Fermentation Process, Germ Theory and Koch’s Postulates Mic 101 Lecture 2: ZNJ Fermentation Process, Germ Theory and Koch’s Postulates

Fermentation Commonly, fermentation means the conversion of carbohydrates to alcohols and carbon dioxide or organic acids yeasts, bacteria, or a combination thereof under anaerobic condition used to produce alcoholic beverages such as wine, beer, and cider

History Ethanol acetaldehyde Glycerol propionaldehyde acroline+phenol When a group of French merchants asked Pasteur to find out why wine and beer soured, they hoped to develop a method that would prevent spoilage when those beverages were shipped long distances. At that time, many scientists believed that air converted the sugars in these fluids into alcohol. Pasteur found instead that microorganisms called yeasts convert the sugars to alcohol in the absence of air. Sugar ethanol + CO2 Souring and spoilage are caused by different microorganisms called bacteria. Ethanol acetaldehyde Glycerol propionaldehyde acroline+phenol

Age of ignorance The early concept For centuries disease was believed to be punishment for ones sins, bad character or poverty etc. It was found to be inconvincible for people that disease was caused by invisible microbe who could travel by air, water to infect plants, animals or humans.

Putting belief to test Robert Koch (1843-1910) Did a series of tests used to assess the germ theory of disease. Definitively demonstrated the link between microbes and infectious diseases Identified causative agents of anthrax and tuberculosis

Statement of the postulates suspected causative agent must be found in every case of the disease and be absent from healthy hosts agent must be isolated and grown in pure culture in the laboratory when the pure culture is introduced into a healthy, susceptible host, the host must get the disease same agent must be re-isolated from diseased experimental host

Contribution to Science Technique developer: In addition to Koch's postulates, Koch played an important role in the development of the use of agar as solid medium. Koch also invented nutrient broth and nutrient agar.

Limitation of Koch’s Postulates 1. The causative organism might not be present in every stage of the disease. Eg. Vibrio cholerae might not be present at the time of extreme vomiting and diarrhoea because the bacteria release toxin which causes the diarrhoea even in the absence of an organism. 2. Many organisms or entities cannot (at the present time) be grown in pure culture, such as prions responsible for Creutzfeldt–Jakob disease. 3. Not all hosts exposed to an infectious agent will acquire the infection. Non-infection may be due to such factors as general health and proper immune functioning; acquired immunity from previous exposure or vaccination; or genetic immunity. It is ethically unacceptable to expose a host experimentally to prove pathogen-disease relationships. In summary, a body of evidence that satisfies Koch's postulates is sufficient but not necessary to establish causation.

Modification of the Postulates New methods has led to revised versions of Koch’s postulates by Fredricks and Relman[11] have suggested the following set of Koch’s postulates for the 21st century: A nucleic acid sequence belonging to a putative pathogen should be present in most cases of an infectious disease. Microbial nucleic acids should be found preferentially in those organs or gross anatomic sites known to be diseased, and not in those organs that lack pathology. Fewer, or no, copies of pathogen-associated nucleic acid sequences should occur in hosts or tissues without disease. With resolution of disease, the copy number of pathogen-associated nucleic acid sequences should decrease or become undetectable. With clinical relapse, the opposite should occur. When sequence detection predates disease, or sequence copy number correlates with severity of disease or pathology, the sequence-disease association is more likely to be a causal relationship. The nature of the microorganism inferred from the available sequence should be consistent with the known biological characteristics of that group of organisms. Tissue-sequence correlates should be sought at the cellular level: efforts should be made to demonstrate specific in situ hybridization of microbial sequence to areas of tissue pathology and to visible microorganisms or to areas where microorganisms are presumed to be located. These sequence-based forms of evidence for microbial causation should be reproducible.

Summary Pasteur and Koch formed the foundation of scientific observations against blind belief. Their work was the first step towards looking for answers through scientific experiments and proofs. Koch’s postulate is a fine documentation of experimental proof between microbes and diseases. His principle observation are applicable with modification in the age of molecular biology.