PURPOSE: To be able to identify whether a given pathogen is responsible for the observed symptoms of a disease.
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. 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.