The Exceptions to Robert Koch’s Postulates

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In part 3 of this series we were introduced to Koch’s experimental requirements (Postulates) in determining the causative agents of most bacterial diseases and indeed they are very valuable in the development of microbiology as a science. Through the years, microbiologists discovered that there are some scientific findings that could not fit to the postulates of Koch; they termed these findings as exceptions to Koch’s Postulates. These exceptions will be discussed in this article.

Exception 1: Some microorganisms could not be cultured in artificial media.

Koch has postulated that the pathogen isolated from the blood of a diseased animal should be cultivated in an artificial culture medium like chicken or beef broth. Microbiologists after Koch found out that there are infectious microorganisms that cannot multiply in artificial media but can only multiply on living cells. This discovery has demanded microbiologists to modify Koch’s postulate and to find alternative means of culturing and detecting certain microorganisms (Alcamo 2002). For example, the failure of microbiologists to isolate the bacterium that causes Legionellosis necessitated them to get a lung tissue sample from an infected person and inject it into guinea pigs. They also got lung tissue from a healthy human and injected it into other guinea pigs. They found out that the guinea pigs inoculated with lung tissue from the infected human developed pneumonia (major symptom of legionellosis) while the others injected with lung tissue from the unafflicted person did not develop pneumonia. Tissue samples from the diseased guinea pigs were cultured in the yolk sacs of chick embryos because the microbes cannot grow in artificial media (e.g. chicken and beef broths). Extremely small microbes were found at the embryos and through the aid of an electron microscope, microbiologists saw rod-shaped bacteria (Ingraham 2000).

Exception 2: Diseases caused by different species of microorganisms could elicit similar symptoms.

In various situations, a human host shows certain signs and symptoms that are associated only with a certain microbe and its disease (Wheelis 2007). For instance, the bacteria responsible for tetanus and diphtheria cause distinctive signs and symptoms that no other microbe can produce. They are unequivocally the only bacteria that produce their respective diseases. However, there are cases wherein different microbes show similar signs and symptoms that you cannot identify what specific microbe causes the disease. For instance, the inflammation of the kidney (nephritis) can involve the action of several different pathogens, all of which cause the same signs and symptoms. Hence, it is often hard to know which particular microbe is causing a disease (Alcamo 2002). Koch believed that the signs and symptoms of anthrax are unique for “anthrax” but actually not.

Exception 3: Some pathogens can cause several disease conditions.

The bacterium Mycobacterium tuberculosis does not only cause lung disease but can also cause diseases in the skin, bones, and internal organs. Moreover, the coccus bacterium Streptococcus pyogenes does not only cause sore throat but can also cause scarlet fever, erysipelas, osteomyelitis (bone inflammation), among other diseases. If diagnosticians use clinical signs and symptoms together with laboratory procedures, the mentioned bacterial infections can usually be distinguished from infections to similar organs by other pathogenic microbes (Alcamo 2002; Ingraham 2000). The pathogen for anthrax that Koch isolated, cultured, and studied, does not only cause gastrointestinal anthrax but also cutaneous and inhalational anthrax.


Alcamo, Edward. 2002. Microbes and Society: An Introduction to Microbiology. Jones & Bartlett Publishers.

Ingraham, John. 2000. Introduction to Microbiology. Pacific Grove, Calif.: Brooks/Cole Pub.

Wheelis, Mark. 2007. Principles of Modern Microbiology. Jones & Bartlett Publishers.