Growth of Yersinia Pseudotuberculosis in Mouse Spleen Despite Loss of a Virulence Plasmid of Mol. WT 47 x 106

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Abstract
A highly virulent strain of Y. pseudotuberculosis (LD50 .apprx. 102 bacteria/mouse) harbored 2 plasmids with MW of 47 .times. 106 and 61 .times. 106. The role of these plasmids in virulence was studied in mice with derived strains cured of plasmids. The plasmid of MW 47 .times. 106 played a major function in virulence. This was shown both by the increase of the LD50 and the lower rate of multiplication in the spleen obtained with strains cured of this plasmid. The plasmids of MW 61 .times. 106 did not play any role in virulence. The strain cured of the plasmid of MW 47 .times. 106 and the plasmid-free strain were able to multiply in the spleens of infected mice during a 7-day period. Thus, virulence factors not associated with plasmids are also responsible for the bacterial growth in tissues in vivo.