Phenotypic Concordance of Chromogenic Agar, Cefoxitin Disk Diffusion and MIC Methods in Clinical MRSA Isolates

Objectives: Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of healthcare- and community-associated infections. Rapid detection is essential for infection control and appropriate antimicrobial therapy. This study evaluated the phenotypic concordance of ChromAgar MRSA, cefoxitin disk diffusion, cefoxitin MIC (minimum inhibitory concentration), and oxacillin MIC methods among MRSA isolates identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS; VITEK MS, bioMérieux, France) Methods: Sixty clinical Staphylococcus aureus isolates categorized as MRSA, along with the American Type Culture Collection (ATCC) reference strains MRSA ATCC 43300 and methicillin-susceptible Staphylococcus aureus (MSSA) ATCC 25923 control strains, were included. Isolates were tested using Condalab ChromAgar MRSA, cefoxitin disk diffusion, cefoxitin MIC, and oxacillin MIC methods according to Clinical and Laboratory Standards Institute (CLSI) M100, 35th edition 2025. Data were analyzed descriptively using frequencies, percentages, medians, and interquartile ranges (IQR). Results: Chromogenic MRSA agar and cefoxitin disk diffusion showed 100% observed agreement among the 60 clinical isolates. All isolates exhibited MRSA-compatible blue colonies on chromogenic agar and were classified as resistant by cefoxitin disk diffusion (60/60, 100.0%). Oxacillin MICs ranged from 0.5 to 64 µg/mL, with a median of 64 µg/mL and an IQR of 16–64 µg/mL; 55 isolates (91.7%) were oxacillin-resistant according to CLSI breakpoints. Cefoxitin MICs ranged from 2 to 64 µg/mL, with a median of 32 µg/mL and an IQR of 16–32 µg/mL; 56 isolates (93.3%) were resistant by cefoxitin MIC interpretation. Oxacillin and cefoxitin MIC classifications were concordant in 53 isolates (88.3%) and discordant in seven (11.7%). The MRSA ATCC control strain showed resistant phenotypes and blue colony formation. In contrast, the MSSA ATCC control strain was susceptible by MIC and disk diffusion and showed a non-MRSA-compatible chromogenic result. Conclusion: ChromAgar MRSA and cefoxitin disk diffusion demonstrated 100% observed agreement among the clinical isolates evaluated in this study. Although cefoxitin and oxacillin MIC methods showed high agreement, discordant results highlighted limitations of oxacillin-based detection alone. These findings demonstrate a high level of phenotypic concordance between cefoxitin-based methods and chromogenic media within the preselected MRSA isolate collection.