Encapsulated anaerobic bacteria in synergistic infections.

Polymicrobial infections can be more virulent than those involving single organisms (4, 29). Studies of the successful transmission of infections in animals that were given defined mixtures of aerobic and anaerobic organisms have been reported by many investigators (28, 38, 41). The key role of Bacteroides melaninogenicus was established by Rosebury et al. (38). However, none of the earlier studies established the role of encapsulated organisms in the infectious process. Encapsulation of anaerobic bacteria has been recognized as an important virulence factor. Several studies demonstrated the pathogenicity of encapsulated anaerobes and their ability to induce abscesses in experimental animals even when inoculated alone. Onderdonk et al. (34) correlated the virulence of B. fragilis strains with the presence of capsule. Encapsulated B. fragilis strains or purified capsular polysaccharide alone induced abscesses, whereas nonencapsulated strains seldom caused abscesses unless they were combined with an aerobic organism. Simon et al. (40) showed that encapsulated Bacteroides strains resisted netrophil-mediated killing, compared with nonencapsulated strains. The susceptibility of pathogenic bacteria to phagocytosis and killing by polymorphonuclear leukocytes and macrophages is important in determining the outcome of the host-pathogen interaction. Ingham et al. (19), Tofte et al. (42), and Jones and Gemmell (21) reported that both phagocytic uptake and killing of facultative bacterial species were impaired at high concentrations of B. fragilis and B. melaninogenicus. The presence of a capsule on B. fragilis was shown to provide the organism with a growth advantage in vivo over nonencapsulated isolates (37). Furthermore, encapsulated strains survived better in vitro than nonencapsulated variants when they were grown in an aerobic environment. Another recently described mechanism of protection independent of encapsulation is the inhibition of polymorphonuclear migration due to the production of succinic acid by Bacteroides sp. (39). The importance of the capsular polysaccharide of B. fragilis as an immunogen was demonstrated when antibodies against it protected animals from early bacteremia (23). However, prevention of the formation of intra-abdominal