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Laura C Huang, Jack Stringham, James Wong, Jorge Maestre, Darlene Miller, Harry W Flynn; Correlation of Phenotypic Expression of Methicillin Resistance with Genotype and In Vitro Susceptibility for mecA+ Staphylococcus epidermidis Endophthalmitis Isolates. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5784.
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© ARVO (1962-2015); The Authors (2016-present)
To correlate the genotypic methicillin resistance conferred by the mecA gene with phenotypic expression of methicillin resistance among Staphylococcus (S) epidermidis isolates from endophthalmitis and in vitro susceptibility to ceftaroline, a new fifth generation cephalosporin, and other commonly used antimicrobials.
Etests and the VITEK 2 system were used to detect the presence and phenotypic expression of methicillin resistance among S. epidermidis isolates recovered from endophthalmitis. Results were compared with mecA genotype using polymerase chain reaction (PCR) and in vitro susceptibility to ceftaroline and vancomycin (Etests) and VITEK 2 breakpoints for gentamicin, daptomycin, linezolid, tigecycline, levofloxacin, and moxifloxacin. Additionally, antibiotic susceptibility was analyzed against the following subgroups: mecA+ with methicillin resistance (group A) and isolates that were mecA+ though clinically methicillin sensitive (group B).
All 32 isolates recovered from 2010-2013 were mecA-genotype positive (100%), however, only 21/32 (65.6%) expressed methicillin resistance by conventional laboratory tests. Isolates were 100% susceptible to vancomycin (MIC90 3 μg/mL, N=32), ceftaroline (MIC90 0.38 μg/mL, N=32), linezolid (N=20), and tigecycline (N=20). The susceptibility of daptomycin (N=20) was 95%. Gentamicin susceptibility overall was 86.7% (26/30) of which group A had 78.9% (15/19) susceptible compared to 100% (11/11) in group B. A correlation between the phenotypic expression of mecA genotype and in vitro susceptibility was demonstrated for the fluoroquinolones. For levofloxacin, group A had 21.1% (4/19) susceptible in contrast to 81.8% (9/11) (p=0.001) in group B. For moxifloxacin, group A had 30% (3/10) susceptible compared to 80% (8/10) (p=0.025) in group B.
Routine laboratory methods may fail to detect heterogeneous and or low level expression of methicillin resistance among mecA+ S. epidermidis genotypes. This may have important implications for the correct selection and management of S. epidermidis endophthalmitis. The new fifth generation cephalosporin, ceftaroline, demonstrated low MIC values and may show promise as a therapeutic agent for methicillin resistant S. epidermidis.
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