The genus Clostridium encompasses more than 60 species that may be commensals of the gut microflora or may cause a variety of infections in humans and animals through the production of a plethora of proteinaceous exotoxins. C. tetani and C. botulinum, for example, cause specific clinical disease by elaborating single but highly potent toxins. In contrast, C. perfringens and C. septicum cause aggressive necrotizing infections that are attributable to multiple toxins, including bacterial proteases, phospholipases, and cytotoxins.
Vegetative cells of Clostridium species are pleomorphic, rod-shaped, and arranged singly or in short chains (Fig. 51-1); the cells have rounded or sometimes pointed ends. Although clostridia stain gram-positive in the early stages of growth, they may appear to be gram-negative or gram-variable later in the growth cycle or in infected tissue specimens. Most strains are motile by means of peritrichous flagella; C. septicum swarms on solid media. Nonmotile species include C. perfringens, C. ramosum, and C. innocuum. Most species are obligately anaerobic, although clostridial tolerance to oxygen varies widely; some species (e.g., C. septicum, C. tertium) will grow but will not sporulate in air.
Scanning electron micrograph of C. perfringens.
Clostridia produce more protein toxins than any other bacterial genus, and more than 25 clostridial toxins lethal to mice have been identified. These proteins include neurotoxins, enterotoxins, cytotoxins, collagenases, permeases, necrotizing toxins, lipases, lecithinases, hemolysins, proteinases, hyaluronidases, DNases, ADP-ribosyltransferases, and neuraminidases. Botulinum and tetanus neurotoxins are the most potent toxins known, with lethal doses of 0.2–10 ng/kg for humans. Epsilon toxin, a 33-kDa protein produced by C. perfringens types B and D, causes edema and hemorrhage in the brain, heart, spinal cord, and kidneys of animals. It is among the most lethal of the clostridial toxins and is considered a potential agent of bioterrorism (Chap. 10). The genomic sequences of some pathogenic clostridia are now available and are likely to facilitate a comprehensive approach to understanding the virulence factors involved in clostridial pathogenesis.
EPIDEMIOLOGY AND TRANSMISSION
Clostridium species are widespread in nature, forming endospores that are commonly found in soil, feces, sewage, and marine sediments. The ecology of C. perfringens in soil is greatly influenced by the degree and duration of animal husbandry in a given location and is relevant to the incidence of gas gangrene caused by contamination of war wounds with soil. For example, the incidence of clostridial gas gangrene is higher in agricultural regions of Europe than in the Sahara Desert of Africa. Similarly, the incidences of tetanus and food-borne botulism are clearly related to the presence of clostridial spores in soil, water, and many foods. Clostridia are present in large numbers in the indigenous microbiota of the intestinal tract of ...