Biological properties of Rickettsia prowazekii on long-term persistence in infected cotton rats.

The biological properties of Rickettsia prowazekii were found to change in the course of long-term persistence in infected cotton rats. The persistent rickettsial populations proved to be heterogeneous with respect to several biological characters. Along with populations possessing typical biological properties, lines of R. prowazekii with lowered antigenicity and virulence and lost ability to be passed in animals were obtained. The latter property was related with rickettsial antigen inducing the formation of complement-fixing antibody. No such relationship was found with antigen inducing the formation of antihaemolysins.