Role of RpoS in Fine-Tuning the Synthesis of Vi Capsular Polysaccharide inSalmonella entericaSerotype Typhi

Abstract

Regulation of the synthesis of Vi polysaccharide, a major virulence determinant inSalmonella entericaserotype Typhi, is under the control of two regulatory systems,ompR-envZandrscB-rscC, which respond to changes in osmolarity. Some serotype Typhi strains exhibit overexpression of Vi polysaccharide, which masks clinical detection of lipopolysaccharide O antigen. This variation in Vi polysaccharide and O antigen display (VW variation) has been observed since the initial studies of serotype Typhi. In this study, we report thatrpoSplays a role in this increased expression in Vi polysaccharide. We constructed a variety of isogenic serotype Typhi mutants that differed in their expression levels of RpoS and examined the role of therpoSproduct in synthesis of Vi polysaccharide under different osmolarity conditions. Vi polysaccharide synthesis was also examined in serotype Typhi mutants in which the native promoter of therpoSwas replaced by anaraCP_BADcassette, so that the expression ofrpoSwas arabinose dependent. The RpoS⁻strains showed increased syntheses of Vi polysaccharide, which at low and medium osmolarities masked O antigen detection. In contrast, RpoS⁺strains showed lower syntheses of Vi polysaccharide, and an increased detection of O antigen was observed. During exponential growth, whenrpoSis unstable or present at low levels, serotype Typhi RpoS⁺strains overexpress the Vi polysaccharide at levels comparable to those for RpoS⁻strains. Our results show that RpoS is another regulator of Vi polysaccharide synthesis and contributes to VW variation in serotype Typhi, which has implications for the development of recombinant attenuatedSalmonellavaccines in humans.