Methicillin-resistant staphylococci in clinical bovine mastitis: occurrence, molecular analysis, and biofilm production

Staphylococcus aureus is an important pathogen that causes mastitis in cattle, and the emergence of methicillin-resistant S. aureus (MRSA) poses a threat to veterinary and human medicine. The aims of the study were to investigate the prevalence of MRSA and methicillin-resistant coagulase-negative staphylococci (MR-CoNS) isolated from clinical mastitis, their ability to form biofilms, and the antimicrobial susceptibility of S. aureus strains. In addition, the Staphylococcal Cassette Chromosome mec (SCCmec) type, spa type and the presence of Panton-Valentine Leucocidin in MRSA were evaluated. A total of 326 staphylococcal strains were screened by multiplex-PCR for S. aureus and Staphylococcus intermedius group (SIG) identification. The S. aureus strains (n = 163) were subjected to phenotypic testing for antimicrobial susceptibility and biofilm formation. Molecular analysis was performed on MRSA mecA-positive strains. Of 163 S. aureus isolates, 142 strains (87.1%) were resistant to at least one antibiotic, and all 19 MRSA strains were resistant to at least four out of five antibiotics tested. All S. aureus strains harboured the icaA gene and were biofilm producers. Nineteen MR-CoNS strains were also isolated. The most prevalent spa types among MRSA were t001 (57.9%) and t037 (31.6%), while one MRSA was type t008 and one was type t041. Most MRSA were SCCmec type I (63.2%) and III (31.6%) and only one strain was type IV. None of the MRSA isolates had the PVL gene. The prevalence of multidrug-resistant S. aureus in bovine mastitis is a serious concern. The finding of MRSA with spa types predominant in humans and infrequent in Italian cows and with SCCmec infrequently found in bovine milk or cheese suggest a human origin of these strains. The ability of MRSA and MR-CoNS involved in bovine mastitis to be transferred to humans and vice versa poses a public health concern.