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How do we know that a bacterium is a pathogen?

In 1890 Robert Koch set out what he considered to be the criteria for
judging whether a given bacterium was the cause of a given disease. Although
these criteria are not without problems (see next page), they brought
clarity to an otherwise confused field.

Koch's postulates

     [*] The bacterium must be present in every case of the disease.


     [*] The bacterium must be isolated from the diseased host & grown in
     pure culture.


     [*] The specific disease must be reproduced when a pure culture of the
     bacterium is inoculated into a healthy susceptible host.


     [*] The bacterium must be recoverable from the experimentally infected

(c) Mark Pallen 1995.
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