In vivo growth and distribution of anthrax bacilli in resistant, susceptible, and immunized hosts.

Differences in in vivo growth rates of Bacillus anthracis in the bodies of infected rats and guinea pigs showed the effect of innate species resistance. The effect of two levels of immunity imposed on these hosts demonstrated that, as the degree of immunity increased, the growth rate of organisms decreased and was accompanied by a decrease in terminal population. It was shown that growth rates in the blood best reflected the population growth in the entire host. Data obtained from the naturally resistant rat that had been immunized with both the Belton-Strange protective antigen and live vaccine demonstrated the role of "toxins" in terminal anthrax. The in vivo morphological changes in anthrax bacilli pointed out errors regarding estimates of true terminal population of organisms when samples for counting were taken postmortem rather than at the time of death.