Genetic basis of R-phase H:z27 antigen expression and phylogenetic relationships of Salmonella enterica serovar Senftenberg

Salmonella enterica serovar Senftenberg (S. Senftenberg) is one of the most common serovars isolated from various animals and humans. According to the White–Kauffmann–Le Minor scheme, its antigenic formula is defined as 1,3,19:g,[s],t:–, whereas 1,3,19:z27:– was formerly designated as serovar Simsbury but later excluded due to interconvertibility with Senftenberg. The H:z27 antigen is now recognized as an R-phase H antigen of S. Senftenberg. To clarify the genetic basis of H:z27 expression, we determined whole-genome sequences of H:z27-expressing isolates. These isolates belonged to sequence type (ST) 185 and harbored a horizontally acquired chromosomal locus containing fljB and fljA homologues, predicted to encode a flagellin and a translational repressor of fliC mRNA, respectively. Deletion of this locus resulted in expression of the H:g,s,t antigen instead of H:z27, as confirmed by serotyping and Western blot analysis. Bayesian temporal phylogenetic analysis suggested that the ST185 lineage acquired this locus around 1871 (95% highest posterior density: 1785–1934).