Search

Increased sensitivity to killing by human serum complement occurs in Salmonella typhimurium strains in which the 95-kilobase virulence plasmid is integrated into the chromosome. This phenotypic change appears to be due to alterations in plasmid gene expression and is reversed by the presence of an autonomous plasmid bearing a cloned region of the virulence plasmid. Accordingly, this region has been termed rsk for reduced serum killing. Sequence analysis of the region reveals that rsk is composed of a series of direct 10-base-pair (bp) repeats with a 21-nucleotide periodicity. Two adjacent repeats are identical, but increasing loss of conservation is apparent with increased distance both 5' and 3' of these highly conserved 10-mers. The smallest isolated sequence which restores the serum-resistant phenotype is only 66 bp long and contains the two identical 10-mers and one degenerate 10-mer (8 of 10 bp conserved) 3' of these. The minimal rsk region of 66 bp does not appear to contain a coding sequence, or a promoter, for a structural gene. It is proposed that the minimal rsk is an isolated regulatory site involved in the regulation of the serum resistance of S. typhimurium. Integration of the 95-kilobase plasmid disrupts the normal regulation of virulence plasmid genes, resulting in an increase in the killing of the bacteria by complement activated by the classical pathway. The introduction of the minimal rsk on a multiple-copy plasmid restores resistance to serum killing, possibly through the titration of a trans-acting regulatory factor.

The association of the virulence plasmid of Salmonella typhimurium with resistance to the bactericidal activity of human serum was studied in chromosomally isogenic pairs of strains differing in their virulence plasmid status. The presence of the plasmid correlated in three pairs of strains with resistance to serum. The absence of the plasmid correlated with increased sensitivity to serum, whereas the reintroduction of the plasmid to the cell resulted in the restoration of resistance to serum. Complement was activated by the classical and alternative pathways equally well by both strains of a pair, but the differential bactericidal action of serum was apparent only after the activation of complement by the classical pathway. No differences in the chemical compositions or in the molecular weight ranges of the lipopolysaccharides were apparent between paired strains. This work confirms the presence of a virulence plasmid-associated mechanism of resistance to serum and distinguishes it from lipopolysaccharide-mediated resistance.

Integration of the Salmonella typhimurium virulence plasmid into the chromosome reduces mouse virulence, serum resistance, and HeLa cell adhesion-invasion while prolonging lag time in minimal medium. The proposed virulence plasmid regulatory element, rsk, partially restores virulence and fully restores the other three phenotypes to wild-type levels. Plasmid curing reduces virulence without affecting the other phenotypes. rsk has no apparent effect on the cured strain.