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Introduction

Robert Koch was a German physician (1843-1910), who was interested in the relationship between microorganisms and disease. Koch formulated four criteria (Koch's postulates), which must be met to prove that a particular microorganism has caused a certain disease (viruses however, require other criteria).

Koch's postulates

1. The microorganism must/should be detected in large numbers in all individuals who suffer from the disease, but it must/should not be present in healthy individuals
2. The microorganism must/should be cultivated in pure culture from samples coming from the sick individual.
3. The cultured microorganism must/should cause the sama disease in healthy individuals (laboratory animals).
4. The microorganism must/should then be cultivated from or detected in the diseased laboratory animal.

In the original version of the postulates must was used, but with today's knowledge, one must use should as there are many exceptions. The first criterion had to be abandoned when it was discovered that there are asymptomatic carriers of certain microorganisms (subclinical infections). The second criterion must sometimes be abandoned because there are microorganisms that cannot be cultivated. The third criterion is not always valid because external factors can affect the results of an experimental infection.

Conclusion

If all criteria are met, a relation between microorganisms and disease has been proven, but if not all criteria are met, then it may still be a connection.

A number of new criteria to prove a relationship between microorganisms and disease was published in 1996 (Fredricks and Relman, Clin. Microbiol. Rev. 9:18-33). These criteria are based on detection of specific DNA sequences associated with disease.

Updated: 2017-09-27.