Koch's Postulates

For his successful determination of the bacterium that causes tuberculosis, Mycobacterium tuberculosis, Koch was awarded the Nobel Prize in Physiology or Medicine in 1905. Koch formulated his famous set of criteria for establishing a causative link between an infectious agent and a disease (Fig. 1.17). These four criteria are known as Koch's postulates:
            1. The microbe is found in all cases of the disease but is absent from healthy individuals.
            2. The microbe is isolated from the diseased host and grown in pure culture.
            3. When the microbe is introduced into a healthy, susceptible host (or animal model), the same disease occurs.
            4. The same strain of microbe is obtained from the newly diseased host. When cultured, the strain shows the same characteristics as before.    

Koch's postulates continue to be used to determine whether a given strain of microbe causes a disease. Mod-em examples include Lyme disease, a tick-borne infection that has become widespread in New England and the Mid-Atlantic states; and hantaviral pneumonia, an emerging disease particularly prevalent among Native Americans in the Southwest. Nevertheless, the postulates remain only a guide; individual diseases and pathogens may confound one or more of the criteria. For example, tuberculosis bacteria are now known to cause symptoms in only 10% of the people infected. If Koch had been able to detect these silent bacilli, they would not have fulfilled his first criterion. In the case of AIDS, the concentration of HIV virus is so low that initially no virus could be detected in patients with fully active symptoms. It took the invention of the polymerase chain reaction (PCR), a method of producing any number of copies of DNA or RNA sequences, to detect the presence of HIV. Another difficulty with AIDS and many other human diseases is the absence of an animal host that exhibits the same disease. In the case of AIDS, even chimpanzees, our closest relatives, are not susceptible, although they exhibit a similar disease from a related pathogen, simian immunodeficiency virus (SIV). Experimentation on humans is prohibited, although in rare instances researchers have voluntarily exposed themselves to a proposed pathogen. For example, Australian researcher Barry Marshall ingested Ileiicobacter pylori to convince skeptical col-leagues that this organism could colonize the extremely acidic stomach. II. pylori turned out to be the causative agent of gastritis and stomach ulcers, conditions that had long been thought to be caused by stress rather than infection. For the discovery of H. pylori, Marshall and col-league Robin Warren won the 2005 Nobel Prize in Physiology or Medicine.