Evidence of correlation between 50-kilobase plasmid of Salmonella choleraesuis and its virulence.

A plasmid of 50 kilobases (kb) was found in all six strains of Salmonella choleraesuis studied, originally isolated from septicemic swine or man. A derivative cured of the 50 kb plasmid was isolated after treatment with novobiocin, and its virulence compared to that of the parent by intraperitoneal injection into mice. The lethality of the parent strain was found higher than that of the cured strain. With a sublethal number of cells of the parent strain injected, the spleen, liver and lymph nodes in various sites were markedly enlarged, and the bacteria were isolated from the heart blood. In contrast to this, neither enlargement of the organs nor recovery of organisms was observed with the cured strain. The Tn1-tagged 50 kb plasmid was next introduced by transformation into the cured strain. The virulence of the plasmid-reintroduced strain was higher than that of the cured strain, and in fact identical to that of the parent. We conclude that the 50 kb plasmid of S. choleraesuis is closely related with the mouse virulence of the organism.