Definition of Koch's postulates
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:
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.
However, Koch's postulates have their limitations and so
may not always be
the last word. They may not hold if:
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.
A harmless bacteria may cause disease if:
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.
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.
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