Antimicrobial Agent Susceptibility and Typing of Staphylococcal Isolates from Subclinical Mastitis in Ewes

Objective was to study susceptibility to antimicrobial agents of 142 staphylococcal isolates from subclinical mastitis in ewes. In total, 41.5% of these were resistant and 5.6% multidrug resistant. More coagulase-negative staphylococci (47.0%) were resistant than Staphylococcus aureus (18.5%) isolates. Resistance was greater to penicillin (22.5%), tetracycline, or ampicillin (18.3%). More biofilm-forming (20.6%) isolates were resistant to tetracycline than nonbiofilm-forming (0.0%) ones. Presence of tetK was associated with presence of icaA in the same strains. Further, 76.6% of resistant isolates versus 57.7% of susceptible ones were recovered immediately postpartum and 23.4% of resistant isolates versus 9.9% of susceptible ones were recovered in farms that practiced routine administration of antimicrobial agents at the end of a lactation period. Most S. aureus (59.3%) were classified in ST133 and most Staphylococcus epidermidis were classified in ST100, ST142, or ST152 (19.0% each). There was no association of sequence types with resistance. Whole genome sequencing showed that, in a Staphylococcus lentus strain, the ermB gene was part of transposon Tn917 integrated into the chromosome; also, a small plasmid was observed in an ermC-carrying Staphylococcus hominis strain and, finally, in an S. aureus and an S. epidermidis strains, small tetK-carrying plasmids (pSau-2716Lar, pSau-3893Lar) of 4.439 kb were found.