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  • Title: Evaluation of disc diffusion tests and agar screening for predicting mecA-mediated oxacillin resistance in Staphylococcus lugdunensis revealed a cefoxitin-susceptible, mecA-positive S. lugdunensis clonal complex 27 clone.
    Author: Ho PL, Liu MC, Tong MK, Fan PM, Tse CW, Wu AK, Cheng VC, Chow KH.
    Journal: J Glob Antimicrob Resist; 2020 Mar; 20():260-265. PubMed ID: 31493529.
    Abstract:
    OBJECTIVES: This study evaluated disc diffusion tests and agar screening for detecting mecA-mediated oxacillin resistance in Staphylococcus lugdunensis (S. lugdunensis). METHODS: Staphylococcus lugdunensis isolates (n = 179) from diverse sources in Hong Kong during 1998-2018 were investigated by disc diffusion tests (cefoxitin and oxacillin) and inoculation onto oxacillin (1 μg/mL and 2 μg/mL) and chromID methicillin-resistant Staphylococcus aureus (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 (CLSI) M100 breakpoints for cefoxitin disc results, there was 88% categorical agreement (CA) and 40% very major error (VME). 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% ME and 0-37% VME). The mecA-positive isolates (n = 21) that could not be detected by the cefoxitin disc test were further characterised. The cefoxitin MIC for all 21 isolates was ≤4 μg/mL. Twenty isolates had an oxacillin MIC of 1-2 μg/mL and one had an oxacillin MIC of 4 μg/mL. All had positive PBP2a results and were typed as clonal cluster 27/SCCmec V. CONCLUSIONS: These findings highlight the need to evaluate phenotypic methods using mecA-positive S. lugdunensis with different oxacillin resistance phenotypes.
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