gms | German Medical Science

Objectives: In 2011 an alternative methicillin resistance gene, mecC, was described in Staphylococcus aureus. Methicillin-resistant S. aureus (MRSA) isolates carrying mecC have been recovered from humans, ruminants, pets, and other animals. Previous studies suggested that the mecC encoded penicillin binding protein (PBP) might exhibit a higher affinity for oxacillin as compared to other β-lactams, in especially to the cephalosporin cefoxitin. This might result in false negative MRSA screening results in test systems using oxacillin for MRSA detection. Thus, we determined susceptibility of clinical mecC positive isolates towards different β-lactam antibiotics, including oxacillin, oxacillin + sulbactam, cefoxitin and the “novel” cephalosporin ceftaroline.

Material and methods: We investigated the susceptibility of 76 clinical mecC positive S. aureus isolates originating from all over Germany towards various β-lactams. Isolates were typed based on spa-typing. Initially, all isolates were tested for their susceptibility towards oxacillin (OXA) via broth microdilution (BMD); additionally they were subjected to an MRSA screening test using oxacillin + sulbactam (OXASu). Cefoxitin (COX) and ceftaroline (CPT) susceptibility was assessed by disc diffusion (DD) methodology according to EUCAST guidelines. Susceptibility towards CPT was additionally tested by BMD for all and by Etest^® for selected isolates.

Results: spa typing revealed for the majority of isolates affiliation to ST130. Our collection included 23 (30%) mecC positive, phenotypically oxacillin-susceptible isolates (MIC range, 0.5–2 mg/L); all isolates with OXA MICs of 0.5–1 mg/L (n=5) revealed also a susceptible OXASu Screeningtest result whereas isolates with an OXA MIC of 2 mg/L were tested resistant. All 76 isolates were resistant towards COX using disk diffusion methodology. With regard to CPT susceptibility testing by disc diffusion, only four isolates revealed “borderline” zone diameters of 21 mm and subsequent broth microdilution revealed a susceptible phenotype for all of them (MIC range, 0.5–1 mg/L). Among the 72 isolates with CPT zone diameters above 22 mm, we detected three non-susceptible isolates (zone diameters: 22-23 mm; MIC: 2 mg/L). MICs for the CPT susceptible isolates were 0.25 mg/L (n=3), 0.5 mg/L (n=32) and 1 mg/L (n=38), respectively. Subsequent CPT Etest^® for selected isolates revealed MICs which were generally 2- to 3-fold lower than those generated by BMD.

Conclusion: Results corroborate the superiority of COX based screening procedures for the detection of mecC positive “low-level” MRSA in comparison to OXA. Additionally we demonstrated a low prevalence of CPT non-susceptibility among mecC positive S. aureus (4% of all isolates investigated) indicating that the variant structure of PbP2 apparently has no influence on its binding affinity for ceftaroline (in contrast to cefoxitin).