J. MED. MICROBIOL.-VOL. 18 (1984). 371-375 0 1984 The Pathological Society of Great Britain and Ireland GROWTH OF YERSINIA PSEUDOTUBERCULOSIS IN MOUSE SPLEEN DESPITE LOSS OF A VIRULENCE PLASMID OF MOL. WT 47 x lo6 M. SIMONET, D. MAZIGH*, P. BERCHE Laboratoire de Microbiologie, Facultk de Midecine Necker-Enfants Malades, 75730 Paris and * Unite‘ d’Ecologie Bactkrienne, Centre National des Yersinia, Institut Pasteur, 75724 Paris Cedex 15, France SUMMARY. A highly virulent strain of Yersinia pseudotuberculosis (LD50 c. lo2 bacterialmouse) harboured two plasmids with mol. wt of 47 x 1 O6 and 6 1 x 1 06. The role of these plasmids in virulence was studied in mice with derived strains cured of plasmids. It was confirmed that the plasmid of mol. wt 47 x lo6 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 the plasmid of mol. wt 47 x lo6.The plasmid of mol. wt 61 x lo6 did not play any role in virulence. This work also demonstrates that the strain cured of the plasmid of mol. wt 47 x lo6 and the plasmid-free strain were able to multiply in the spleens of infected mice during a 7-day period. This suggests that virulence factors not associated with plasmids are also responsible for the bacterial growth in tissues in uiuo. INTRODUCTION Yersinia pseudotuberculosis causes epizootic outbreaks with various patterns of disease in birds and in domestic and wild mammals. It may also cause disease in man (Mollaret, 1965). Several authors have reported that plasmids with mol. wt between 42 x lo6 and 47 x lo6 are associated with the virulence of the various species of Yersinia, i.e., Y. enterocolitica, Y . pestis and Y. pseudotuberculosis (Gemski, Lazere and Casey, 1980a; Gemski et al., 1980b; Ben-Gurion and Shafferman, 1981; Portnoy and Falkow, 1981). It has been shown that the presence of these plasmids is related to the synthesis of V and W antigens which appear to be essential for the virulence of the genus Yersinia (Brubaker, 1972). Pathogenicity of Y . pseudotuberculosis has been studied by inducing keratoconjunctivitis in the guinea pig (Gemski et al., 1980b; Maki et al., 1983) and by assessing mortality in the mouse (Ben-Gurion and Shafferman, 1981; Bolin, Norlander and Wolf-Watz, 1982). The purpose of this work was to study the virulence of Y . pseudotuberculosis strains with or without a plasmid of mol. wt 47 x lo6 by following the kinetics of bacterial growth in the spleens and survival of infected mice. Received 12 Mar. 1984; accepted 30 Apr. 1984 371