Improved sensitivity for meticillin resistance detection in coagulase-negative staphylococci by moxalactam antibiotic discs or a cefoxitin investigation zone

Meticillin resistance in coagulase-negative staphylococci (CoNS) is very common and is usually mediated by mecA encoding penicillin-binding protein 2a (PBP2a; Fleer et al., 2012). Difficulties in the detection of meticillin resistance in CoNS are due to the variable phenotypic resistance expression of mecA, which is characterized by the presence of a small oxacillinresistant subpopulation and/or transcriptional repression of mecA frequently found in CoNS (Dickinson & Archer, 2000; Finan et al., 2002; Nijjar et al., 2014). Therefore, the cefoxitin disc diffusion method, currently defined as a standard procedure by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the Clinical and Laboratory Standards Institute (CLSI), may fail to detect meticillin resistance in these isolates (CLSI, 2016; EUCAST, 2016). The Committee for Antibiotic Susceptibility Testing of the French Society of Microbiology (CA-SFM) introduced a 30 μg moxalactam disc in 2005, which has been proposed to be a better predictor than cefoxitin for the detection of meticillin resistance in CoNS (CA-SFM, 2012; Join-Lambert et al., 2007). We evaluated the effectiveness of various single and combined methods for the detection of meticillin resistance in CoNS, and derived a practical diagnostic approach.