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.