Koch's Postulates

Introduction:

Koch's postulates are a set of experimental steps designed to prove beyond almost any level of doubt that a specific organism is the cause of a specific infectious disease. Developed by Robert Koch in 1876, Koch was able to prove that a specific organism (Bacillus anthracis) was linked with a particular disease (anthrax), and subsequently proposed these postulates. These postulates are: that the causative agent should always be found in hosts suffering the disease and not in healthy individuals; that the causative organism must be isolated from the diseased host and grown in pure culture; that the same disease must result when the isolated organism is inoculated into a healthy host; and that the organism must be re-isolated from the experimentally infected host and it must be the same as what was originally isolated. If all of these postulates are satisfied, it is safe to say that the organism being studied is in fact the cause of the disease.

Yoghurt can be considered the diseased state of milk (the "disease" being lactic acidosis) and as such the fermentation of yoghurt by certain organisms (Streptococcus thermophillus and Lactobacillus bulgaricus) can be studied in such a way to confirm that these organisms are the cause of the disease by way of Koch's postulates.

Streptococcus thermophillus and Lactobacillus bulgaricus are both gram-positive, lactic acid bacteria, which means that they utilize lactose in the environment as energy source and produce lactic acid. Despite the energetic similarity, the two strains of bacteria require rather different conditions to achieve optimum growth: while Lactobacillus bulgaricus prefers low pH environment, the acidity, in turn, inhibits the growth of Streptococcus thermophillus. On the other hand, certain amino acids (e.g. histidine and glycine) are essential for the growth of Streptococcus thermophillus but could be produced by Lactobacillus bulgaricus using the nutrients obtained from milk. In spite of and because of the differences, the two strains of bacteria exhibit a symbiotic relationship and are used together as a mixed culture in yoghurt production which accelerates the lactic acid production and coagulation of milk (Lourens-Hattingh & Viljoen, 2001).

Aim:

To prove, using Koch's postulates, that the interaction between a microorganism (the bacteria) and the host (milk) is the cause of the disease (lactic acidosis).

Experimental Strategy:

Experiment 1:

This experiment is to confirm that the microorganisms are present in the diseased clinical case, thus addressing the first 2 postulates.

I would first make fixed bacterial smear of the yoghurt sample and conduct a gram stain to confirm the presence of microorganism. Then, I would inoculate