VIRULENCE OF PPST(-) STRAINS OF YERSINIA-PESTIS FOR GUINEA-PIGS() ANDPPST()

Guinea-pigs were infected subcutaneously or by respiratory challenge w ith plasmid-containing (pPst(+)pCad(+)pFra(+)) Yersinia pestis strain 358 and its pPst(-)pCad(+)pFra(+), pPst(+)pCad(+)pFra(-) and pPst(-)pC ad(+)pFra(-) derivatives, grown in vitro at 28 degrees C or at 37 degr ees C, Lack of plasmid pPst did not lead to an increase in LD50 with e ither route of challenge, When the virulence of the four Y. pestis str ains grown at the two temperatures was compared, the LD50 values of th ose grown at 37 degrees C were lower Respiratory challenge with cultur es grown at 37 degrees C mimics the man-to-man pneumonic plague cycle, The average LD50 values decreased c. two-fold and 10-fold for pPst(+) and pPst(-) Y. pestis variants, respectively. The data suggest that h istorical epidemic outbreaks of pneumonic plague in the human populati on residing in the Caucasus region where there are natural plague foci in common voles may have been caused by pPst(-) Y. pestis strains.