Pathogenicity of piliated and encapsulatedBacteroides fragilis

Abstract
Of 20 isolates of theBacteroides fragilis group, only three were encapsulated (C+) and these induced abscesses in mice. After coinoculation with live, formalized or capsular material from other bacteria seven other isolates formed abscesses. TheBacteroides recovered from these abscesses were C+; those that had been inoculated withKlebsiella had pili-like and bacteriophage-like structures. Once encapsulated, theBacteroides isolates induced abscesses when injected alone.