For many years, people believed that life was able to spring from non-life, an idea that is referred to as spontaneous generation. While it now seems obvious to most humans that life can only come from life and that a non-living object will never spontaneously create life, spontaneous generation was quite difficult for scientists to disprove due to its wide-held and long-standing belief. Frances Redi was among the first scientists to perform experiments that disproved spontaneous generation. Using three jars of meat – one open, one with a mesh lid, and one tightly covered – he was able to show that rotting meat did not create flies but rather that the flies themselves created new flies when they had access to rotting meat. Although Redi’s experiment was easily repeatable by others, many still objected to a total refutation of spontaneous generation, claiming that flies and other large organisms might be too complex but that smaller or simpler organisms could still come from non-life objects. John Needham and Lazzaro Spallanzani both attempted to disprove spontaneous generation by boiling various broths to kill any microorganisms that might be present and then showing that no life forms reappeared if the flasks were sealed correctly. Arguments against Needham and Spallanzani included the fact that sealed flasks did not allow air, then called the “vital force,” to enter the flasks and thus would disallow spontaneous generation and the fact that some flasks did show the growth of organisms after sealing, which we now know would have been due to microbial contamination between the time of boiling and the time of sealing the contents. It was finally Louis Pasteur who was able to convince most of the world that spontaneous generation is an invalid theory. Pasteur used flasks with long, bent necks in his experiments with boiled broths. The open neck of the flask allowed air to enter but the curved nature of the necks created pockets which captured air-borne microorganisms so they were not allowed to enter the broths. Although Pasteur’s broths would become contaminated if they were tipped so that the broth touched the bends in the flask neck, if the flasks were kept upright they remained sterile and free-of-life indefinitely. Finally, people began to accept the idea that life only comes from life and the theory of biogenesis was born.
After spontaneous generation was disproved, some scientists began noticing that various microorganisms affected the chemical and physical properties of other organic materials. In one such case, Pasteur famously proved that the presence of bacteria caused spoilage, yielding vinegar instead of the intoxicating wine. Then, Joseph Lister was able to show great success in lowering the death rates in maternity wards and other surgical arenas by using disinfectants in both the air and on surgical instruments. While proofs like these lead some to begin wondering about the causal effects of diseases and infections, many people were still reluctant to believe that disease and infection were not due strictly to “moral” causes or the “immoral” actions of the affected person. It was finally up to Robert Koch to develop the germ theory of disease. After considering all of the data before him, Koch theorized that it is microorganisms that are the cause of many diseases. By testing mice that he had injected with anthrax from sick or dead cows, Koch not only developed his still-adhered-to postulates, but he also cemented the germ theory of disease into microbiology. In his work with anthrax, and in later work with such infectious diseases as tuberculosis and cholera, Koch followed four basic steps. First, Koch observed the microbes in the diseased animals to see which were in all the blood samples. He then cultivated that microbe in a pure form. After injecting healthy animals with the cultivated microbe, he was able to observe the once-healthy animals become sick and then was able to find the same microbe in the newly sick animals’ blood as was in the blood of the originally sick animals. Because Of Redi, Needham, Spallanzani, and Pasteur’s previous experiments disproving spontaneous generation, Koch was able to develop his germ theory of disease without having to fight the idea that the microbes in the blood of the newly-sick animals were spontaneously generated, but rather he was able to state that the microbes were the cause of the new infections.