nodule formation and development.
Lactococcus…to detect the presence of antibiotics in milk.
Photo bacterium... to detect toxic pollutants.
E.coli……………to detect the presence of mercury in soil samples.
Biosensors only help to detect the presence of pollutants and separate processes are required to remove these
pollutants. They also find application in medical or diagnostics field or in food and fermentation operations for
measurement of specific parameters. In food industry, biosensors can be incorporated into food packages to monitor
temperature abuse, loss of shelf life, microbial contamination, and to provide visual indicator to consumers of the
state of the product at the time of purchase.
Bioremediation: The concept of using microorganisms to remove pollutants is known as bioremediation. We can
use either indigenous microorganisms or genetically engineered microorganisms. Pseudomonas and Bacillus are
two most important and most commonly used bacteria for bioremedical purposes. These bacteria can either turn
pollutants into energy source that they consume or alternately they produce enzymes that break down these
pollutants into less harmful molecular products. Studies are being carried out to develop efficient microbial
inoculants for bioremediation of effluents from distillery, tanneries, textile and food processing industries etc.
Greenhouse Gas bioremediation: There is great concern about the greenhouse effect. Water, methane and carbon
dioxide in earth atmosphere are important as they absorb infra-red radiations. This process is important for
maintaining the temperature of the earth’s surface. In the recent years because of increased burning of the fossil
fuels, carbon dioxide content in the atmosphere has increased many folds and has caused rise in the temperature on
the earth as a result of the greenhouse effect. Global warming leading to melting of the icy polar caps, rise in sea
level and flooding of major coastal areas, besides warmer air is causing chronic draughts in major crop growing
areas as a result of modified global air circulation pattern.
Some scientists believe that nature automatically takes care of the problem and they emphasize that extra carbon
dioxide stimulates plant growth. They also believe that increase in crop productivity after the industrial revolution
may infact be due to the presence of CO in abundance in the atmosphere and may not be due to the use of
2
fertilizers and pesticides. This increased plant growth may be the cause of increased methane in the atmosphere as
methane is five times more effective as a greenhouse effect gas as compared to CO
2.
Concentration of methane has increased alarmingly in the atmosphere and has infact doubled in 100 years which
may be due to more plant growth resulting in more decaying plant material. When dead plants decay under
anaerobic conditions bacteria break down organic matter and release CO and H which is combined to form CH
2
2
4
and H O by the activities of anaerobic methanogenic bacteria.
2
The use of aerobic methanotrophic bacteria which oxidize methane is important and is being studied for greenhouse
gas bioremediation. The activities of methanogens and methanotrophic bacteria should be balanced under ideal
conditions and the use of chemical fertilizers and pesticides may have inhibited the growth of methanotrophs. There
is a need to determine the optimum growth conditions for these bacteria for use in greenhouse gas bioremediation.