Enhancement of growth of aerobic and facultative bacteria in mixed infections with Bacteroides species

The potential for mutual enhancement of growth of the Bacteroides fragilis and B. melaninogenicus groups and the aerobic and facultative organisms commonly isolated with them in mixed infections was evaluated. Enhancement was studied by measuring the relative increase in CFU of the two bacterial components inducing subcutaneous abscesses in mice. Of the 42 combinations between three isolates each of the B. fragilis and B. melaninogenicus groups and seven aerobic or facultative organisms, Bacteroides spp. were enhanced in only 8 and inhibited in 4. The aerobic and facultative bacteria were enhanced in 31 of the 42 combinations and depressed in 2. The organisms uniformly enhanced by all of the Bacteroides spp. were group A streptococci and Escherichia coli (all six instances), followed by Staphylococcus aureus and Klebsiella pneumoniae (five of six instances), Pseudomonas aeruginosa (four instances), group D streptococci (in three instances only by the B. fragilis group), and Haemophilus influenzae (one instance). It is apparent that the growth rate of facultative and aerobic bacteria is enhanced much more in mixed infections with Bacteroides spp. than that of their anaerobic counterparts.