A highly virulent strain of (LD50 10 bacteria/mouse) harboured two plasmids with mol. wt of 47 x 10 and 61 x 10. 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 10 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 10. The plasmid of mol. wt 61 x 10 did not play any role in virulence. This work also demonstrates that the strain cured of the plasmid of mol. wt 47 x 10 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 .


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