The Characterization of Bacteroides fragilis Bacteriophage Recovered from Animal Sera: observations on the Nature of Bacteroides Phage Carrier Cultures

SUMMARY A group of bacteriophages active on Bacteroides fragilis have been isolated from a number of animal sera including calf, foetal calf, lamb, chicken and rabbit. The phages are specific for Bacteroides fragilis; no lytic activity was detected on 36 strains of Bacteroides oralis, 6 strains of Bacteroides melaninogenicus and seven strains of the genus Fusobacterium. Some characteristics of these bacteriophages such as adsorption constants and single-step growth experiments as well as electron micrographs are presented. The B. fragilis host-phage system readily develops a stable phage-bacterium association, i.e. phage carrier cultures. Broth cultures infected with phage at high multiplicities, demonstrated active bacterial growth coincident with phage production. Cultivation of phage carrier strain 12 in medium containing phage antisera eliminated phage carriage. The phage-cured cultures yielded a mixture of phage-sensitive and phage-resistant clones and evidence was obtained of a rapid rate of alternation between these two phenotypes. The simultaneous production of both phage and bacteria in B. fragilis carrier cultures may be ascribed to the continuous development of phage-sensitive organisms from a phage-resistant population.