Infectivity of organisms recovered from polymicrobial abscesses

The ability to cause subcutaneous abscesses in mice was used to identify the pathogens among the bacteria recovered from 13 clinical abscesses. A total of 35 isolates (30 anaerobes and 5 aerobes), 16 of which were encapsulated, were recovered from these abscesses. Encapsulated organisms included eight Bacteroides spp. (three Bacteroides asaccharolyticus and one strain each of Bacteroides oralis, Bacteroides intermedius, Bacteroides biacutus, Bacteroides vulgatus, and Bacteroides ruminicola subsp. brevis), three anaerobic gram-positive cocci, two Clostridium spp., and two strains of Escherichia coli. Single organisms, or combinations thereof, obtained from these abscesses were inoculated subcutaneously into mice. All but one of the encapsulated organisms were able to cause abscesses by themselves and were recovered from the abscesses when inoculated with other organisms. Seven non-encapsulated organisms which were also recovered mixed with the encapsulated organisms were never able to cause abscesses nor could they be recovered after they were injected alone. Thirteen strains that did not induce abscesses when injected by themselves into mice survived when injected with other organisms which were encapsulated. In three instances, pairs of non-encapsulated organisms belonging to the last group were able to induce an abscess and survive in it when inoculated together. It was shown that the possession of a capsule by a clinical isolate increases the likelihood that it is a major contributor to the infectious process.