Disk diffusion testing is widely used to detect methicillin resistance in staphylococci, and cefoxitin is currently considered the best marker formecA-mediated methicillin resistance. In low-inoculum diffusion testing (colony suspension at 106CFU/ml), the addition of moxalactam in combination with cefoxitin has been reported to improve on cefoxitin alone for the detection of methicillin-heteroresistant staphylococci. However, moxalactam is absent from EUCAST and CLSI guidelines, which use high-inoculum diffusion testing (colony suspension at 108CFU/ml), calling into question the potential interest of including moxalactam in their recommendations. The inhibition zone diameters of cefoxitin and moxalactam, alone and in combination, were evaluated for concordance withmecAandmecCpositivity in a large collection of clinicalStaphylococcusisolates (611Staphylococcus aureus,Staphylococcuslugdunensis, andStaphylococcus saprophyticusisolates and 307 coagulase-negative staphylococci other thanS. lugdunensisandS. saprophyticusisolates, of which 22% and 53% weremecA-positive, respectively) and in 25mecC-positiveS. aureusisolates using high-inoculum diffusion testing. Receiver operating characteristic, sensitivity, and specificity analyses indicated that the detection ofmecA- andmecC-positive and negative isolates did not improve with moxalactam, either alone or in combination with cefoxitin, compared to cefoxitin alone. These findings were similar in both theS. aureus/S. lugdunensis/S. saprophyticusgroup and in the coagulase-negative staphylococci group. Our results do not support the use of moxalactam as an additional marker of methicillin resistance when testing with high-inoculum disk diffusion.
Comparison of the BD Phoenix System with the Cefoxitin Disk Diffusion Test for Detection of Methicillin Resistance in Staphylococcus aureus and Coagulase-Negative Staphylococci
