Role of encapsulated anaerobic bacteria in synergistic infections

The effect of encapsulation on the virulence, survival, and protection of anaerobic bacteria from phagocytosis is reviewed. Support for the importance of encapsulated Bacteroides sp. and anaerobic and facultative Gram-positive cocci (AFGPC) was provided by their higher recovery rate in oropharyngeal infections, compared to their number in the normal oral flora. Studies of the pathogenicity of anaerobic bacteria of the Bacteroides, Fusobacterium, and Clostridium genera and AFGPC are also presented. The organisms were inoculated into mice and their ability to induce subcutaneous abscesses was determined. Encapsulated Bacteroides, Fusobacteria, and AFGPC generally induced abscesses, whereas unencapsulated organisms did not. However, many of the strains that had only a minimal number of encapsulated organisms (less than 1%) survived in the abscess, and became heavily encapsulated when inoculated with other viable or nonviable encapsulated bacteria. These strains were thereafter able to induce abscesses when injected alone. Encapsulated Bacteroides sp. and anaerobic cocci induced bacteremia and translocation, and increased the mortality of the infected animals more often than did the unencapsulated form of the same strains. The relative importance of encapsulated anaerobes in relation to their aerobic and facultative counterparts in mixed infection was studied, using selective antimicrobial therapy and quantitative cultures of abscesses induced in mice. With few exceptions, possession of a capsule made Bacteroides sp. more important than their aerobic counterparts. Synergistic potentials were seen between encapsulated Bacteroides sp. and all tested aerobic bacteria and most AFGPC, and between most AFGPC and Pseudomonas aeruginosa or Staphylococcus aureus. These studies demonstrated the importance of encapsulated anaerobes in mixed infections.