What are Koch's postulates?

Definition

Koch’s postulates are a set of experimental guidelines used to determine if a particular microorganism is the causative agent of a particular disease.

Historical Overview

In the nineteenth century, Robert Koch, a German physician and
bacteriologist, played a significant role in determining the etiology (cause) of
an infectious disease. Through his work with Bacillus anthracis
(the causative agent of anthrax), he linked a specific
microorganism to a specific infectious disease. Koch conducted experiments showing
that B. anthracis was always present in diseased animals, that
healthy animals inoculated with the bacterium would develop the disease, and that
cultivation of the bacterium in artificial media followed by inoculation resulted
in the disease.

Koch also discovered the causative organisms for several other diseases,
including tuberculosis and cholera. In describing the etiology of
tuberculosis, Koch proposed a set of guidelines for establishing a cause and
effect relationship between a given microorganism and a specific disease. These
scientific criteria are known as Koch’s postulates.

The Postulates

Koch’s postulates are a set of four experimental criteria used to establish the
etiology of a disease. The first criterion states that the pathogen must
be present in all infected persons and absent in all healthy persons. The second
criterion states that the pathogen must be isolated from the diseased person and
cultivated in the laboratory. The third criterion states that the cultivated
pathogen must cause the disease in a healthy person after inoculation. The fourth
criterion states that the pathogen must be isolated again from the infected person
and identified as identical to the original isolate.

Exceptions

There are some exceptions to Koch’s postulates. Certain pathogens and
fastidious microorganisms have complex and unusual growth requirements and can
survive only within living host cells. Such microorganisms cannot be cultured on
artificial media. Numerous pathogens infect a specific species only while others
become transformed in vitro. Some infectious diseases have unclear origins while
others cause multiple disease conditions. Many infections develop from the
combined effects of several different microorganisms. Various diseases do not
originate from a microorganism and may be the result of poor nutrition,
chromosomal abnormality, organ failure, or environmental influences. These
exceptions have stimulated the need for modifications to Koch’s postulates.

Impact

Koch’s contributions were invaluable in the advancement of medical microbiology and in the understanding of the nature of a disease. Koch’s postulates still provide the essential principles for determining the causative agents of emerging infectious diseases and the basic foundation within which to address disease and public health.

Bibliography

Daniel, Wayne W. Biostatistics: A Foundation for Analysis in the Health Sciences. 9th ed. Hoboken, N.J.: John Wiley & Sons, 2009.

Engelkirk, Paul G., and Gwendolyn R. W. Burton. Burton’s Microbiology for the Health Sciences. 8th ed. Baltimore: Lippincott Williams & Wilkins, 2007.

Hardy, Simon P. Human Microbiology. New York: Taylor and Francis, 2003.

Murray, Patrick R., Ken S. Rosenthal, and Michael A. Pfaller. Medical Microbiology. 6th ed. Philadelphia: Mosby/Elsevier, 2009.

Straus, Eugene, and Alex Straus. Medical Marvels: The One Hundred Greatest Advances in Medicine. Amherst, N.Y.: Prometheus Books, 2006.

Tortora, Gerard J., Berdell R. Funke, and Christine L. Case. “Koch’s Postulates.” In Microbiology: An Introduction. 10th ed. San Francisco: Benjamin Cummings, 2010.