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Although many diseases has well established pathologies by the middle of the 19th century, scientists still argued what was the causative agent of infectious disease. For example, despite the well established epidemiology of cholera, some scientists believed that it was the smell associated with the disease (i.e. sewage) that set off the symptoms, while others, based on the work of Anton van Leeuwenhoek and his microscope, believed microscopic pieces of matter were responsible.
However, microscopic living things existed in all individuals, and were largely impossible to isolate in sufficient quantities. They were also difficult to grow outside the body, as many of the organisms that were suspected to cause human disease would not grow well outside it. Even if they could be grown, they were once again difficult to isolate and identify, and would soon become contaminated with other microscopic organisms.
To attempt to deal with these difficulties, Koch developed the modern blood culture, which is still the basis for modern bacteriological research. He mixed blood (human or animal) with gelatine. To this mixture, a small sample of the organism was added. The bacteria ate the blood, but because of the gelatine, they could not move freely through the medium and instead formed large colonies of millions of bacteria that could be easily viewed through a microscope. Moreover, any contamination that existed would be isolated in the area it landed rather than existing throughout the entire sample.
In this way, he isolated three bacteria those responsible for anthrax, tuberculosis and cholera. In tests on animals, he demonstrated his four postulates, which, with modifications, are still in use today:
- That the bacteria will only appear in persons with the disease;
- That the bacteria can be grown in pure culture;
- That that anyone given the bacteria from the culture will develop the disease; and
- That you can once again grow the bacteria from a pure culture of a person so infected.
Although the finding was very controversial at the time, Koch proved that cholera is transmitted by a bacteria in fecal material and not from the smell of sewage.
For his work on tuberculosis, Koch received the Nobel Prize for Physiology and Medicine in 1905.