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The discovery of microbial world immediately raised questions regarding the origin of microorganisms. It was by then recognized that living organisms such as plants and animals do not originate spontaneously. Yet, some believed that these microorganisms arose spontaneously and this theory came to be known as the theory of spontaneous origin or abiogenesis.
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One of the first to provide evidence that microorganisms do not arise spontaneously in organic infusions was the Italian, Lazzaro spallanzani, who conducted number of experiments on this problem. He could show repeatedly that the growth of microorganisms in infusions can be prevented by heating. However, if such infusions were exposed to air after heating, these underwent spoilage. He concluded that, air contained microorganisms and these were carried into the infusions which were kept open after heating and subsequently caused spoilage. To further prove his point, he sealed the flasks after heating and when this was done, the infusions remained sterile. He concluded that microorganisms do not grow in the infusions, if the infusions are boiled and sealed to prevent the entry of air.
Francesco Redi (1665):
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Who put the theory of spontaneous generation. He demonstrated that the developing maggots coming from meat were the larvae of flies. The maggots could not develop when the meat was protected by placing it in vessel covered with fine gauze. He disproved the spontaneous formation of living being from non-living matter and is known as the doctrine of spontaneous generation or abiogenesis.
Louis Pasteur(1822-1895):
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Louis Pasteur began his brilliant career as professor of chemistry at the University Lille, French. A principal industry of France being the manufacture of wines and beer. Pasteur studied the methods and processes involved in order to help his neighbours produce a consistently good product. He found that fermentation of fruits and grains, resulting in alcohol was brought about by microbes by examining many batches of “Ferment”, he found microbes of different sorts. In good lots one type predominated, and in the poor products another kind was present. By proper selection of the microbe, the manufacturer might be assured of a consistently good and uniform product. Pasteur suggested that the undesirable types of microbes might be removed by heating- not enough to hurt flavour of the fruit juice, but enough to destroy a very high percentage of the microbial populations. He found that holding the juices at a temperatures of 62.80C (1450F) for half an hour did the job. Today pasteurization is widely used in fermentation industries, but most familiar with it in the dairy industry.
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Pasteur’s success in solving the problem of fermentation led the French government to request that he investigate pebrine, a silkworm disease that was ruining an important French industry. For several years Pasteur struggled with this problem, heartaches and disappointments following one after another. Eventually he isolated the parasite causing the disease. He also showed that silkworm farmers could eliminate the disease by using only healthy, disease free caterpillars for breeding stock.
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Turning from silk to wool, Pasteur next tackled the problem of anthrax, a disease of cattle, sheep and sometimes human beings. In 1880, Pasteur isolated the organism causing chicken chlorera. In one of his inoculation experiments, he found that the old cultures of the organism, when used as inocula, were non-infective, which incidentally led to the discovery of an avirulent bacterium, which could induce antibody formation in an experimental animal. He showed through further studies that the presence of antibodies in the animal would prevent subsequent infections by the virulent culture of the pathogen. His subsequent studies resulted in the development of ‘attenuated cultures’ or ‘vaccines’ for the prevention of anthrax.
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The success attained in these studies enabled him to concentrate on human diseases and so he turned his attention to rabies a disease transmitted to man by dog bite. He prepared vaccines against rabies from infected rabbits. In one of his experimental studies with the anthrax bacillus, he observed that a contaminating organisms in the bacterial culture completely suppressed the pathogenic quality of the culture and he stated that if the phenomenon could be explored, it would be of great use in curing diseases of man and animal. The phenomenon referred by Pasteur is ‘antibiosis’ and subsequent investigation on this have led to the discovery of antibiotics, the ‘wonder drugs’ now widely used to cure diseases of man and animal.
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Louis Pasteur showed that growth did not occur in infusions which had been heated but exposed to air provided the incoming air was treated to remove the microorganisms. He first demonstrated that air contains microscopically observable organised structures. He aspirated a large quantity of air through a tube that contained a plug of gun cotton to serve as a filter. The gun cotton was then removed and dissolved in a mixture of alcohol-ether and the sediment was examined microscopically. He found that this sediment contained not only organic matter but also a large number of small round structures which were identical to microorganisms.
 
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To prove that infusions which are once heated can be kept sterile, if kept away from air containing microorganisms, he sterilized infusions in flasks with open S-shaped tubes. These flasks containing sterile infusions had free access to air but the air has to pass through the S-shaped tube. Dust particles and microorganisms in the air settled at the bend and did not reach the infusion, thus keeping the infusion sterile.
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Support to Pasteur’s conclusion, Johns Tyndall—1828, who showed that sterile infusions placed in a dust free chamber could remain sterile indefinitely even if kept exposed to air.
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Microorganisms exists in two forms-a heat labile form (vegetative) and heat resistant form (endospores). Intermittent heating and cooling to preserve food materials – Tyndallization.
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