Factors influencing the loss of virulence in Pasteurella pestis.

It was found in a previous investigation that various virulent strains of Pasteurella pestis showed a reduction in virulence in aerated broth at 37 C whenever several generations of growth occurred either from small inocula (104 cells/ml) or after repeated passage using larger inocula (Fukui, et al., 1957b). This attenuation seemed to be caused by growth conditions favoring the selection of avirulent mutants present in the inoculum or arising during growth. This report presents additional evidence that the loss of virulence in aerated broth cultures is due to selective conditions favoring the growth of avirulent mutants and, further, that this selection can be prevented either by addition of certain bacterial culture filtrates, or simply by adjustment of the pH to 7.8 to 8. The selection of avirulent mutants can also be reduced or prevented if the broth culture is incubated under reduced oxygen tension or in a cellophane sac placed in the peritoneal cavity of a guinea pig.