German physician Robert Koch (1843–1910) demonstration the first direct of the role of bacteria in causing disease. Koch injected healthy mice with material from diseased animals, and the mice became ill. After transferring anthrax by inoculation through a series of 20 mice, he incubated a piece of spleen containing the anthrax bacillus in beef serum. The bacilli grew, reproduced, and produced endospores. When the isolated bacilli or their spores were injected into healthy mice, anthrax developed. His criteria for proving the causal relationship between a microorganism and a specific disease are known as Koch’s postulates. Koch’s proof that B. anthracis caused anthrax was independently confirmed by Pasteur and his coworkers. They discovered that after burial of dead animals, anthrax spores survived and were brought to the surface by earthworms.
Healthy animals then ingested the spores and became ill. After completing his anthrax studies, Koch fully outlined his postulates in his work on the cause of tuberculosis.
Koch’s Application of His Postulates Demonstrated That Mycobacterium
tuberculosis Is the Causative Agent of Tuberculosis
- The microorganism must be present in every case of the disease but absent from healthy organisms. Koch developed a staining technique to examine human tissue.
- tuberculosis cells could be identified in diseased tissue.
- The suspected microorganisms must be isolated and grown in a pure culture.
Koch grew M. tuberculosis in pure culture on coagulated blood serum.
- The same disease must result when the isolated microorganism is inoculated into a healthy host.
Koch injected cells from the pure culture of M. tuberculosis into guinea pigs. The guinea pigs subsequently died of tuberculosis.
- The same microorganism must be isolated again from the diseased host. Koch isolated M. tuberculosis from the dead guinea pigs and was able to again culture the microbe in pure culture on coagulated blood serum.
Although the criteria that Koch developed for proving a causal relationship between a microorganism and a specific disease have been of great importance in medical microbiology.
A Molecular Approach to Koch’s Postulates
- Inactivation of the gene or genes associated with the suspected virulence trait should substantially decrease
- Replacement of the mutated gene with the normal wildtype gene should fully restore pathogenicity.
- The gene should be expressed at some point during the infection and disease process.
- Antibodies or immune system cells directed against the gene products should protect the host.
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