the strains responsible for most disease in civilian populations. Also, the strains used in the live oral vaccine are not significantly attenuated. While no spread of infection has been found in military use, spread was observed among children when the live oral vaccines were tested (Foy and Grayston, 1982). The potential problem of respiratory illness developing in contacts of recipients of the oral unattenuated adenovirus vaccine and questions about the oncogenic potential of adenoviruses remain obstacles to future vaccine development.
Bacteroides fragilis This organism is regarded as the most important anaerobe in clinical medicine because of its prevalence in intra-abdominal sepsis, its frequent association with bacteremia, and its resistance to antibiotics. B. fragilis is also the only anaerobic bacterium capable in experimental animal model systems of inducing abscess alone rather than in combination with a facultative aerobic organism (Washington, 1971). The organism has a polysaccharide capsule that is immunogenic, and animal experiments have shown that vaccination induces antigen specific T-cell dependent immunity to abscesses. The principal factor limiting pursuit of a polysaccharide vaccine for this organism is that only 70 percent of intra-abdominal infections involve B. fragilis (Shapiro et al., 1982). In animal models, however, immunization with capsular polysaccharide leads to development of an antigen specific lymphokine capable of protecting against abscesses caused by B. fragilis. Pursuit of a vaccine of cellular rather than bacterial origin offers promise for reducing the incidence of intra-abdominal abscesses (Kasper, personal communication, 1984).
Clostridium botulinum The disease caused by this organism is relatively rare in the United States (50 to 100 cases per year) and most cases occur in young infants. A vaccine is available, but its efficacy in children under age six months (against the infant botulism form of the disease) is uncertain. Use of the vaccine is now restricted to older, high-risk individuals, such as those doing experimental work with the organism.
Clostridium difficile This agent is the most common cause of antibiotic-associated colitis. Its two toxins, designated A and B, are immunogenic, but it is not yet known if antigenic experience confers protection. The high prevalence and serious morbidity associated with C. difficile disease make it a likely candidate for vaccine development in the future.
Clostridium perfringens This organism is a common cause of food-borne outbreaks of gastrointestinal illness, but the disease is usually brief, self-limited, and not life threatening except in some regions. The enterotoxin is immunogenic and a vaccine is available outside the United States.