Characterization of Staphylococcus aureus from Bovine Mastitis: Genetic Markers and Antibiotic Resistance

Staphylococcus aureus is a significant contributor to subclinical bovine mastitis, causing economic losses in the dairy sector. This study aimed to isolate and characterize S. aureus from 51 subclinical bovine milk samples. Bacterial colonies were cultured on Mannitol Salt Agar (MSA) using ten-fold serial dilutions, and isolates were identified based on catalase and coagulase tests. Antibiotic resistance screening was conducted on MSA supplemented with 16 µg/mL penicillin G. Molecular characterization employed PCR to detect the nuc, coa, clfa, mecA, blaZ, and se genes. Antibiotic susceptibility testing followed the disc diffusion method, and penicillin G minimum inhibitory concentrations (MICs) were determined using broth microdilution. Sequence and phylogenetic analyses of the blaZ gene were performed to investigate genetic variations and their association with resistance. Results showed that 14 isolates harbored resistance genes from milk samples with high somatic cell counts, though the se gene was absent. All isolates demonstrated 100 % susceptibility to gentamicin, streptomycin, rifampin, ampicillin, and chloramphenicol, while exhibiting complete resistance to penicillin, cefotetan, and nalidixic acid. The minimum inhibitory concentration (MIC) of penicillin G varied widely, ranging from 16 to 512 µg/mL. Phylogenetic analysis identified three distinct groups; however, no significant correlation was found between blaZ variations and resistance levels. These findings emphasizing the importance of robust surveillance and responsible antibiotic use to control bovine mastitis in Thailand. HIGHLIGHTS Only 293 colonies (6.87 %) were identified as S. aureus, with 14 penicillin-resistant S. aureus were able to grow on MSA supplemented with 16 µg/mL penicillin G, highlighting regional differences and antibiotic resistance concerns. Five distinct patterns were identified with complete resistance to penicillin, cefotetan, and nalidixic acid, emphasizing the challenge of treating bovine mastitis caused by these isolates. All the isolates possessed the nuc and coa genes, with subsets carrying the clfa and mecA genes, suggesting varying pathogenicity and methicillin resistance. Minimum inhibitory concentration (MIC) for penicillin ranged from 16 to 512 µg/mL, with widespread presence of the blaZ gene supporting β-lactamase-mediated resistance. Phylogenetic analysis revealed 3 distinct clusters of blaZ gene sequences, offering new insights into genetic variability and its limited correlation with phenotypic resistance levels. GRAPHICAL ABSTRACT