FACTORS INFLUENCING THE LOSS OF VIRULENCE IN PASTEURELLA PESTIS

Abstract

Additional evidence is presented which shows that reduction of virulence of P. pestis grown in aerated heart infusion broth incubated at 370C is a result of selective conditions favoring the growth of avirulent cells. These selective conditions can be reduced or eliminated by the addition of filtrates from avirulent broth cultures and three other genera of bacteria to the heart infusion medium. No loss of virulence in broth cultures occurred when the initial pH of the medium was adjusted to 7.8-8. Selection of avirulent mutants did not occur in cultures grown in a cellophane sac in the peritoneal cavity of a guinea pig. Preliminary evidence suggests that reductive conditions may play a role in the maintenance of virulence of P. pestis cuItures.