Simultaneously screening for methicillin-resistant Staphylococcus aureus and its susceptibility to potentiated penicillins

Abstract

Introduction. We recently revealed that a significant proportion of clinical methicillin-resistant Staphylococcus aureus (MRSA) isolates are susceptible to pencillins and clavulanic acid (potentiated penicillins), including widely available combinations such as co-amoxiclav. These isolates also showed increased susceptibility to oxacillin on Iso-Sensitest Agar (ISA). Hypothesis/Gap Statement. The increased susceptibility to oxacillin displayed on ISA by these MRSA isolates may be used to distinguish them from the resistant ones. Aim. We aimed to develop a method to simultaneously screen a S. aureus clinical isolate for its susceptibility to methicillin and potentiated penicillins. Methodology. A double-disc diffusion method using 10 µg cefoxitin and 1 µg oxacillin discs on ISA was developed and tested against a panel of 120 whole genome-sequenced MRSA isolates. The sensitivity of the method was compared with that of previously published genotypic and phenotypic methods. In addition, double-disc diffusion was performed for all isolates on Müller–Hinton agar (MHA) following the European Committee on Antimicrobial Susceptibility Testing (EUCAST) protocol. Results. All isolates (120/120) were reconfirmed to be phenotypically MRSA, as indicated by the result of cefoxitin disc diffusion testing. All isolates (40/40) that had a pencillins and clavulanic acid (Pen–Clav)-resistant genotype were not inhibited by oxacillin, while 77/80 (96.3 %) isolates that had a Pen–Clav-susceptible genotype were inhibited by oxacillin on ISA. The results also showed that the EUCAST method using MHA correctly identified all isolates as MRSA but failed to distinguish the Pen–Clav-susceptible isolates from the Pen–Clav-resistant isolates. Conclusions. This double-disc diffusion method using ISA could be used to accurately screen for clinical MRSA isolates and determine their susceptibility to Pen–Clav simultaneously, rapidly identifying MRSA infections that might be suitable for treatment with potentiated penicillins.