Molecular characterization of equine Staphylococcus aureus isolates with a reduced oxacillin susceptibility

Detection of borderline oxacillin-resistant Staphylococcus aureus (BORSA) represents a challenge to both, veterinary and human laboratories. Between 2015 and 2017, 19 equine S. aureus with elevated minimal inhibitory concentrations for oxacillin were detected in routine diagnostics. The aim of this study was to characterize these isolates to reveal factors possibly associated with the BORSA phenotypes. All S. aureus were subjected to antimicrobial susceptibility testing and whole genome sequencing (WGS). A quantifiable beta-lactamase activity assay was performed for a representative subset of 13 isolates. WGS data analysis of the 19 BORSA isolates revealed two different genomic lineages [sequence type (ST)1 and ST1660)] being associated with the particular phenotypes. The cgMLST revealed a close relatedness of all isolates belonging to either ST1 or ST1660. Phenotypic resistance to beta-lactams, aminoglycosides, tetracyclines, fluoroquinolones and sulfamethoxazole/trimethoprim was determined, and the corresponding resistance genes were detected. For the penicillin-binding proteins 1-4, amino acid substitutions were predicted using WGSs. Since neither transglycosylase nor transpeptidase domains were affected, these alterations might not explain the BORSA phenotypes. Moreover, beta-lactamase activity was found to be associated with an inducible blaZ gene, however, lineage-specific differences regarding expression profiles were noted.