2 Julian I. Rood The key features that delineate members of the genus Clostridium are that they are Gram-positive rods that are anaerobic and form heat-resistant endospores. By and large these features define the genus, although there are some clostridia that stain Gram-negative and some clostridia that can grow in the presence of oxygen. Most members of this genus are commensal or soil bacteria that do not cause disease, but we tend to focus our attention on the pathogenic clostridia. The genus is extremely diverse and, by normal taxonomic criteria, should be divided into several different genera (Chapter 1). However, the established role of several clostridial species in some of the major diseases of humans and animals, including tetanus, botulism, gas gangrene, and various enteric and enterotoxemic syndromes, has precluded what would otherwise be a sensible and scientifically sound reclassification (Chapter 1). Recently, the movement of pathogens such as Clostridium difficile and Clostridium sordellii into the genera Peptoclostridium and Paeniclostridium, respectively, has been suggested, but these proposals are yet to be adopted formally. Bacterial metabolism is the process by which bacteria obtain nutrients and energy from the environment or their host, enabling them to grow and multiply. It is beyond the scope of this chapter to describe this process in detail, since entire books can and have been written on the topic. The key issue that will be discussed here is what, in general terms, distinguishes the metabolism of anaerobic bacteria from that of aerobic and facultative anaerobic bacteria. Aerobes are defined as bacteria that are unable to grow in the absence of oxygen. Facultative anaerobes can grow in the presence or absence of oxygen, usually growing more rapidly under aerobic conditions. Anaerobic bacteria are unable to grow in the presence of oxygen, but grow very well under anaerobic conditions. Anaerobic bacteria can be divided further into two major groups: strict anaerobes, which are killed by exposure to oxygen, and aerotolerant anaerobes, which can only grow anaerobically but are not killed by exposure to oxygen. Aerobic bacteria obtain most of their energy, in the form of ATP, in a highly efficient manner by the passage of electrons through the membrane-bound electron-transport chain, culminating in the use of oxygen as a terminal electron acceptor. This process is known as aerobic respiration. In an aerobic environment, facultative anaerobes such as Escherichia coli or Salmonella spp. use the electron-transport chain to produce ATP. In the absence of oxygen, they are reliant on the far less efficient substrate-level phosphorylation process or the use of an alternative electron acceptor. Anaerobic bacteria may still be able to obtain their energy from the electron-transport chain by use of an alternative terminal electron acceptor, usually an inorganic compound such as a nitrate or sulfate. This process is known as anaerobic respiration. Alternatively, they may carry out anaerobic fermentation and obtain all of their ATP from substrate-level phosphorylation, with oxidized NAD regenerated by the reduction of intermediates in the glycolytic pathway to ionized carboxylic acids such as acetate, lactate, or butyrate. Such organisms may significantly increase the throughput of sugars through the glycolytic pathway and therefore do not necessarily grow at a slower rate than aerobic bacteria, even though the output from aerobic respiration (38 moles of ATP per mole of glucose catabolized to CO2) is far greater than that from fermentation (2 moles of ATP per mole of glucose partially catabolized to a mixture of alcohols and/or organic acids). Like many other bacteria, the clostridia are not restricted to metabolizing sugars to obtain their energy. They can ferment other compounds such as amino acids to obtain both their carbon and energy. For example, C. difficile uses the Stickland reaction in which pairs of amino acids are fermented in a coupled reaction, with one amino acid acting as an electron donor and the other amino acid acting as an electron acceptor. Clostridial diseases and infections can be divided into three major types: neurotoxic diseases, histotoxic diseases, and enteric diseases. Although the focus of this book is clostridial diseases of animals, the clostridia are also important human pathogens. The major clostridial diseases of humans are botulism, tetanus, gas gangrene, food poisoning, pseudomembranous colitis, and antibiotic-associated diarrhea. The major clostridial diseases of animals are outlined in Chapter 3 (Table 3.1) and described in subsequent chapters. In both humans and animals, botulism and tetanus are caused by Clostridium botulinum and Clostridium tetani, respectively. Traumatic gas gangrene or clostridial myonecrosis in humans is primarily mediated by Clostridium perfringens and non-traumatic gas gangrene by Clostridium septicum, although other clostridia such as Clostridium novyi and C. sordellii can cause severe histotoxic infections in humans and animals. Enterotoxin (CPE)-producing strains of C. perfringens are now the second major cause of human food poisoning in the U.S.A., and can also cause non-food-borne gastrointestinal disease. The major cause of human antibiotic-associated diarrhea and a broader range of enteric infections, including pseudomembranous colitis and toxic megacolon, is the major nosocomial pathogen, C. difficile.
General Physiological and Virulence Properties of the Pathogenic Clostridia
Introduction
Anaerobic metabolism
Major clostridial diseases
The onset of clostridial infections
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