With the surprising existence of these microbes discovered, considerable debate as to their origin developed over the next several centuries. Since ancient times, and extending well into the Renaissance, the concept of spontaneous generation had been widely accepted with some forms of life, including weeds and vermin. The fundamental premise of this theory was that nonliving matter developed into viable life-forms through some
form of unknown, spontaneous transformation. Leeuwenhoek offered a competing suggestion that these microbes were formed from "seeds" or "germs" released by his animalcules.
An Italian physician, Francesco Redi, had offered support for such a view with his studies (circa 1665) of the growth of maggots on putrefying meat. As opposed to spontaneous growth, his findings showed that these maggots actually represented the larval stages of flies that laid eggs on the unprotected surfaces. Meats held either in a closed vessel or covered with fine gauze (protected from direct fly contact) exhibited no such growth. Italian naturalist Lazzaro Spallanzini, roughly five decades later, used heat to prevent the appearance of animalcules in sealed infusions. Spurred by a prize of 12,000 francs established by Napoleon Bonaparte to find a means of preserving fresh foods for his soldiers, French inventor Francois Appert used Spallanzini's heating strategy in 1795 to develop a commercial appertization process for preparing canned foods. In 1856 the eminent French chemist Louis Pasteur (Figure 10.4) similarly applied this knowledge about the use of controlled heating, developing his pasteurization process as a means of carefully protecting wines.
The success Pasteur enjoyed with pasteurization prompted his subsequent research focus on microbial metabolism, by which he finally laid the spontaneous generation premise to rest. Using swan-necked flasks (Figure 10.5), he showed that sterile solutions would not yield any "germ" growth unless reexposed to the particles within contaminated air (proving that it was not the air itself that "generated" new life).
During the era of inquiry regarding spontaneous generation, British naturalist John Tuberville Needham had conducted another sterilization study (1740), whose results at the time were, considered inconsistent and inconclusive. Since his heated broths still generated new cells, some suspected that his methods had been flawed. However, several years after the spontaneous generation concept had effectively been laid to rest, English physicist John Tyndall (1870) discovered that hay infusions subjected to prolonged boiling (and seemingly sterilized) could, in fact, still germinate new viable cells. What he found, though, was that these cultures had not developed through spontaneous means or improper handling, but rather, from the growth of heat-resistant agents. Tyndall's work was confirmed by German botanist Ferdinand Cohn, who visually confirmed the presence of these heat-resistant bodies, known today as endospores. Tyndall's subsequent work with these sporulating cells also led to his development of a sterilization procedure (tyndallization) by which discontinuous heating effectively kills all cells, even those able to develop such spores.
Inspired by Pasteur's efforts, Cohn published a three-volume treatise on bacteria in 1872 that many consider to be the true origin of the field of bacteriology. This book offered several classic insights, including the first attempt at bacterial classification and the first description of bacterial spores.
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