Is the Cefoxitin Disk Test Reliable Enough To Detect Oxacillin Resistance in Coagulase-Negative Staphylococci?

Coagulase-negative staphylococci (CoNS) represent an important etiology of nosocomial bloodstream infections (2, 4, 6). The introduction of empirical treatment has been a crucial step in reducing morbidity and mortality caused by such infections and in controlling the spread of resistant strains (1, 6). It has been advocated that the cefoxitin disk would be more sensitive than the 1-μg oxacillin disk to predict the oxacillin heteroresistance phenotype among staphylococci (1, 2, 4, 7, 8). Thus, the National Committee for Clinical Laboratory Standards (NCCLS) has recommended its use for detection of oxacillin resistance (7). A total of 5 of 241 (2.2%) CoNS displaying cefoxitin inhibition zones of ≥25 mm and resistance to oxacillin by disk diffusion test were isolated by the clinical laboratory of the Hospital São Paulo, a tertiary university hospital located in São Paulo, Brazil, during September and October 2004. The bacterial strains were isolated from diverse body sites and referred to the Laboratório Especial de Microbiologia Clínica of Universidade Federal de São Paulo for further characterization. The identification of the CoNS was confirmed based on the morphology of bacterial colonies on blood agar, Gram stain, 3% H2O2 catalase and by agglutination tests (Staphy-test; PROBAC, São Paulo, Brazil). The strains were tested for coagulase activity by slide tests with rabbit plasma (Difco Laboratories, Detroit, MI). The final identification was carried out using the GPI card Vitek (BioMerieux, Hazelwood, MI). The antimicrobial susceptibility testing of six antimicrobial agents was performed using Etest (AB Biodisk, Solna, Sweden). The results were interpreted according to the NCCLS guidelines (7). Staphylococcus aureus ATCC 25923 and ATCC 29213 were used as quality control strains. The mecA gene was detected by PCR technique according to the method described previously (6). The genetic relatedness of the five CoNS was evaluated by automated ribotyping using the RiboPrinter microbial characterization system (Qualicon, Wilmington, DE). Isolates were considered to belong to the same ribogroup if their similarity coefficients were ≥0.93. According to the Vitek system, all five isolates were identified as Staphylococcus epidermidis. They were susceptible to linezolid, vancomycin, and teicoplanin. These isolates were confirmed to be resistant to oxacillin and exhibited cefoxitin inhibition zones that were ≥25 mm, as shown in Table 1. The presence of the mecA gene was detected in all five CoNS, which displayed distinct ribotyping patterns.