The pathogenicity of Bacteroides fragilis and related species estimated by intracutaneous infection in the guinea-pig.

The pathogenicity of 42 strains of Bacteroides of human origins was estimated by intracutaneous injection of bacterial suspensions into guinea-pig skin. Comparisons of living and heat-killed suspensions revealed that B. fragilis strains maintained themselves and possibly multiplied in the skin, whereas the lesions induced by non-fragilis strains appeared to be due mainly to toxicity. Measurement of skin pathogenicity in terms of the number of viable organisms in the inoculum that produced a lesion 10 mm in diameter showed that B. fragilis was, on average, 17 times as pathogenic as non-fragilis strains. Skin tests of pathogens may be of value in the analysis of virulence factors of Bacteroides and possibly of other anaerobic organisms.