Antimicrobial and Methicillin Resistance Pattern of Potential Mastitis-Inducing Staphylococcus aureus and Coagulase-Negative Staphylococci Isolates from the Mammary Secretion of Dairy Goats

SIMPLE SUMMARY: Staphylococcus spp. constitutes an important pathogenic microbe in goat farming due to its association with udder infection that results in decreased milk production and/or quality. This pathogen can be resistant to antibiotics as well as infect humans, thus, posing a public health concern. This study investigated the percentage and composition of staphylococci that are present in goat milk and evaluated the resistance pattern by testing different antibiotics. A further focus was given on the presence of resistance to a critically important antibiotic, namely methicillin. Staphylococcus spp. isolates were recovered from 45.9% of the milk samples which were differentiated into two groups, coagulase-negative staphylococci (CoNS, 72.3%) and Staphylococcus aureus (27.7%). Both groups were most commonly resistant to penicillins. CoNS exhibited resistance to a broader range of antibiotics than S. aureus with resistant CoNS being in higher number as well. Of the nine S. aureus and CoNS suspected of methicillin resistance, eight carried the mecA gene which is linked to this type of antibiotic resistance. The observed presence of resistance to antibiotics, especially methicillin, in staphylococci isolated from goat milk poses a concern for animal and human health. ABSTRACT: Staphylococcus spp. is an important mastitis-inducing zoonotic pathogen in goats and is associated with antimicrobial resistance (AMR). The objectives of this study were to determine the prevalence and composition of staphylococci in individual mammary secretion (MS) samples of clinically healthy goats and to evaluate the phenotypic AMR pattern and the presence of methicillin resistance in the Staphylococcus spp. strains. Staphylococcus spp. isolates (n = 101) from the MS samples (n = 220) were identified to species level using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The antimicrobial susceptibility testing included a disk diffusion assay and the determination of the minimum inhibitory concentrations (MIC) of resistant strains (n = 46). Presumptive methicillin-resistant strains (n = 9) were assessed for the presence of mecA, mecC and SCCmec/orfx genes. Staphylococcus spp. isolates were recovered from 45.9% of the MS samples, of which, 72.3% was identified as coagulase-negative staphylococci (CoNS), with the remaining being Staphylococcus aureus. CoNS and S. aureus were most commonly resistant to ampicillin (56.2% and 57.1%, respectively), penicillin (26.0% and 39.3%, respectively), amoxicillin (26 % and 25 %, respectively) and cephalexin (12.3% and 25%, respectively) in the disk diffusion method. CoNS exhibited a broader AMR pattern and a higher percentage of resistant strains than S. aureus in the disk diffusion and MIC methods. Of the nine oxacillin- and cefoxitin-resistant strains, three S. aureus and five CoNS strains carried the mecA gene and, thus, were identified as methicillin-resistant. The mecC gene was not found in any of the studied strains. The presence of AMR and methicillin resistance in caprine S. aureus and CoNS poses a concern for animal and public health.