Evaluation of disc diffusion tests and agar screening for prediction of mecA-mediated oxacillin resistance in Staphylococcus lugdunensis reveal a cefoxitin-susceptible, mecA-positive S. lugdunensis clonal complex 27 clone.
J Glob Antimicrob Resist. 2019 Sep 04;:
Authors: Ho PL, Liu MC, Tong MK, Fan PM, Tse CW, Wu AK, Cheng VC, Chow KH
OBJECTIVES: This study evaluated disc diffusion tests and agar screening for detection of mecA-mediated oxacillin resistance in Staphylococcus lugdunensis.
METHODS: S. lugdunensis isolates (n = 179) from diverse sources in Hong Kong, 1998-2018 were investigated by disc diffusion tests (cefoxitin and oxacillin) and inoculation onto oxacillin (1 µg/ml and 2 µg/ml) and chromID MRSA agars. The results were compared with mecA PCR as the reference. Isolates with discordant results were further tested by MIC and penicillin binding protein 2a (PBP2a) assays.
RESULTS: Cefoxitin and oxacillin zone diameters were not distributed in ways that allowed reliable division of the mecA-positive (n = 52) and mecA-negative (n = 127) isolates. On applying the 2019 Clinical Laboratory Standards Institute M100 breakpoints for cefoxitin disc results, there was 88% categorical agreement (CA) and 40% very major errors (VMEs). Screening using 2 µg/ml oxacillin agar reliably differentiated mecA-positive and mecA-negative isolates (100% CA) without any major error (ME) or VME results. The performance of screening using 1 µg/ml oxacillin agar or ChromID MRSA agar was variable (74%-89% CA, 0-38% MEs and 0-37% VMEs). The mecA-positive isolates (n = 21) that could not be detected by the cefoxitin disc test were further characterized. Cefoxitin MIC for all 21 isolates were ≤4 µg/ml. Twenty isolates had oxacillin MIC of 1-2 µg/ml and one had oxacillin MIC of 4 µg/ml. All had positive PBP2a results and were typed as clonal cluster (CC) 27/SCCmec V.
CONCLUSIONS: Our findings highlight the need to evaluate phenotypic methods using mecA positive S. lugdunensis with different oxacillin resistance phenotypes.
PMID: 31493529 [PubMed - as supplied by publisher]