Koch's postulates

In 1890 the German physician and bacteriologist Robert Koch set out his celebrated criteria for judging whether a given bacteria is the cause of a given disease.

Koch's criteria brought some much-needed scientific clarity to what was then a very confused field.

Koch's postulates are as follows:</P> > >The bacteria must be present in every case of the disease.

>The bacteria must be isolated from the host with the disease and grown in pure culture.

>The specific disease must be reproduced when a pure culture of the bacteria is inoculated into a healthy susceptible host.

>The bacteria must be recoverable from the experimentally infected host.</LI></UL> However, Koch's postulates have their limitations and so may not always be the last word.

They may not hold if:</P> > >The particular bacteria (such as the one that causes leprosy) cannot be 'grown in pure culture' in the laboratory.

>There is no animal model of infection with that particular bacteria.</LI></UL> A harmless bacteria may cause disease if:</P> > >It has acquired extra virulence factors making it pathogenic.

>It gains access to deep tissues via trauma, surgery, an IV line, etc.

>It infects an immunocompromised patient.

>Not all people infected by a bacteria may develop disease-subclinical infection is usually more common than clinically obvious infection.</LI></UL> Despite such limitations, Koch's postulates are still a useful benchmark in judging whether there is a cause-and-effect relationship between a bacteria (or any other type of microorganism) and a clinical disease.