Pathogenicity of capsulate and non-capsulate members of Bacteroides fragilis and B. melaninogenicus groups in mixed infection with Escherichia coli and Streptococcus pyogenes

The relationships between capsulate and non-capsulate Bacteroides fragilis strains and Escherichia coli, and between capsulate and non-capsulate strains of the B. melaninogenicus group and Streptococcus pyogenes, were studied in a subcutaneous abscess model in mice. Selective antimicrobial agents directed against either aerobic or anaerobic bacteria were used alone or in combination to explore the effect of eradication of one component of the mixed infection. Single agent therapy effective against both aerobic and anaerobic flora was also employed. Single therapy of mixed infection directed at the elimination of only one organism (S. pyogenes, E. coli or Bacteroides sp.) caused significant reductions in the numbers of sensitive organisms and also smaller yet significant decreases in the numbers of insensitive organisms. However, the abscesses were not eliminated after such therapy. Combination therapy or use of a single agent (cefoxitin) directed against the aerobic and anaerobic components of the infection was more effective. Non-capsulate Bacteroides spp. became capsulate after passage in mice mixed with either S. pyogenes or E. coli. Therapy directed at the elimination of S. pyogenes and E. coli did not prevent the emergence of capsulate Bacteroides spp. These data demonstrate the synergy between all members of the B. fragilis group and E. coli and between the B. melaninogenicus group and S. pyogenes, and reiterate the need to direct antimicrobial therapy at the eradication of the aerobic and anaerobic components of mixed infections.