Prevalence of Borderline Oxacillin-Resistant Staphylococcus aureus Strains (BORSA) and Patients’ Clinical Outcome in A Tertiary Teaching Hospital in East Coast Malaysia
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Abstract
Background: Borderline Oxacillin-Resistant Staphylococcus aureus (BORSA) is an emerging S. aureus strain that exhibits reduced susceptibility to penicillinase-stable penicillins (PRPs) with oxacillin MICs between 2 and 4 μg/mL. This resistance is attributed to blaZ hyperproduction or point mutations in penicillin-binding proteins. Their MIC pattern makes clinical management of infected patients challenging. This study aims to estimate the prevalence of BORSA and describe patient clinical outcomes based on prescribed antibiotic regimens.
Methods: A total of 307 archived isolates collected over six months from June to December 2024 from clinical cases of S. aureus were screened using cefoxitin and oxacillin disc diffusion. Potential BORSA isolates were confirmed by gradient diffusion testing. Nitrocefin and double-disc synergy tests confirmed β-lactamase production. Patients’ clinical and demographic data were retrospectively reviewed and analysed for outcomes according to the prescribed antibiotic regimen.
Results: The prevalence of BORSA was 6.5% (n = 20). All isolates exhibited β-lactamase production. Most isolates (65%, n = 13) were recovered from bloodstream infections, predominantly affecting adults (mean age 48.5 years) with comorbidities, specifically diabetes mellitus (65%, n = 13) and renal pathologies (45%, n = 9). Resistance to tetracycline and fusidic acid was observed in 65% (n = 13) and 25% (n = 5) of isolates, respectively. Patients managed with β-lactam-based regimens achieved clinical resolution, whereas treatment with vancomycin resulted in a 50% mortality rate.
Conclusion: The prevalence of BORSA at our hospital was 6.5%. Phenotypic evidence suggests this resistance is driven by β-lactamase hyperproduction. Accurate identification is clinically critical to avoid unnecessary use of vancomycin or therapeutic failure with standard penicillin.
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