A novel PhoP-regulated locus encoding the cytolysin ClyA and the secreted invasin TaiA of Salmonella enterica serovar Typhi is involved in virulence

Abstract

Salmonella entericaserovar Typhi causes a human-restricted systemic infection called typhoid fever. We have identified a Typhi genomic region encoding two ORFs, STY1498 and STY1499, that are expressed during infection of human macrophages and organized in an operon. STY1498 corresponds toclyA, which encodes a pore-forming cytolysin, and STY1499 encodes a 27 kDa protein, without any attributed function, which we have named TaiA (Typhi-associated invasin A). In order to evaluate the roles of these genes in Typhi pathogenesis, isogenic Typhi strains harbouring a non-polar mutation of eitherclyAortaiAwere constructed. In macrophages,taiAwas involved in increasing phagocytosis, astaiAdeletion reduced bacterial uptake, whereasclyAreduced or controlled bacterial growth, asclyAdeletion enhanced Typhi survival within macrophages without affecting cytotoxicity. In epithelial cells, deletion oftaiAhad no effect on invasion, whereas deletion ofclyAenhanced the Typhi invasion rate, and reduced cytotoxicity. Overexpression oftaiAin Typhi or inEscherichia coliresulted in a higher invasion rate of epithelial cells. We have demonstrated that TaiA is secreted independently of both theSalmonellapathogenicity island (SPI)-1 and the SPI-2 type three secretion systems. We have shown that this operon is regulated by the virulence-associated regulator PhoP. Moreover, our results revealed that products of this operon might be involved in promoting the use of macrophages as a sheltered reservoir for Typhi and allowing long-term persistence inside the host.