Aspirates obtained from infections adjacent to mucous membrane surfaces generally contain a complex bacterial population consisting of several species.1 The relative importance of the various bacteria isolated in clinical specimens is not clear.
Polymicrobic infections are more pathogenic for experimental animals than those involving single organisms.2 Studies of the successful transmission of infections in animals inoculated with defined mixtures of aerobic and anaerobic organisms, including the Bacteroides melaninogenicus group, have been reported by many investigators.3,4 Except for some strains of B melaninogenicus,5 most organisms in these mixtures were not pathogenic alone,3 although these were required for the pathogenicity of the mixture.
Several recent studies demonstrated the pathogenicity of encapsulated anaerobes and their ability to induce abscesses alone. Onderdonk et al6,7 correlated the virulence of Bacteroides fragilis strains with the presence of capsule, and Simon et al8 described decreased phagocytosis of the encapsulated B