9 Exceptions to Koch's Postulates

What are the 4  Koch's postulates?

Koch's postulates are a set of criteria used to establish a causal relationship between a microorganism and a disease.

Koch's postulate

Postulate 1. The microorganism must be present in every case of the disease but absent from healthy organisms.

Koch's Experiment:

  • Koch developed a staining technique to examine human tissue.
  • Mycobacterium tuberculosis cells could be identified in diseased tissue.

Exceptions 1: In the case of some diseases, microorganism may be present even without showing any signs or symptoms of disease. Polio, AIDS and Cholera can be carried asymptomatically, which violates the first postulate.

Postulate 2. The microorganism must be isolated from the diseased host and grown in pure culture.

Koch's Experiment:

Koch grew M. tuberculosis in pure culture on coagulated blood serum.

Exceptions 2:  Some microorganism cannot be cultured in an artificial medium

Example: Treponema pallidum (Syphilis), Mycobacterium leprae (leprosy)

Rickettsia, chlamydia multiply only within cell

Exceptions 3:  Subcellular disease-causing particle cannot be cultured in an artificial medium Viruses, prions etc

Exceptions 4:  A diseases may be caused by many causative microorganism

Diarrhea and Meningitis are caused by bacterial, viral, fungal, and parasitic pathogens.

Exception 5: Subcellular disease-causing particle cannot be cultured in an artificial medium such as viruses, prions etc.

Exception 6: Some microorganism can cause many diseases

Streptococcus pyogenes can cause sore throat but also cause scarlet fever and bone inflammation. Mycobacterium tuberculosis can cause disease symptoms in lungs, bone and even skin

Postulate 3. The same disease must result when the isolated microorganism is inoculated into a healthy host.

Koch's Experiment:

  • Koch injected cells from the pure culture of M.tuberculosis into guinea pigs.
  • The guinea pigs subsequently died of tuberculosis

Exception 7: Koch himself proved in regard to both tuberculosis and cholera, not all organisms exposed to an infectious agent will acquire the infection

Exception 8: For human limited diseases like AIDS caused by HIV virus, without a suitable nonhuman host, a researcher cannot evaluate postulate 3 as deliberately infecting humans presents obvious ethical concerns.

Postulate 4. The same microorganism must be isolated again from the diseased host.

Koch's Experiment:

  • Koch isolated M. tuberculosis from the dead guinea pigs and was able to again culture the microbe in pure culture on coagulated blood serum.

Exception 9: Many infections by microbes, considered the underlying cause of a disease, are absent from the lesions that ultimately develop.

Group A streptococcus infection is considered to be the underlying cause of rheumatic fever. The infection is long gone prior to the appearance of the valvular and endocardial lesions of rheumatic fever.

Human papillomavirus (HPV) is considered to be the underlying cause of nearly all cases of squamous carcinoma of the uterine cervix. After many years; following the early papillomavirus infections, may lack recoverable virus. 

Difference between Koch's Postulates and Molecular Koch's postulate

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