Molecular Characterization of Equine Staphylococcus aureus Isolates Exhibiting Reduced Oxacillin Susceptibility

Abstract

The 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 identify factors possibly associated with the BORSA phenotype. All S. aureus were subjected to antimicrobial susceptibility testing and whole genome sequencing (WGS). A quantifiable β-lactamase activity assay was performed for a representative subset of 13 isolates. The WGS data analysis of the 19 BORSA isolates identified two different genomic lineages, sequence type (ST) 1 and ST1660. The core genome multilocus sequence typing (cgMLST) revealed a close relatedness of all isolates belonging to either ST1 or ST1660. The WGS analysis identified the resistance genes aadD, dfrG, tet(L), and/or blaZ and aacA-aphD. Phenotypic resistance to penicillins, aminoglycosides, tetracyclines, fluoroquinolones and sulfamethoxazole/trimethoprim was observed in the respective isolates. For the penicillin-binding proteins 1–4, amino acid substitutions were predicted using WGS data. Since neither transglycosylase nor transpeptidase domains were affected, these alterations might not explain the BORSA phenotype. Moreover, β-lactamase activity was found to be associated with an inducible blaZ gene. The lineage-specific differences regarding the expression profiles were noted.

Multidrug-resistant Staphylococcus cohnii and Staphylococcus urealyticus isolates from German dairy farms exhibit resistance to beta-lactam antibiotics and divergent penicillin-binding proteins

“…However, further in depth analysis of S. cohnii blaZ genes using NCBI BLAST revealed that the sequences only show low identities (< 80%) to any other published staphylococcal blaZ gene. Along with the mecA and mecC gene, the femA and gdpP genes were associated with beta-lactam antibiotic resistance 23 and both genes were detected in the isolates of our study. In general, however, data availability regarding AMR and its respective genes in S. cohnii and S. urealyticus is extremely limited and for this reason it is unclear how reliable the annotation and function of the so far published S. cohnii blaZ genes is.…”

“…A borderline oxacillin-resistance in S. aureus isolates due to modified PBPs was previously suggested in a review about methicillin resistance in staphylococci 45 . Moreover, studies recently reported borderline cefoxitin- and oxacillin-resistant S. aureus isolates with substituted PBP amino acid sequences 23 , 46 , 47 . However, in both cases, the borderline resistance was associated with the hyperproduction of the BlaZ beta-lactamase.…”

“…With the exception of S. cohnii isolate SC7, not only the mecA and mecC genes, but also mecB and mecD were absent from all genomes. In addition, there were no femB, femC, femD, mgrA or mprF genes which might be associated with beta-lactam resistance 23 . Only the femA and gdpP genes were detected.…”

Methicillin-Resistant Staphylococci and Macrococci at the Interface of Human and Animal Health

“…Scholtzek et al [ 35 ] characterized equine S. aureus isolates that show elevated minimal inhibitory concentrations (MICs) for oxacillin but could not be categorized as MRSA. Those so-called borderline oxacillin-resistant S. aureus (BORSA) strains pose a challenge for routine laboratories in both human and veterinary medicine if no special procedures and molecular confirmation are practiced.…”

“…Instead, they are characterized by hyper-production of beta-lactamases or their resistance is associated with point mutations of the PBPs occurring usually in S. aureus . By core genome multilocus sequence typing, Scholtzek et al observed the close relatedness of the isolates belonging to either ST1 or ST1660 [ 35 ]. The beta-lactamase activity of the 19 isolates included was found to be associated with an inducible blaZ gene.…”