Characterization of a new variant of IS257 that has displaced the capsule genes within bovine isolates of Staphylococcus aureus.

Many bovine Staphylococcus aureus isolates from Argentina are nontypeable (NT), i.e., they do not produce serotype 5 or 8 capsular polysaccharides (CPs). Some of these NT strains have a deletion of the cap5(8) gene cluster mediated by a variant of IS257, now designated IScap. IScap showed 93% amino acid identity to S. aureus ORF49 but only 85% identity to IS431 from S. aureus N315 and 88% identity to an IS257-like element from bovine strain RF122. Thirty-six (53%) of 68 bovine isolates, drawn from a previously described S. aureus strain collection, carried some variant of IS257, including IScap. Of these 36 IS+ isolates, 6 were CP5+, 1 was CP8+, and 29 were NT. Forty-four of the 68 isolates were NT, and 24 of these 44 NT isolates (55%) exhibited IScap-mediated deletion of the cap5(8) gene cluster. IScap was not found among 20 human NT S. aureus isolates bearing the cap5HIJK genes, which suggests that IScap-mediated deletion of the capsule locus is restricted to bovine strains of S. aureus. We were unable to identify a precursor strain in which IScap flanked the cap5(8) capsule locus, nor were we able to select for deletion of the cap5(8) locus in vitro. Our results support the hypothesis that deletion of the cap5 locus occurred in the distant past and that the relative abundance of these NT strains may be a result of their ability to persist in subclinical mastitis infection in cows.