Multidrug resistant and Methicillin Resistant Staphylococcus aureus (MRSA) isolated from raw sheep’s milk

Multidrug resistant (MDR) and Methicillin Resistant Staphylococcus aureus (MRSA) contamination in milk and dairy products can origin from animals, farm environment, from human in contact with animals or food handlers. Therefore, milk and dairy products could represent a potential source of antibiotic resistant S. aureus strains that could reach human through the food chain. Most of the reports on the occurrence and characterization of MDR S. aureus and MRSA refer to dairy cows, while little information is cu ently available on st ains isolated f om s eep’s milk. T e aim of t e p esent study was to evaluate the presence of MDR S. aureus and MRSA harboring mecA and mecC genes in aw s eep’s milk. enotypic esistance to antibiotics and t e p esence of t e genetic determinants were also investigated. Bulk tank milk samples and milking machines filters were collected from 17 Sardinian dairy sheep farms. In addition, 3 filters from one cheese-making plant collecting milk from the investigated farms were sampled. The detection of Coagulase Positive Staphylococci was performed according to ISO 6888- 1:1999 and the potential presence of MRSA assessed using ChromID MRSA Smart agar plates. Isolates were submitted to PCR for species identification. Minimum Inhibitory Concentration (MIC) for Ampicillin (AM), Cephalothin (CEF), Cefoxitin (FOX), Erythromycin (E), Oxacillin (OX), Penicillin (P), Tetracycline (TE) and Vancomycin (VA) was determined using broth microdilution method (CLSI M07,M100, 2015). The detection of the genes mecA, mecC, blaZ, ermA-B-C, vanA, tetK-M-S-W encoding antibiotic resistance was performed as previously described (Spanu et al., 2014). In this study, 118 S. aureus strains were collected, 65 strains from 17 positive milk filters, 38 from 7 bulk tank milk samples and 15 from cheese-making plant filters. Twelve strains (10.2%) were resistant at least to one of the β-lactam antibiotics tested and 6 isolates showed multiple resistance against AM, FOX, OX and/or P. Among these, 3 strains were identified as MRSA with MIC values of 16-32 µg/mL for OX and 64 µg/mL for FOX. Interestingly, 2 out of 3 MRSA were also resistant to E (8 µg/mL), despite only 1 strain carried blaZ, mecA, mecC and ermB-C genes. All the isolates were susceptible to CEF and VA and did not carry the correspondent resistant genes. On the other hand, although resistance to TE was not found, 15 and 7 S. aureus strains carried tetM and tetK genes, respectively. The results of the present study suggest the emergence of MDR S. aureus also in small ruminants dairy chain which pose a potential public health hazard for the spreading of MRSA strains.