Coagulase negative staphylococci from ovine milk: Genotypic and phenotypic characterization of susceptibility to antibiotics, disinfectants and biofilm production

Coagulase negative staphylococci (CoNS) represent the main cause of subclinical mastitis in ruminants. Resistance to antimicrobials, as well as biofilm production play a key role in environmental persistence. In this work, CoNS isolates from 120 samples of ovine bulk-tank milk collected in Tuscany and Latium (Italy) were identified via PCR-RFLP and characterized for their susceptibility to antibiotics, disinfectants and ability to form biofilm. Out of 73 isolates, S. epidermidis was the prevalent species. Thirty-nine (53.4 %) isolates were resistant to at least one of the tested antibiotics. The highest resistance rates were recorded against ampicillin (39.7 %), and tetracycline (10.9 %). As for genetic profiles, 44 (60.3 %) isolates were positive for at least one of the targeted resistance genes. The most detected was blaZ (46.6 %), while 6 isolates (8.2 %) showed the simultaneous presence of blaZ and tet genes. Despite the common presence of at least one of the targeted biofilm production determinants (83.5 % isolates), none of them were positive when grown on Congo Red Agar. Only 9 isolates (12.3 %) were positive for disinfectant resistance genes (qac), among which smr-qacC’ was the prevalent (9.6 %). Twenty isolates were selected to determine MIC and MBC against benzalkonium chloride and chlorhexidine digluconate. For both disinfectants, 12/20 (60 %) isolates showed reduced susceptibility. This study highlights the role of CoNS as potential carriers of a wide range of environmental persistence factors. These include not only antibiotic resistance and biofilm production genes, but in some cases also biocide resistance genes, which could confer unspecific additional resistance mechanisms against a broad range of antimicrobials.