Studies on the relation of cultural conditions and virulence of Pasteurella pestis

The influence of cultural conditions upon the loss of virulence in laboratory cultures of Pas-teurella pestis has been noted in the literature. Devignat and Schoetter (1942) reported that 2 highly virulent strains became avirulent after 30 to 36 days when serially transferred in aerated broth at 37 C. Pollitzer (1954) summarized additional conditions under which loss in viru-lence occurred. These included repeated sub-culture at weekly intervals, action of bacterio-phage, cultivation on bile media, prolonged incubation of the broth cultures at 32 C (Otten's procedure), growth in alcohol-broth at 32 C for long periods of time, and passage of virulent strains through immune animals. The latter 3 conditions were used by Jawetz and Meyer (1943) in producing avirulent strains for their studies on immunity. Data are presented in this paper which indicate that the virulence of P. pestis strains was main-tained when serially transferred in aerated broth at 26 C, or on agar slants at 26 or 37 C. However, a striking reduction in virulence was observed when P. pestis strains were cultured in aerated broth at 37 C from large inocula and serially transferred, or from small inocula without sub-sequent transfer. This attenuation of plague cultures was due to the selection of avirulent variants present in the inoculum or arising during growth. MATERIAL AND METHODS Three strains of virulent P. pestis originally isolated from human cases were employed: (1