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

Koch's postulates:

  • 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.

#immunity #ppp