Reduced vancomycin susceptibility in Staphylococcus aureus isolated from sheep with sub-clinical mastitis

Reduced antimicrobial susceptibility in Staphylococcus aureus is a serious threat both in human and veterinary medicine. Animals have been recently suggested as methicillin-resistant S. aureus reservoir in the community. Vancomycin-resistant S. aureus are emerging pathogens that have the potential to become more prevalent in humans. The contact with animals is a risk factor for S. aureus infection in humans, but also the consumption of raw milk-based products must be considered. The aim of this work was to evaluate the susceptibility of S. aureus strains isolated from sheep to antimicrobial drugs used in humans or in veterinary medicine. Eighty-six S. aureus, were isolated from milk of sheep with sub-clinical mastitis, identified by PCR and analyzed by Kirby-Bauer disk diffusion method for clindamycin (C), erythromycin (E), gentamycin (G), methicillin (M), oxytetracyclin (O), vancomycin (V) and quinupristin/dalfopristin (Q/D). On the basis of the disk diffusion test the resistant strains were 16 (18.6%) for C, 14 (16.3%) for E, 6 (6.9%) for G, 2 (2.3%) for M, 10 (11.6%) for O, 33 (38.4%) for V, and 4 (4.6%) for Q/D. E, G, O and C are usually used in farm animals and the S. aureus phenotypic resistance to these antibiotics is not surprising. The most unexpected result was the high percentage (38.4%) of resistance to V, a drug used in humans but not in sheep. Furthermore, 4 strains were resistant to Q/D, an antimicrobial never used in animals. The molecular basis of the reduced antimicrobial susceptibility observed has to be investigated in order to verify whether sheep is a reservoir for genetically vancomycin resistant S. aureus. The use of antimicrobials for veterinary-use may select genetic factors responsible for multi-resistance and thus also for resistance to drugs used only in humans. An adequate use of antibiotics in animals is essential to prevent the selection and spread of genetically multi-resistant strains both in animals and humans.