The observation of microscopic organisms led priests and philosophers to wonder where they came from. In the eighteenth century, scientists and church leaders intensely debated the question of spontaneous generation, the theory that living creatures such as maggots could arise spontaneously, without parental organisms. Chemists of the day tended to support spontaneous generation, at it appeared similar to the changes in matter that could occur when chemicals were mixed. Christian church leaders, however, supported the biblical view that all organisms have "parents" going back to the first week of creation. The Italian priest Francesco Redi (1626-1697) showed that maggots in decaying meat were the offspring of flies. Meat kept in a sealed container, excluding flies, did not produce maggots. Thus, Redi's experiment argued against spontaneous generation for macroscopic organisms. The meat still putrefied, however, producing microbes that seemed to arise "without parents." To disprove spontaneous generation of microbes, another Italian priest, Lazzaro Spallanzani (1729-1799), showed that a sealed flask of meat broth sterilized by boiling failed to grow microbes. Spallanzani also noticed that microbes often appeared in pairs. Were these two parental microbes coupling to produce offspring, or did one microbe become two? By long and tenacious observation, Spallanzani watched a single microbe grow in size until it split in two. Thus, he demonstrated cell fission, the process by which cells arise by the splitting of preexisting cells. Even Spallanzani's experiments, however, did not put the matter to rest. Proponents of spontaneous generation argued that the microbes in the priest's flask lacked access to oxygen and therefore could not grow. The pursuit of this question was left to future microbiologists, including the famous French microbiologist Louis Pasteur (1822-1895) (Fig. 1.14A). In addressing spontaneous generation and related questions, Pasteur and his contemporaries laid the foundations for modern microbiology.
Louis Pasteur reveals the biochemical basis of microbial growth. Pasteur began his scientific career as a chemist and wrote his doctoral thesis on the structure of organic crystals. He discovered the fundamental chemical property of chirality, the fact that some organic molecules exist in two forms that differ only by mirror symmetry. In other words, the two structures are mirror images of one another, like the right and left hands. Pasteur found that when microbes were cultured on a nutrient substance containing both mirror forms, only one mirror form was consumed. He concluded that the metabolic preference for one mirror form was a fundamental property of life. Subsequent research has confirmed that most biological molecules, such as DNA and proteins, occur in only one of their mirror forms. As a chemist, Pasteur was asked to help with a widespread problem encountered by French manufacturers of wine and beer. The production of alcoholic beverages is now known to occur by fermentation, a process by which microbes gain energy by converting sugars into alcohol. In the time of Pasteur, however, the conversion of grapes or grain to alcohol was believed to be a spontaneous chemical process. No one could explain why some fermentation mixtures produced vinegar (acetic acid) instead of alcohol. Pasteur discovered that fermentation is actually caused by living yeast, a single-celled fungus. In the absence of oxygen, yeast produces alcohol as a terminal waste product. But when the yeast culture is contaminated with bacteria, the bacteria outgrow the yeast and produce acetic acid instead of alcohol. (Fermentative metabolism is discussed further in Chapter 13.) Pasteur's work on fermentation led him to test a key claim made by proponents of spontaneous generation. The proponents claimed that Spallanzani's failure to find spontaneous appearance of microbes was due to lack of oxygen. From his studies of yeast fermentation, Pasteur knew that some microbial species do not require oxygen for growth. So he devised an unsealed flask with a long, bent "swan neck" that admitted air but kept the boiled contents free of microbes (Fig. 1.148). The famous swan-necked flasks remained free of microbial growth for many years; but when a flask was tilted to enable contact of broth with microbe containing dust, growth occurred immediately. Thus, Pasteur disproved that lack of oxygen was the reason for the failure of spontaneous generation in Spallanzani's flasks. But even Pasteur's work did not prove that microbial growth requires preexisting microbes. The Irish scientist John Tyndall (1820-1893) attempted the same experiment as Pasteur, but sometimes found the opposite result. Tyndall found that the broth sometimes gave rise to microbes, no matter how long it was sterilized by boiling. The microbes appear because some kinds of organic matter, particularly hay infusion, arecontaminated with a heat resistant form of bacteria called endospores (or spores). The spore form can be eliminated only by repeated cycles of boiling and resting, in which the spores germinate to the growing, vegetative form that is killed at 100°C. It was later discovered that endospores could be killed by boiling under pressure, as in a pressure cooker, which generates higher temperatures than can be obtained at atmospheric pressure. The steam pressure device called the autoclave became a standard method for the sterilization of materials required for the controlled study of microbes. (Microbial control and antisepsis are discussed further in Chapter 5.) Although spontaneous generation has been discredited as a continual source of microbes, at some point in the past the first living organisms must have originated from nonliving materials. The origin of life is explored in Special Topic 1.1, and discussed further in Chapter 17.
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