Phenotypic and genotypic antimicrobial resistance in Staphylococcus spp. isolated from raw milk of cows, goats, and ewes

Abstract

Background: Mastitis is a major disease in dairy production, causing economic losses and increasing concern due to antimicrobial-resistant bacteria. The presence of resistant Staphylococcus spp. in raw milk from clinically healthy animals may represent a potential public health risk and a challenge for control strategies under a One Health framework. Objective: To evaluate antimicrobial resistance patterns and resistance genes in Staphylococcus spp. isolated from raw milk of clinically healthy cows, goats, and ewes from three dairy farms included in this study. Methodology: Seventy-five isolates (25 per species) were analyzed using disk diffusion antibiograms and PCR detection of blaZ, mecA, ermB, and tetK genes. Phenotypic–genotypic agreement and interspecies differences were statistically evaluated. Results: High phenotypic resistance to β-lactams was observed, especially in cows (ampicillin 96%, penicillin 92%) and ewes (penicillin 60%). Oxacillin resistance was higher in goats and ewes (52%) than in cows (8%). Resistance to erythromycin and tetracycline was moderate to low. Gene detection showed high prevalence of mecA (84–100%) and blaZ (80% in cows and goats; 20% in ewes), with significant interspecies differences in blaZ and tetK (p < 0.001). Overall phenotype–genotype agreement was low, although blaZ showed the highest diagnostic value. Frequent codetection of resistance genes was observed among isolates, indicating the coexistence of multiple resistance determinants within the same bacterial populations. Implications: The low phenotype–genotype concordance highlights limitations of relying on a single diagnostic approach and suggests the need for integrated surveillance combining phenotypic and molecular tools. Conclusion: Antimicrobial-resistant Staphylococcus spp. are widely present in raw milk from clinically healthy ruminants, with significant interspecies differences. The present study revealed marked differences in antimicrobial resistance profiles and resistance gene distribution among Staphylococcus spp. isolated from cows, goats, and ewes. The low phenotype–genotype concordance observed for several antibiotic–gene pairs highlights the complexity of antimicrobial resistance and supports the complementary use of phenotypic and molecular approaches for surveillance and characterization of resistant isolates.