Hypermutable Staphylococcus aureus strains present at high frequency in subclinical bovine mastitis isolates are associated with the development of antibiotic resistance.

Hypermutable bacterial strains with greatly elevated spontaneous mutation rates have been described at high frequencies in various clinically important species, particularly in cystic fibrosis (CF) patients. It has been suggested that such strains can play a major role in the development of chronic respiratory infections. Nevertheless, little information is available regarding the potential association between hypermutation and other chronic infection settings. Here, we investigated the mutation frequencies of 261 Staphylococcus aureus isolates from bovine mastitis cases. The comparative analysis revealed that the subclinical mastitis (SM) isolates harbored significantly more hypermutators than the clinical mastitis (CM) isolates (26/141 versus 0/120, P<0.001, Fisher's exact test). Analysis of mutS and mutL genes, which are major components of the methyl mismatch repair (MMR) system, revealed that 13 of the 14 genetically unrelated hypermutators showed alterations in their deduced MutS and/or MutL amino acid sequences. The hypermutators were much more frequently found to be resistant to all of the 7 antibiotics tested (except sulfafurazole) than the nonmutators. Moreover, the proportion of hypermutators harboring multi-drug resistance was significantly higher than that of the nonmutators as well (P<0.001). Taken together, this work provides evidence that hypermutability plays an important role in antibiotic resistance development during long-term persistence of S. aureus, and reveals that the link between hypermutation and chronic infections appears not to be restricted to respiratory infections alone.