Molecular characterization of vancomycin-resistant Staphylococcus aureus isolated from bovine milk

Vancomycin-resistant Staphylococcus aureus (VRSA) is a zoonotic life-threatening pathogen. Vancomycin exhibits anti-bacterial activity by inhibiting peptidoglycan synthesis by binding to the D-ala-D-ala terminus of the peptidoglycan. But in VRSA, D-ala-D-ala is replaced by D-ala-D-lactate due to the presence of vanA, vanB or vanD genes. This study was intended to identify the molecular prevalence of VRSA in 768 bovine milk samples, risk factor association, antibiogram profile and bioinformatics analysis of VRSA by targeting vanB gene. Out of a total of 248 S. aureus isolates from mastitic milk samples, the phenotypic and genotypic prevalence of VRSA was estimated to be 17.74% and 10.89%, respectively. Farm-level risk factors including use of improper milking technique, lack of milker's care during milking, unhygienic conditions during milking and no dry cow therapy were found to be significantly associated (p < 0.05). Anti-microbial susceptibility testing of VRSA isolates exhibited the highest resistance to oxytetracycline, followed by oxacillin and Trimethoprim+sulfamethoxazole. The current study sequences showed more resemblance with reported sequences from Iraq (MN747834) and Egypt (MK095504, MK087830), which belong to vanB gene from S. aureus as compared to sequences from other countries, which belong to vanB gene from the genus Enterococcus. The Genetic Algorithm for Recombination Detection (GARD) found 234 potential breakpoints, translating into a search room of 123,883,305 models with up to 4 breakpoints. The phylogenetic motif profiling method discovered evolutionarily conserved residues across target genes' homologous protein sequences. These residues were discovered to be conserved in drug-resistant target proteins over the evolutionary process and may play a key role in their function. The current study revealed a molecular prevalence of VRSA in dairy animals, along with molecular analysis of vancomycin resistance in S. aureus by targeting the vanB gene using standard bioinformatics tools. The occurrence of VRSA in animals requires serious attention because this pathogen has zoonotic potential, so it can become a greater risk to consumer health.