Genetic Variation in the blaZ Gene Leading to the BORSA Phenotype in Staphylococcus aureus

BACKGROUND/OBJECTIVES: Staphylococcus aureus is a leading cause of bacteraemia in Danish hospitals. Approximately 70% of clinical S. aureus isolates are penicillin-resistant, which is predominantly due to blaZ-mediated β-lactamase production.

METHODS: A collection of 489 S. aureus strains derived from bacteraemia were cultured and their genomes sequenced.

RESULTS: From this collection, 71% of isolates were methicillin-susceptible S. aureus (MSSA) harbouring blaZ. While most isolates contained the blaZ gene belonging to the well-characterised A, B, C and D variants, three strains (1%) produced a BlaZ protein characterised by having threonine residues on both positions 128 and 216 and, therefore, belonged to neither of the established blaZ variants. We named this variant, variant F. We report that clinical isolates expressing blaZ variant F were resistant to oxacillin. The β-lactamase production phenotype in isolates carrying either of the A, B, C or D variants was only weakly discernible on MIC gradient strip and disk diffusion tests. When the β-lactamases were expressed either from a T7 promoter or from their endogenous promoters in Escherichia coli, variant F was significantly better at degrading ampicillin than variant A. We also showed that variant F conferred oxacillin resistance when expressed in an isogenic S. aureus strain, while variant A did not. Finally, we demonstrated that the F variant threonine 216 played a role in the enzyme's superior activity.

CONCLUSIONS: Our findings demonstrate that the new F variant of BlaZ is sufficient to render S. aureus a BORSA strain, which is superior in the degradation of common anti-staphylococcal β-lactam antibiotics, such as benzylpenicillin, cloxacillin, and oxacillin. It is sensitive to β-lactamase inhibitors and rapidly degrades nitrocefin. We provide a genetic explanation for the borderline oxacillin-resistant S. aureus (BORSA) phenotype.