Clostridia: Sporeforming Anaerobic Bacilli

Excerpt

Of the anaerobes that infect humans, the clostridia are the most widely studied. They are involved in a variety of human diseases, the most important of which are gas gangrene, tetanus, botulism, pseudomembranous colitis and food poisoning. In most cases, clostridia are opportunistic pathogens; that is, one or more species establishes a nidus of infection in a particular site in a compromised host. All pathogenic clostridial species produce protein exotoxins (such as botulinum and tetanus toxins) that play an important role in pathogenesis.

Most generalizations about Clostridium have exceptions. The clostridia are classically anaerobic rods, but some species can become aerotolerant on subculture; a few species (C carnis, C histolyticum, and C tertium) can grow under aerobic conditions. Most species are Gram-positive, but a few are Gram-negative. Also, many Gram-positive species easily lose the Gram reaction, resulting in Gram-negative cultures.

The clostridia form characteristic spores, the position of which is useful in species identification; however, some species do not sporulate unless exposed to exacting cultural conditions. Many clostridia are transient or permanent members of the normal flora of the human skin and the gastrointestinal tracts of humans and animals. Unlike typical members of the human bacterial flora, most clostridia can also be found worldwide in the soil.

Because clostridia are ubiquitous saprophytes, many isolated from clinical specimens are accidental contaminants and not involved in a disease process. Because these organisms are normally found on the skin, even a pure culture of clostridia isolated from blood may have no clinical significance. In determining the importance of a clinical isolate of clostridia, the clinician should consider the frequency of isolation of the species, the presence of other microbes of pathogenic potential, and the clinical symptoms of the patient. Many clostridial infections can be controlled by antibiotic therapy (e.g., penicillin, chloramphenicol, vancomycin, metronidazole) accompanied, in some cases, by tissue debridement. Antitoxin therapy and toxoid immunization are clearly useful in some clostridial infections, such as tetanus.