The rpoS mutant allele of Salmonella typhi Ty2 is identical to that of the live typhoid vaccine Ty21a.

Salmonella requires its alternative sigma factor sigma S (RpoS) for virulence in mice. rpoS mutants can be frequently isolated from highly passaged Salmonella laboratory strains. In particular, the live typhoid oral vaccine Salmonella typhi Ty21a and its parental strain Ty2, a 'wild-type' strain widely used for vaccine development, are rpoS mutants. Here, we show that the nucleotide sequence of the rpoS mutant allele of Ty2 is identical to that of the rpoS mutant allele of Ty21a. This demonstrates that the rpoS mutation arose in Ty2 before the isolation of Ty21a in 1975, an observation that may have implications for vaccine research.