Emergence of multidrug-resistant sequence type 45 strains among mecA-positive borderline oxacillin-resistant Staphylococcus aureus causing bacteraemia in a medical centre in Taiwan.

Clinical data related to bacteraemia caused by mecA-positive borderline oxacillin-resistant Staphylococcus aureus (BORSA) isolates [minimum inhibitory concentrations (MICs) of 0.5-4 mg/L] are limited. Patients aged ≥15 years with bacteraemia due to BORSA who were admitted to the emergency department of a Taiwanese tertiary hospital between January 2001 and December 2015 were evaluated. Clinical characteristics of patients with bacteraemia caused by BORSA, methicillin-susceptible S. aureus and methicillin-resistant S. aureus (MRSA) were compared. MICs of 12 antibiotics were determined by agar dilution. Genetic characteristics of 64 available BORSA isolates were analysed by detection of the SCCmec gene, the mecA promoter region sequence and multilocus sequence typing (MLST). Pulsed-field gel electrophoresis (PFGE) was performed to identify the clonality of the emerging ST isolates. The quinolone resistance genes were analysed for the emerging ciprofloxacin-resistant ST45 clone isolates. Among the 65 BORSA bacteraemic patients, in-hospital mortality was 24.6%, significantly lower than that of MRSA patients (38.5%) (P = 0.03). Underlying conditions were similar between BORSA and MRSA patients, but with less dialysis in the former (P = 0.01). Multivariate analysis revealed that septic shock [odds ratio (OR) = 15.95] and bacteraemia originating from lower respiratory tract infection (OR = 5.78) were two independent risk factors for 30-day mortality. The ST45 clone with high-level ciprofloxacin resistance (MICs of 8-128 mg/L) replaced ST59 as the predominant clone since 2012, although no major clustering was detected. In conclusion, the emerging multidrug-resistant ST45 clone has replaced ST59 as the most common among BORSA isolates.