Biofilm production, molecular attributes and antibiotic resistance of Staphylococcus aureus isolated from bovine milk

The formation of biofilm and resistance to certain antibiotics by staphylococci are acknowledged as a significant virulence factors. We focused on monitoring phenotype antibiotic resistance and biofilm production, including some genes encoding for cell wall-anchored surface proteins of Staphylococcus aureus (S. aureus) from bovine milk with subclinical mastitis. A total of 230 lactating cows were selected for the detection of mastitis. Of the tested dairy cows, 82 (35.7%) were positive on the California Mastitis Test (CMT). From a total of 105 CMT-positive quarter milk samples, 87 cases were identified as bacterial pathogens of the mammary gland. The most common udder pathogens among all infected quarters were coagulase-negative staphylococci (50.6%) and S. aureus (20.7%). Of all isolates of S. aureus (18), 10 isolates showed resistance to ampicillin, followed by resistance to erythromycin (10/18), to piperacillin + tazobactam, clindamycin (9/18), to oxacillin, ampicillin + sulbactam and to cefoxitin (8/18). One strain was resistant to chloramphenicol. Strong biofilm production was detected in 5 S. aureus; half of the strains were weak producers of biofilm, and 4 isolates had moderate formation capacity. The genes isdA, isdB and agrI were detected in all examined isolates from cows with subclinical mastitis. In our study, fnbpA was present in 17 isolates. The majority of S. aureus exhibited antibiotic resistance, biofilm production and the presence of virulence genes, hence constituting a potential public health threat.