THE MICROBIAL MENACE
PASTEUR AND KOCH - BACTERIOLOGY -
GERM THEORY OF DISEASE
In the field of observation, chance favors only those
who are prepared... .
-Louis Pasteur, 1854
Though Leeuwenhoek's microscope made it possible to see bacteria,
bacteriology did not emerge as a science until almost two centuries
later. Viruses, almost all of which are smaller than the smallest
bacteria, were suspected as a cause of disease in the 19th century
and even named (virus being a name for "poison"), but were not
specifically seen until well into the 20th century with the advent of
Louis Pasteur came to his formulation of the germ theory of disease
through his studies of fermentation for the French wine industry and
through subsequent studies of silkworm diseases for the silk industry.
His investigation of fermentation focused on the role of living
organisms. These discoveries helped Pasteur formulate his "germ theory
of disease" and led directly to the development of a process in which
heat is used to kill bacteria. The process, known as pasteurization,
is still used. Pasteur also developed a vaccine for rabies.
Robert Koch confirmed Pasteur's germ theory and took it several
steps further. His investigations began with a study of Bacillus
anthracis, which causes a disease affecting cattle. He cultured the
anthrax bacillus and later used the same techniques in tracking and
culturing the organism responsible for tuberculosis. Koch won a Nobel
Prize for his work on tuberculosis, but is perhaps better remembered for
his formulation of four basic principles or postulates of bacteriology.
Left: Joseph Meister came to Pasteur after being bitten by a rabid dog.
Pasteur treated him with a rabies vaccine, an unorthodox use of vaccines in that
infection had probably already occurred. However, the treatment apparently succeeded.
The rabies virus would not be identified for another half a century.
Right: Vaccine as a treatment.
- The organism must be present in every case of the disease
- The organism must not be present in any other disease as an agent not responsible for disease
- The organism must be capable of being isolated
- After growth in pure culture, the organism must be able to produce the disease
Koch's postulates, formalized in 1882:
Koch's postulates ushered in a sustained era of pathogen research and
identification. In their day, these postulates were enormously helpful
to epidemiologists helping to formalize standards by which new pathogens
could be identified. Koch's postulates are still taught to medical
students, but modern medical science is starting to pass them up. Most of
the pathogens that can be identified by Koch's postulates are believed
to have been identified, and interest is turning to diseases with
multiple causes or factors and to infectious agents that cannot be
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