BSc Introduction to Microbiology Notes Study Material
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BSc Introduction to Microbiology Notes Study Material
What is Microbiology?
Microbiology is the study of microorganisms, that is the organisms that are of microscopic dimensions. These organisms are too small to be clearly perceived by the unaided human eye. If an object has a diameter of less than 0.1 mm, the eye can not perceive (or more correctly resolve) it at all, and very little detail can be perceived in an object with a diameter of 1 mm. Roughly speaking organisms with a diameter of 1 mm or less are microorganisms and fall into the broad domain of microbiology. Since most microorganisms are only a few thousandths of a millimeter in size, they can only be seen with the aid of a microscope.
As a direct consequence of the invisibility of microbes to the naked eye and the need for special techniques to study them, microbiology was the last of the three major divisions in biology (the other two are botany and zoology) to develop. Though it was late to arrive, this is a discipline that has been quick to develop. (BSc Introduction to Microbiology Notes Study Material)
At present, there is general agreement to include five major groups as microorganisms: the subdivisions of virology, bacteriology, mycology, phycology and protozoology (the studies of viruses, bacteria, fungi, algae, and protozoa respectively).
Traditionally, all organisms had been included under the disciplines of either botany or zoology. Bacteria, algae, and fungi have in the past been considered to be part of the Plant Kingdom while protozoa have been included in the Animal Kingdom. This view cannot be supported from taxonomic standpoint microbiology encompasses the study of groups of organisms in all three divisions of biology it may be argued that it covers greater biological diversity than the other two divisions. (BSc Introduction to Microbiology Notes Study Material)
Microbes and Man
Despite their small size, microbes are of immense importance to man. They cause disease, are a source of various foods and medicines, and dispose of our wastes. They are in fact responsible for the very air we breathe since free molecular oxygen was completely absent from the pre-biotic atmosphere and this gas has accumulated only as a product of the metabolism of primeval photosynthetic bacteria.
Mankind has made use of microorganisms, or their biochemical activities, since long before he even knew of their existence. In 6000 BC the ancient Babylonians and Sumerians were brewing beer much as we do today and the Egyptians were baking leaven bread 2000 years later.
Despite the antiquity of these microbiological practices, the first documentation of the structure of microorganisms did not occur until the advent of the first microscopes in the seventeenth century. Although the spontaneous generation of mice from old rags and of maggots from meat had been disproved by this time, it was not until the last century that Louis Pasteur showed that microbes were not produced de novo from mud or decaying organic matter. (BSc Introduction to Microbiology Notes Study Material)
If Pasteur was the founder of industrial microbiology, Robert Koch was the forefather of medical microbiology. Koch, a German doctor could show in 1876 that the causal agent of bovine anthrax was a bacterium, Bacillus anthracis. His remarkable work remains a landmark in microbiology about the nature of the disease and in proving the pathogenicity of a disease-causing agent in its host through Koch’s postulates.
We live in a time when microbiology has come of age. Microbial products are being produced on a large scale by industrial microbiologists -roughly 14,000 tons of penicillin, 300 million tons of the flavor enhancer MSG (monosodium glutamate), and 1450 million tons of vinegar being manufactured per annum. We use microbes to make beer, wine, cheese, yogurt, sauerkraut, soya sauce, antibiotics, pesticides, gels, etc. Microbiological reactions are used in signage triages in transforming the chemical structure of drugs, clean clothes (bacterial enzymes used in biological detergents), and even to extract metals like copper and uranium from their mineral ores.
New technologies like gene cloning have been developed within the last decade where these microbes may be used as factory cells for the synthesis of valuable pharmaceutical products like human insulin, hormones, antiviral drugs, and vaccines. When somatostatin, a growth hormone secreted by the brain, was first purified it took 500,000 sheep brains to produce a 5 mg quantity. In 1977, genetic engineers produced an equal amount of this growth hormone from two gallons of bacterial culture at a cost of fewer than one hundred rupees.
History of Microbiology
The period between 1860 and 1910 may be aptly called the golden age of microbiology. It was in this period that this discipline had its firm foundations.
Microbiology is said to have its roots in the great expansion and development of the biological sciences that took place after 1850. It was in this period that Gregor Mendel established the principles of genetics. That Claude Bernard designed his classic experiments in human physiology. And that Charles Darwin formulated the process of natural selection inspiring him for publishing On the Origin of Species. It was also during this period that Louis Pasteur and Robert Koch developed the techniques for the isolation and identification of the microbes of disease.
The development of microbiology as a discipline could be traced along the following historical eras.
[I] Discovery era
This period concerns the discovery of the microbial world, which has been dominated by van Leeuwenhoek.
- Marcello Malpighi. He was a professor at the University of Bologna. During the course of his studies on biological specimens, he described the cells of the brain, spleen, and kidney tissues, with a rudimentary lens. He discovered the breathing system of insects and depicted the development of the chick embryo. (BSc Introduction to Microbiology Notes Study Material)
- Anton van Leeuwenhoek. Anton van Leeuwenhoek is said to be the discoverer of the microbial world. He was a draper and haberdasher, and the owner of a dry goods shop in Delft, Holland. Leeuwenhoek was a qualified surveyor and the town’s official wine taster. In his spare time, he ground pieces of glass into fine lenses, placing them between two silver or brass plates riveted together. With his crude lens system, van Leeuwenhoek could see clearly the objects magnified about 200 times. He observed hair fibers, plant structures, crystals, an insect’s eye, and a variety of fluids including blood, and even scrappings from his own teeth.
During the course of his work, van Leeuwenhoek’s greatest claim to fame rests on the discovery of the tiny microbes that he called animalcules. He found them first in rainwater, then in pond water, and later in material from his own teeth. and eventually in most of the specimens he examined. They astonished him at first, then delighted him, and finally perplexed him as he sought to explain where they came from and what use they had. (BSc Introduction to Microbiology Notes Study Material)
All the main kinds of unicellular microbes that we know today–protozoa, algae, yeasts, and bacteria—were first described by him. He first described the microbes in 1676. For some years previously he had been communicating with the Royal Society of London. His eighth letter, sent in 1676 and published in 1677 had special significance as it contained his first detailed descriptions of the microorganisms. His sketches were elegant in detail and clarity. In 1683, an important letter sketched the rod, sphere, and spiral forms of bacteria and their movement.
It was the first description of bacteria and how van Leeuwenhoek perceived them. The material examined was the scrappings of his own teeth mixed with pure (without any animals) rainwater. This matter was seen to contain many small living animals. In all, he wrote over 200 letters during a 50-year period. (BSc Introduction to Microbiology Notes Study Material)
He had been a very suspicious and secretive person and invited no one to work with him nor he could tell anyone how he ground his lenses or constructed his microscopes. The science of microbiology, therefore, developed slowly from this point due to two main reasons : (i) there were no microscopes to be had, nor was any technology available for an efficient lens system, and (ii) microbes were idle curiosities, and nothing more. The disease was associated with magic and mysticism. The development of microbiology was thus delayed until both the instrumentation and an appreciation of microbes and disease came together. This could occur only during the mid-1800s.
[II] Transition period
Though there were a number of significant developments in microbiology during van Leeuwenhoek’s time and in later years, two noteworthy contributions enhanced the interest in microbes and their relation to disease. These are :
(1) The controversy over a spontaneous generation: includes experimentations mainly of Francesco Redi, John Needham, Lazzaro Spallanzani, and Nicolas Appert, and
(2) Disease transmission: includes the work of Ignaz Semmelweis and John Snow.
- Francesco Redi. The belief in the spontaneous formation of living beings from nonliving matter is known as the doctrine of spontaneous generation or abiogenesis, which has had a long existence. This doctrine was accepted without question until the Renaissance. Till Renaissance, people had noted that lifeless objects often supported live organisms. Slime seemed to give rise to toads, rags to mice, and meat to maggots.
Francesco Redi, an Italian physician on the basis of the results of his experiments in 1665. Put the theory of spontaneous generation to rest. He showed that the developing maggots coming from meat were the larvae of flies The maggots could not appear when meat was protected by placing it in a vessel closed with fine gauze. So that flies were unable to deposit their eggs on it.
- John Needham. His observations of animalcules in mutton broth in 1748, however, aroused interest in the theory of spontaneous generation. He proposed that tiny organisms arose spontaneously in his mutton gray. He had covered the flasks with corks as done by Redi and had even heated some flasks. Still, the microbes appeared. (BSc Introduction to Microbiology Notes Study Material)
- Lazzaro Spallanzani. This Italian naturalist, in 1767, attempted to refute Needham’s work. He boiled the flasks for longer time periods, and also sealed them shut by melting the glass in the neck. His control flasks, boiled only briefly or stoppered with porous corks, showed the microbes, whereas the experimental flasks were free of any life.
Needham, however, countered that the life force had been killed by the heat, and life could not be expected to occur. However, in the end, Spallanzani’s proofs were accepted. Spontaneous generation was, however, still persisting in the minds of people, and this doctrine could be completely discarded only when Louis Pasteur, about a hundred years later appeared on this front.
- Nicolas Appert. By 1805 this French wine-maker showed that he could preserve soups and liquids by heating them extensively in thick champagne bottles. This was infact an outgrowth of Spallanzani’s work.
- Ignaz Semmelweis. The work of two persons during the mid-1800s is significant. It shows a growing awareness of the mode of disease transmission. Semmelweis, a Hungarian physician postulated in 1847 that the blood of victims of puerperal fever contained the causal agent of the disease, and that spread occurred when the physician came in contact with the blood. John Snow was another worker.
- John Snow. He, an English doctor traced cholera to the municipal water supply of London during an 1854 epidemic. The epidemic could be limited by carefully avoiding the water. He believed that something in the environment was responsible for the infection.
By this time, in 1859, Darwin could bring his On the Origin of Species. And Europeans showed that the human body could be conceived as a creature susceptible to the laws of nature. The attention now centered around the idea that microbes may be involved in a disease. The disease may be a biological phenomenon rather than due to magic or mysticism.
- To this point, we have seen how the early discoveries in microbiology occurred in isolated instances with little direction or continuity. Even after Leeuwenhoek’s elegant descriptions, microbes were considered merely interesting creatures of nature. How spontaneous generation focused iteration on file 3.54 microbes and bridges the gap to the 1850s. Semmelweis and Snow were among the first to suspect the involvement of a specific agent in disease though skepticism still remained strong.
Now we would look forward to the experiments of Louis Pasteur and Robert Koch, during the beginning of the golden age of microbiology. Here we find indisputable proof that microbes cause disease. More important, there was an acceptance of their work by the scientific community and a willingness to continue and expand the work. Here, we see the beginnings of microbiology as a discipline of biology.
[III] Golden age of microbiology (1860-1910)
- Louis Pasteur. By 1860 some scientists had begun to realize that there is a causal relationship between the development of microorganisms in organic infusions and the chemical changes that take place in these infusions: microbes are the agents that bring about the chemical changes. The great pioneer in these studies was a French, Louis Pasteur. He made a series of experiments. His main contributions to the development of microbiology are as follows:
(i) That tartaric acid, an organic compound, formed two types of crystals, that he could separate microscopically.
(ii) Chemical changes, such as fermentation are vital processes, brought about by the activity of microbes such as yeasts and bacteria, and they could be a cause of disease.
(iii) First demonstrated that air does contain microorganisms.
(iv) Discovery of anaerobic life, and introduction of the terms aerobic and anaerobic.
(v) Discovery of immunity in sheep against anthrax.
Pasteur’s main contributions are presented here :
(a) Fermentation process. Pasteur became interested in why the local wines in France were turning sour. The popular belief was that the fermentation of grape juice to wine was a natural chemical process, involving the breakdown of the protein albumin. However, Pasteur could observe under his microscope some oval yeast cells, and barely some rods commonly called bacteria. He believed that yeasts played a major role in fermentation. (BSc Introduction to Microbiology Notes Study Material)
In a series of classic experiments Pasteur first showed that alcohol would be produced from grape-yeast mixtures. Using heat, he then destroyed the yeasts, whereupon alcohol failed to appear in the grape juice. However, when yeasts were returned to the flasks, fermentation took place, and wine was formed. Moreover, if care was taken to eliminate the bacteria, the wine would not become sour.
(i) Albumin solution —–> Incubation —-> No fermentation.
(ii) Grape juice ——- Incubation ——–> No fermentation.
(iii) Grape juice + Pure yeasts —–> Incubation ——->Wine + Yeasts.
(iv) Grape juice + Yeasts ———>Heat ——-> No wine.
(v) Grape juice + Yeasts + Bacteria ——->Incubation ——–>Sour wine.
(vi) Grape juice + Yeasts + Bacteria. ———> Heat ———>Yeasts added—–>Incubation ———>Good wine.
Pasteur’s work thus seemed to indicate that the microscopic bacteria were tiny chemical factories that could bring about important changes. The yeasts appeared to be vital to the fermentation process and the bar made the wine sour. Pasteur’s work appeared to demonstrate that the mich could be a cause of disease, for if they could make the wine sick, perhaps could also make the body ill. His experiments thus revealed the role of micro in the transformation of organic matter, and perhaps also in causing disease.
(b) Pasteur and disease (swan-necked flasks). As a result of some experiments, outlined below. Pasteur believed that bacteria were in the soil and air and thus their spread could be controlled. If so, then perhaps disease could also be controlled.
(i) Flask containing sterile broth —> Opened to air —–> Life appears.
(ii) Flask containing sterile broth —> Neck sealed —–> No life appears.
(iii) Flask containing sterile broth —> Its side arm opened to air but heated—–> No life appears.
Others believed that bacteria arose spontaneously from organic matter. As in the sick body, as long as the life force was present. Pasteur took the view of spontaneous generation seriously in order to develop his own theory of disease. He first showed that as the air became purer, fewer microbes could be located. His swan-necked flasks provided the final defeat for the spontaneous generation theory. The flasks were left open to allow entry of any life force present in untreated air. As shown in Figure, the curvature of the neck prevented the microbes from reaching the broth. (BSc Introduction to Microbiology Notes Study Material)
The flasks remained sterile. Thus spontaneous generation was eliminated as a viable theory, and the idea that microbes existed in the environment was strengthened. Within a short period of seven years, Pasteur lost his three daughters namely Jeanne, Camille, and Cecille, who died of typhoid fever (1859), septicemia (1865), and typhoid fever (1866) respectively. He thus returned to the study of human diseases.
- Robert Koch. The German doctor, Robert Koch is credited for his following main contributions to the development of microbiology:
(a) Final proof that bacteria could be isolated and shown to cause disease while working on anthrax disease of animals. Thus he presented his germ theory of disease.
(b) Developing the series of procedures’ postulates, by which a specific organism could be related to a specific disease.
(c) Development of pure culture techniques.
Koch had observed thread-like organisms in the blood of animals that had died of anthrax, a disease that was a serious threat to farmers killing their sheep and cattle herds periodically. He injected laboratory animals with the bacteria-laden blood of dead sheep. He then performed autopsies and noted how the same symptoms appeared regularly. Koch isolated a few of the bacilli in the clear sterile vitreous humor of an ox’s eye and watched their multiplication. He injected laboratory mice with a sliver of wood containing spores and observed as anthrax symptoms developed in the animals. (BSc Introduction to Microbiology Notes Study Material)
On autopsy, the blood was swarming with the threadlike bacteria, and he reisolated them in the vitreous humor. The cycle was now complete. In 1867 Koch presented his germ theory of disease. Ironically, van Leeuwenhoek had first described the microorganisms 200 years earlier. Koch had now proved that they caused disease. (BSc Introduction to Microbiology Notes Study Material)
The sequence of procedures by which Koch established his germ theory of disease came to be known as Koch’s postulates. These procedures became a guide for relating a specific organism to a single disease. (BSc Introduction to Microbiology Notes Study Material)
Koch accidentally observed that a slice of potato contained masses of bacteria in colonies, each colony separate and distinct from the others, and each composed of billions of rods. It occurred to him that bacteria could grow and multiply on solid surfaces, and he added gelatin to his broth to prepare a solid culture medium. When seeded onto the surface, bacteria formed visible colonies and each contained only one type of organism. Inoculations of laboratory animals could now be made with the assurance that only one type of microbe was involved.
This was the breakthrough that had eluded Pasteur and that was to spark further development of microbiology.
- Competition period. At some period in history, Pasteur and Koch might have become friends and colleagues in their search for the agents of disease. However, the period following the 1870 Franco-Prussian War did not allow it. Both France and Germany were undergoing unification, and heroes played an important role in the spirit of nationalism. Pasteur thus became a symbol of French achievement and Koch, his German counterpart. This developed a professional rivalry that was to last until the 1900s.
Koch announced his germ theory of disease in 1876. By 1878 Pasteur had adopted Koch’s methods, had verified Koch’s results, and had gone several steps beyond. Pasteur discovered that bacteria were sensitive to temperature since chickens did not acquire anthrax at their normal body temperature of 42°C but did so when they were cooled to 37°C. He also recovered anthrax spores from the soil and demonstrated that cattle became infected by grazing in contaminated fields. This explained the periodic recurrence of the disease. Pasteur suggested that dead animals be buried deeply in soil unfit for grazing and where earthworms would not bring the spores to the surface.
A remarkable observation was made that a group of test animals failed to acquire anthrax after being inoculated routinely. Pasteur gave them a second inoculation with fresh and particularly virulent bacilli. Still the animals lived. Pasteur then discovered that the first injection was with a culture of bacilli that had been grown above normal temperature for several days.
He reasoned that the heated bacteria, instead of killing the animals had apparently made them resistant to the second injection. At that moment there was formulated a basic principle of immunity, which was that modified bacteria might be used to build protection against infectious bacteria. This is one of the foundations on which immunization with vaccines is based.
Pasteur’s work put France once more at the forefront of science. However, it was Koch’s student, Georg Gaffky, who was the first to successfully grow the typhoid bacillus. Friederich Loeffler, an assistant of Koch, isolated the organism of diphtheria.
Attention shifted again to France when Emile Roux and Alexander Yersin of Pasteur’s lab. proved that diphtheria infections were due to a poisonous toxin produced by the bacterium. Emil von Behring. Koch’s associate was able to treat diphtheria successfully in 1890 by injecting a patient with antitoxin, me protein synthesized by the body in response to the toxin. von Behring was rewarded with the first Nobel Prize in Physiology or Medicine. (BSc Introduction to Microbiology Notes Study Material)
Later, Shibasaburo Kitasato came from Japan to study with Koch. He successfully grew the tetanus bacillus, and also experimented with plague bacillus. In 1884, Elie Metchnikoff, a native of Ukraine, had come to Pasteur’s lab. Discovered that white blood cells engulf foreign organisms in the blood and gave the name phagocytosis to the process. In the same year Christian Gram, a Danish physician working in Berlin, noted that certain stained bacteria were losing their purple colour when treated with alcohol, whereas others retained it.
He suggested that this procedure might be used to differentiate bacteria, and there evolved the Gram stain technique, the most important staining procedure in microbiology. (BSc Introduction to Microbiology Notes Study Material)
Pasteur was almost at the climax of his career in 1885 when he successfully treated young Joseph Meister for rabies. He developed a vaccine for the rabies virus he was never to see. It was an important application in medicine. There were given funds generously, including Russia for the establishment of the Pasteur Institute in Paris over which Pasteur presided till his death ten years later.
Koch, also reaching the height of his influence, isolated the bacillus of tuberculosis in 1882. He continued work on this disease. His work was interrupted in 1883 when he visited Egypt and India where cholera was raging. He isolated a comma-shaped microbe as the causative agent and confirmed the 1854 theory of John Snow that water was important to the transmission of the disease. In 1885 Koch was appointed to the University of Berlin. And in 1890 was made Director of the Institute for Infectious Diseases. He continued his work on tuberculosis and in 1905 was honoured with the Nobel Prize.
- Other pioneers of microbiology. During the golden age of microbiology, there were some other workers who contributed to the development of microbiology, in France, Germany, Italy, England, and the U.S.A.
- End of the golden age. A dramatic turn in microbiology research was signaled by the death of Robert Koch in 1910 and the advent of World War I. The Pasteur Institute was closed, and the German laboratories converted to the production of blood components used to treat war infections. Thus came to an end what many have called the Golden Age of Microbiology, a fifty-year period from the 1860 work of Pasteur on alcohol fermentation to the death of Koch. (BSc Introduction to Microbiology Notes Study Material)
This age has witnessed a series of discoveries unparalleled in medicine, most of which involved the identification of agents of disease. There was awareness of the relationship of microbes to disease and methods of transmission. This led to sterile practices in hospitals, increased research in pest control, and the care in preparation and consumption of food.
In 1892 a Russian, Dmitri Iwanowski reported that sap from a diseased plant, even after it was filtered could cause mosaic disease in tobacco plants. He suggested a toxin as the agent of disease and used the word virus in his reports. The virus was a common word of that period. Later yellow fever virus, the measles virus, the rabies virus, and other viruses became the focus of attention in microbiology. Finally, the viruses were visualized in the 1940s and shown as a unique group of infectious agents.
Today we live in a Golden Age of Virology where many viral diseases are disappearing from the fabric of life much as bacterial diseases declined at the turn of the century. (BSc Introduction to Microbiology Notes Study Material)
[IV] Microorganisms as geochemical agents
It may be seen that in the last decades of the 19th century, microbiologists’ interests centered on the role of microbes as agents of infectious disease. Still, some scientists could carry forward the work initiated by Pasteur on fermentation processes.
They could clearly demonstrate that microorganisms can serve as specific agents for large-scale chemical transformations and that the microbial world as a whole might well be responsible for a wide variety of other geochemical changes. The work of S. Winogradsky and M.W. Beijerinck was largely responsible to establish that microbes play cardinal roles in the biologically important cycles of matter on earth—the cycles of carbon, nitrogen, and sulphur. (BSc Introduction to Microbiology Notes Study Material)
Many groups of microbes with a wide range of physiologic diversity are specialized to bring about chemical transformations and play vital parts in the turnover of matter on earth. Winogradsky discovered chemoautotrophic bacteria. Which grows in completely inorganic environments, obtaining energy by oxidation of reduced inorganic compounds and using Co, as a carbon source. He found for example autotrophic sulfur bacteria and nitrifying bacteria. (BSc Introduction to Microbiology Notes Study Material)
The two scientists are also known for their contributions to the role played by microbes in the fixation of atmospheric nitrogen. This is done by both, free-living bacteria and those in symbiotic associations. (BSc Introduction to Microbiology Notes Study Material)
Winogradsky and Beijernick also developed a new and important technique for the isolation of a particular kind of microorganism from a mixed population through modification of such factors as the carbon source, the energy supply, the temperature, and the pH value of the growth medium. The technique is known as enrichment culture. (BSc Introduction to Microbiology Notes Study Material)
[V] Growth of microbiology in the twentieth century
During the last decades of the 19th-century microbiology became an established discipline with a distinct set of concepts and techniques. During the same period, the science of general biology also emerged, with the published work of Charles Darwin. However, microbiology and general biology had little coherence. For half a century after the death of Pasteur in 1895, microbiology and general biology developed almost complete independence from one another. (BSc Introduction to Microbiology Notes Study Material)
Microbiology contributed significantly to the development of a new discipline of biochemistry. The discovery of cell-free alcoholic fermentation by Buchner in 1897 provided the key to chemical analyses of energy-yielding processes. In the first two decades of the 20th century parallel studies on glycolysis by muscle and alcoholic fermentation by yeast gradually revealed their fundamental similarity.
There was thus found a common ground for vertebrate physiologists and microbial biochemists. Later vitamins required by animals proved chemically identical to the growth factors required by some yeasts and bacteria. These and some other such discoveries on precursors of coenzymes etc., from 1920 to 1935 could demonstrate the basic similarities of all living systems at the metabolic level. This doctrine proclaimed by biochemists and microbiologists came under the slogan the unity of biochemistry.
The second great advance of biology in the early 20th century—the creation of a new discipline of genetics—had no immediate impact on microbiology. The first important contact between genetics and microbiology occurred in 1941. When Beadle and Tatum succeeded in isolating a series of biochemical mutants from the fungus, Neurospora. This opened the way to analysis of the consequences of mutation in biochemical terms and Neurospora. They joined the fruit fly and the maize plant as a material of choice for genetic research.
In 1943, studies on mutation in bacteria by Delbruck and Luria engineered genetic work on these microbes. Soon after several mechanisms of genetic transfer were shown to exist in bacteria. Significantly different from sexual recombination in plants and animals. In 1944, the work of Avery, McLeod, and McCarty on the process of transformation in bacteria, revealed that genetic matter in a living organism is DNA.
The confluence of microbiology, genetics, and biochemistry between 1940 and 1945 brought to an end the long isolation of microbiology from the main currents of biological thought. It also set the stage for the second major revolution in biology. Microbiology made many contributions of fundamental importance: the advent of molecular biology. (BSc Introduction to Microbiology Notes Study Material)
To sum up, the development of microbiology as a discipline may well be followed along the following main lines:
(1) The discovery of the microbial world.
(2) The controversy over spontaneous generation.
(3) The discovery of the role of microorganisms in transformations of organic matter.
(4) The discovery of the role of microorganisms in the causation of disease.
(5) The development of pure culture methods.
(6) Microorganisms as geochemical agents.
(7) The Growth of microbiology in the 20th century.
BSc Introduction to Microbiology Notes Study Material