Disk Diffusion Testing for Detection of Methicillin-Resistant Staphylococci: Does Moxalactam Improve upon Cefoxitin?

Abstract

dDisk diffusion testing is widely used to detect methicillin resistance in staphylococci, and cefoxitin is currently considered the best marker for mecA-mediated methicillin resistance. In low-inoculum diffusion testing (colony suspension at 10 6 CFU/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 highinoculum diffusion testing (colony suspension at 10 8 CFU/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 with mecA and mecC positivity in a large collection of clinical Staphylococcus isolates (611 Staphylococcus aureus, Staphylococcus lugdunensis, and Staphylococcus saprophyticus isolates and 307 coagulase-negative staphylococci other than S. lugdunensis and S. saprophyticus isolates, of which 22% and 53% were mecA-positive, respectively) and in 25 mecC-positive S. aureus isolates using high-inoculum diffusion testing. Receiver operating characteristic, sensitivity, and specificity analyses indicated that the detection of mecA-and mecC-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 the S. aureus/S. lugdunensis/S. saprophyticus group 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.

Evaluation of various phenotypic methods with genotypic screening for detection of methicillin-resistant Staphylococcus aureus

“…High specificity shown by CFD and oxacillin MIC was due to the presence of inducers such as cefoxitin and NaCl which increase the identification of resistance. Cefoxitin antibiotic is found to be the best inducer for PBP2a, PBP3, and PBP4 proteins [26]. The other two phenotypic methods in this study showed a low reliability, where OSA test could detect MRSA only in 18 mecA-positive S. aureus, and showed false positive for 15 non-mecA S. aureus, whereas oxacillin disk diffusion test detected MRSA in 18 mecA-positive S. aureus and showed a false positive for 14 non-mecA S. aureus.…”

Phenotypic analysis and antibiotic susceptibility of methicillin-resistant Staphylococcus aureus in Kiambu County, Kenya

“…Detection of methicillin resistance by S. aureus was carried out by disk-diffusion (Kirby-Bauer) technique [13,14] with 30 µg cefoxitin (Liofilchem, Roseto degli Abruzzi, Italy) according to the Clinical Laboratory Standard Institute (CLSI) guidelines [15].…”

“…The antimicrobial susceptibility testing of S. aureus isolates to antibiotics was carried out by disk diffusion (Kirby-Bauer) [13,14] according to the CLSI guidelines [15]. Multi discs with nine (9, including cefoxitin) antimicrobial agents (Himedia Laboratories, Mumbai India) were used, namely: penicillin G -1 i.u.…”

Inanimate Surfaces and Air Contamination with Multidrug Resistant Species of Staphylococcus in the Neonatal Intensive Care Unit Environment

“…The previous use of antimicrobials, usually broad-spectrum, increases hospital costs and drives the emergence of resistant microorganisms, as it exerts a selective pressure in the ICU environment, selecting MDR isolates. It is also associated with death as a consequence of inadequate therapy [ 35 , 36 ]. Thus, we can observe the occurrence of a cycle, as MDR microorganisms from neonatal infections are deposited on surfaces exposed to air or directly by the hands of health professionals.…”