Phenotypic and genotypic antimicrobial resistance of staphylococci from bovine milk

The aim of this study was to examine phenotypic and genotypic antimicrobial resistance of staphylococci from milk samples from cows with subclinical and clinical mastitis and from cows without mastitis symptoms to methicillin, tetracyclines, macrolides and lincosamides (ML). Of 207 strains, including 34 S. aureus and 173 coagulase-negative staphylococci (CNS), 11 (6.4%) CNS strains were phenotypically resistant to methicillin. The mecA gene was detected by PCR only in two S. xylosus strains and one strain of S. epidermidis and S. simulans. No methicillin-resistant S. aureus strains were observed. In methicillin-resistant strains with mecA, gene resistance to other investigated antibiotics was not observed. Phenotypic resistance to tetracycline was detected in 11.0% of CNS strains and 47.4% of them carried the tetK gene. Of 173 CNS strains studied, 27 (15.6%) were resistant to at least one ML antibiotic. The resistance gene ermC was detected in 55.5% of the 27 ML-resistant strains. The ermA and ermB genes were detected in 14.8% and 11.1% of ML-resistant CNS strains, respectively. Antimicrobial resistance to methicillin, tetracyclines and macrolides was detected more frequently in staphylococcal strains from clinical mastitis compared to animals with subclinical symptoms and without mastitis, while the resistance to lincosamides showed a similar frequency in all groups of cows. In conclusion, CNS species from bovine milk differ in phenotypic and genotypic antimicrobial resistance profiles, and the use of PCR technique alone for the detection of methicillin, macrolide, lincosamide and tetyracycline resistance in CNS from cattle is not reliable.