CONICET | Buscador de Institutos y Recursos Humanos

Background: Oxacillin resistance in S. aureus is mediated by PBP2a encoded by mecA gene. Majority of these isolates displays a heterogeneous resistance and very low level methicillin resistant S. aureus can be misdiagnosed as methicillin susceptible S. aureus. We investigated one of these heteroresistant strains that could only be detected by PBP2a latex agglutination test. Methods: Identification was done using standard biochemical tests. In vitro susceptibility testing and screening for detection of MRSA was performed according to CLSI guidelines. PBP2a was detected using a PBP2a latex agglutination test kit as recommended by the manufacturer (BioMerioux). mecA was detected by PCR. Typing of staphylococcal cassette chromosome (SCCmec) was performed by detection and sequence of ccr genes. A heavy inoculum was streaked on MHA plates containing cefoxitin (8µg/ml) or rifampin (32µg/ml) to select the homotypic population. Genotyping was performed by spa typing and PFGE. Results: Inhibition zones were 20 mm for oxacillin and 22 mm for cefoxitin. No growth was observed in oxacillin agar screen plate (MHA with 4% NaCl and 6 μg/ml of oxacillin). PBP2a was positive and we confirmed methicillin resistance amplifying mecA coding gene. ccrC2 (SCCmecVII) was identified by sequencing. Isogenicity between heterotypic and homotypic populations was confirmed by spa type (t127) and PFGE. Conclusions: Neither oxacillin/cefoxitin disk diffusion tests nor the oxacillin agar screen method could detect mecA mediated oxacillin resistance. PBP2a detection was the unique phenotypic test useful to predict β-lactams resistance in this isolate. However, this commercial kit is not available for every clinical lab in developing countries. Despite 30 μg cefoxitin disk diffusion method has been proposed as an efficient method for the detection of methicillin resistance, in this case, did not permit MRSA recognition.

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