Basic Microbiology - Koch's postulates

Basic Microbiology - Koch's postulates     

                   The first direct demonstration that bacteria cause disease came from the study of anthrax by the German physician Robert Koch (1843–1910).The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884 and refined and published by Koch in 1890.1905 he was awarded the Nobel Prize in Physiology or Medicine. Koch’s postulates are the criteria that establish a causative relationship between a microbe and a disease, there are four exceptions to Koch’s postulates.

Koch Experiment 

           Koch injected healthy mice with material from diseased animals, and the mice became ill. After transferring anthrax by inoculation through a series of 20 mice, he incubated a piece of spleen containing the anthrax bacillus in beef serum. The bacteria grew, reproduced, and produced endospores. When isolated bacteria or their spores were injected into healthy mice, anthrax developed. His criteria for proving the causal relationship between a microorganism and a specific disease are known as Koch’s postulates. After completing his anthrax studies, Koch fully outlined his postulates in his work on the cause of tuberculosis (figure 1.15). In 1884 he reported that this disease was caused by the rod-shaped bacterium Mycobacterium tuberculosis.

                While Koch’s postulates are still widely used, their application is at times not feasible. For instance, organisms such as Mycobacterium leprae, the causative agent of leprosy, cannot be isolated in pure culture. Some human diseases are so deadly (e.g., Ebola virus disease) that it would be unethical to use humans as the experimental organism; if an appropriate animal model does not exist, the postulates cannot be fully met.

What are Koch’s Postulates


Koch’s postulates refer to the four criteria established by Robert Koch to identify the causative agent for a particular disease. They include:

  1. The pathogenic microorganism must be present in all cases of the disease;
  2. The pathogenic microorganism can be isolated from the infected host and grown in a pure culture;
  3. The pathogenic microorganism grown in the culture must be able to cause the disease once inoculated into a healthy, susceptible, laboratory organism;
  4. The pathogenic microorganism re-isolated from the second host organism must show the original characteristics of the inoculated pathogen.

Exceptions to Koch’s Postulates

                  Koch’s postulates do not fit with some scientific findings. The exceptions to Koch’s postulates are the following five findings that are in contrary to Koch’s postulates.
  1. Some microorganisms that cause diseases have never been cultivated under laboratory conditions. In Koch’s postulates, the blood-born pathogenic microorganisms are cultured in artificial media. However, some microorganisms cannot multiply in artificial growth media. For example, leprae can only grow in armadillos, a type of New World placental mammal.
  2. Some diseases are caused by several microorganisms. Microorganisms cause distinctive signs and symptoms in the host as in tetanus and diphtheria. However, several microorganisms may elicit the same disease conditions in various situations. For example, kidney inflammation (nephritis) and pneumonia can be caused by several types of pathogens.
  3. Some microorganisms cause several diseases. For example, tuberculosis can cause several disease conditions in lungs, internal organs, bones, skin, etc. S. pyogenescan cause sore throat, erysipelas, scarlet fever, and osteomyelitis. The symptoms of M. tuberculosis are shown in figure.
  4. Some diseases have variable signs and symptoms among patients. For example, tetanus shows variable disease conditions among different individuals.
  5. Ethical considerations prohibit the experiments when humans are the only host for a particular disease. If there is an investigation/research on HIV that infects humans, it is unethical to purposely infect a human with HIV.

Conclusion

                                    Koch’s postulates state the relationship between a disease-causing microbe and its disease. However, some of the later findings on disease-causing pathogens are contrary to Koch’s postulates. They are the five exceptions to Koch’s Postulates.

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