Characterization of a New Variant of IS 257 That Has Displaced the Capsule Genes within Bovine Isolates of Staphylococcus aureus

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Abstract
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 IS 257 , now designated IS cap . IS cap showed 93% amino acid identity to S. aureus ORF49 but only 85% identity to IS 431 from S. aureus N315 and 88% identity to an IS 257 -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 IS 257 , including IS cap. 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 IS cap -mediated deletion of the cap5 ( 8 ) gene cluster. IS cap was not found among 20 human NT S. aureus isolates bearing the cap5HIJK genes, which suggests that IS cap -mediated deletion of the capsule locus is restricted to bovine strains of S. aureus . We were unable to identify a precursor strain in which IS cap 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.