Salmonella enteritidis has a homologue of tolC that is required for virulence in BALB/c mice

The ability of Salmonella to invade tissue culture cells is correlated with virulence. Therefore, the tissue culture invasion model has been used extensively to study this process and to identify the bacterial genes involved and their products. Described here is the further characterization of a Salmonella enteritidis mutant (SM6T) originally identified as non-invasive for tissue culture cells. A chromosomal DNA fragment complementing this defect was cloned and sequenced. The derived protein sequence is 89% identical to TolC from Escherichia coli , an outer membrane protein required for the signal peptide-independent transport of α-haemolysin and colicin V. Therefore, sinA was renamed tolC and is referred to in this text as tolC ^s to distinguish it from tolC of E. coli TolC^s and TolC are functionally similar since tolC can complement the invasion-defective phenotype of a tolC^s mutant, and tolC^s is required for export of α-haemolysin by Salmonella . The tolC ^s mutant is avirulent for mice when administered by the oral route, suggesting that the gene is important for virulence. Further characterization of the tolC^s mutant indicated that like tolC mutants it is more sensitive than the wild-type strain to various detergents, antibiotics and dyes. This mutant is more sensitive to Triton X-100 only when associated with the monolayer, and the invasion-defective phenotype appears to be an artifact of this sensitivity. In addition, the tolC^s mutant is more sensitive to the bactericidal activity of human serum. Therefore, the avirulent phenotype could be the result of an inability to secrete a necessary virulence factor, or an increased sensitivity to complement and detergents as a result of a subtle alteration in the lipopolysaccharide (LPS) associated with tolC mutations.