MICROBIC VIRULENCE AND HOST SUSCEPTIBILITY IN MOUSE TYPHOID INFECTION.

Mice bred at The Rockefeller Institute vary in their susceptibility to mouse typhoid infection caused by a certain strain of Bacillus pestis caviae. This graded variation may be roughly analyzed as follows: in any series infected per os with a fixed dose, 20 to 30 per cent show no sign of infection, no positive blood cultures, and no agglutinins; 5 or 10 per cent present symptoms of disease, positive blood cultures, and then recover with or without homologous agglutinins; 70 or 80 per cent develop positive blood cultures and succumb in a more or less constant ratio relative to time. The strain of Bacillus pestis caviae employed throughout a 10 month series of experiments has shown no permanent change in virulence. Blood cultures taken from infected mice early in disease, shortly after death, and 6 days after death, chronic stool carrier cultures, and chronic septicemia cultures, all show approximately the same degree of virulence.