Leeuwenhoek and the Discovery of The CellsEdit
Prior to Leeuwenhoek, there had been little or notion of the idea that microscopic living things could exist. Part of this was due to the fact that most microscopes of the time were not strong enough to see them, though they did exist . However, Leeuwenhoek was skillful enough to make such a powerful microscope, and observed protazoans in pond water .
Not only did he observe them, but he also was able to deduce that they were living because they were motile, and only living things have the ablility to move by their own power. Leeuwenhoek's deductions, and his creative use of technology to explore new avenues, led to the recognition of cells as the building blocks of life, which would have profound influence on biological study and knowledge.
Before Louis Pasteur and other scientists proved them wrong, the mainstream belief in science was that living things arose spontaneously from non-living things. This was particularly true of the "cells" which Leuvenhok discovered, because the cells were comparatively simple and therefore it appeared logical that they arose naturally from their environments . Another scientist named Lazzaro Spallanzani had previously proven that many bacteria (though not all) are killed by boiling and, if sealed from the air, they will not regrow in the container . However, contemporaries refused to deny spontaneous generation, saying that since the organisms spontaneously arose from air, sealing off the container made Lazzaro's hypothesis invalid.
Pasteur put this to rest by designing an experiment. He designed an apparatus, called a swan-neck flask, in which a sterile liquid was exposed to the air but no bacteria could reach it. Bacteria and the dust they lived on were trapped in the swan neck, and sterile air reached the liquid. Since the liquid did not become contaminated with bacteria, Pasteur proved that they did not spontaneously arise from the air. This led to the death of the theory of spontaneous generation and to further studies about how bacteria do reproduce. These studies are important because they have lead to an understanding of how many antibiotics, including Penicillin, work.
Robert Koch and his Four PostulatesEdit
Before scientists like Pasteur and Koch arrived on the scientific scene to prove them wrong, many scientists had false beliefs concerning the nature of disease. One of these beliefs was that infectious bacteria spontaneously generated in a human body as a result of diseases  . While Pasteur proved that spontaneous generation was impossible in the air, Koch devised a scheme by which the cause of diseases in people (and animals) could be experimentally tested.
His scheme revolved around the notion that, if a person is able to prove that one organism is common to all cases of a disease, and if it can be shown that no outside factor causes the disease, then the organism must, indeed, cause the disease. To make it experimentally rigorous, Koch needed a control scheme, so he devised a set of "postulates" to prove that the organism in question actually caused the disease :
- The organism had to be present in any case of the illness in question.
- It had to be possible to take the organism out of the patient and purify it so that only one species was present. This prevents the possibility that an organism other than the one being observed actually causes the disease.
- The now-purified organism had to cause the disease when injected into a healthy animal. This prevents the possibility of something before the bacteria causing the disease, and debunks the possibility that the toxins came first and caused formation of the bacteria.
- As a final check, one must be able to re-isolate the organism from the newly-sick animal.
Koch used this to prove that Bacillus anthracis causes anthrax , and it has been used to establish the causes of a large number of other bacterial diseases as well.