STRAIN DIFFERENCES IN EXPRESSION OF VIRULENCE BY THE 90 KILOBASE PAIR VIRULENCE PLASMID OF SALMONELLA SEROVAR TYPHIMURIUM

A number of plasmidless strains were obtained by curing the 90 kilobase pair (kb) virulence plasmid from six strains, C5, TML, W118, SR11, LT2 and Fisher, of Salmonella serovar Typhimurium. A number of transposon (Tn5) tagged 90 kb plasmids, also derived from these Typhimurium strains, were then transferred back into these plasmidless strains. Plasmid-cured strains, reconstituted strains, and the parental strains were tested for their virulence in BALB/c mice. There were two groups of Typhimurium strains: one required the 90 kb plasmid to express high virulence (LD50 < 50 bacteria), and the other, regardless of the presence or absence of the 90 kb plasmid, maintained the same level of virulence at LD50 = 10 to 7 × 105 bacteria. Among the plasmidless strains, there were strains with a virulence level as low as LD50 = 107 bacteria, which was unaffected by the presence of the 90 kb plasmid. © 1991.