In the absence of electron acceptors such as oxygen, nitrate, and sulfate, Clostridium, Fusobaterium, and a few other anaerobes can ferment amino acids. This fermentation occurs during anaerobic and putrefaction processes. The most important mechanism for amino acid degradation is Stickland fermentation; during Stickland fermentation of two amino acids, one serves as the electron donor while the other serves as the acceptor. All amino acids are classified into electron acceptors or donors on the basis of Stickland fermentation, and only tryptophan and tyrosine can behave both as an acceptor and a donor. In addition to decarboxylation of various amino acids by this mechanism, the subsequent reactions yield a variety of products that can have unpleasant odors. In the absence of fermentable carbohydrates and in rich protein substrates, a large number of Clostridium species -such as C. botulinum, C. tetani, and C. perfringens - can generate ATP from amino acid fermentation. The ATP yield here is 1 mol per 3 mol of amino acid used, and thus the reaction is highly advantageous for organisms that can grow in rich anaerobic protein environments. The main products of the Stickland reaction are NH3, CO2, short-chain fatty acids and H2, and the minor products are hydrogen sulfide, methyl mercaptane, phenols, and alcohols, which together with fatty acids form a typical putrefying odor. The favorite habitat of Clostridium is the soil, in anaerobic scrap previously colonized by aerobic bacteria, and Clostridium species can also colonize the mammalian intestine. Moreover, these species can produce infectious diseases, such as botulism caused by C. botulinum, tetanus by C. tetani, and gangrene by C. perfringens. Clostridium sporogenes, a typical soil bacteria, can also participate in peritoneal infection that can occur as a result of the proliferation of food pathogens, intestinal obstruction, or mesenteric thrombosis.
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