Antimicrobial susceptibility of Staphylococcus aureus causing bovine mastitis in Argentine dairy herds

We assessed the in vitro activity of selected antimicrobial agents against 95 Staphylococcus aureus strains causing both clinical and subclinical bovine mastitis belonging to 61 dairy farms from the Central dairy area of Argentina. Minimal inhibitory concentrations (MICs) of penicillin, oxacillin, gentamicin, erythromycin, enrofloxacin and florfenicol were estimated. In addition, the agar diffusion test was performed. MIC50 and MIC90 were as follows: penicillin, 0.05 and 4 microg/ml; oxacillin, 0.25 and 0.25 microg/ml; gentamicin, 0.25 and 0.5 microg/ml; erythromycin 0.125 and 0.25 microg/ml; enrofloxacin 0.25 and 0.5 microg/ml, and florfenicol 4 and 8 microg/ml. Beta-lactamase activity was detected in 89% of 46 penicillin-resistant strains. Apart from penicillin, antimicrobial resistance in S. aureus causing bovine mastitis remains rare in Argentine dairy farms.