Synergistic aerobic and anaerobic infections.

Encapsulation affects the virulence and survival of anaerobic bacteria and their protection from phagocytosis. More encapsulated Bacteroides strains and anaerobic and facultative gram-positive cocci are isolated from patients with clinical infections than from healthy people. The pathogenicity of Bacteroides, Fusobacterium, Clostridium, and cocci isolates was demonstrated by their ability to induce subcutaneous abscesses in mice. Encapsulated Bacteroides, Fusobacterium, and cocci isolates generally induced abscesses, whereas nonencapsulated organisms did not. When strains that had fewer than 1% encapsulated organisms were inoculated with other viable or nonviable encapsulated bacteria, many survived in the abscesses and became heavily encapsulated. These strains were then able to induce abscesses when injected alone. Encapsulated Bacteroides species and anaerobic cocci induced bacteremia and translocation and increased the mortality in infected animals more often than did nonencapsulated forms of the same strain. In studies of selective antimicrobial therapy and quantitative cultures of abscesses, it was determined that possession of a capsule generally made Bacteroides species more important in mixed infections than their aerobic counterparts. In vivo synergy was seen between encapsulated Bacteroides species and all tested aerobic bacteria and most anaerobic and facultative gram-positive cocci as well as between most of these cocci and Pseudomonas aeruginosa or Staphylococcus aureus. It is concluded that encapsulated anaerobic bacteria have an important pathogenic role in polymicrobial infections.