Revised interpretation of oxacillin MICs for Staphylococcus epidermidis based on mecA detection

Area of Science:

• Clinical microbiology
• Antimicrobial resistance

Background:

• Staphylococcus epidermidis is a common cause of nosocomial infections.
• Methicillin resistance in staphylococci is a growing public health concern.
• Accurate detection of methicillin resistance is crucial for effective treatment.

Purpose of the Study:

• To compare oxacillin resistance rates between Staphylococcus epidermidis and Staphylococcus aureus.
• To evaluate the correlation between mecA genotype and antimicrobial susceptibility in S. epidermidis.
• To assess the adequacy of current National Committee for Clinical Laboratory Standards (NCCLS) interpretive criteria for methicillin resistance in S. epidermidis.

Main Methods:

• Antimicrobial susceptibility testing using oxacillin Minimum Inhibitory Concentration (MIC) determination.
• Phenotypic oxacillin disk diffusion testing.
• Detection of the mecA gene by genotypic methods.

Main Results:

• A higher proportion of S. epidermidis strains exhibited oxacillin MICs of 1-2 µg/ml compared to S. aureus.
• mecA-negative S. epidermidis strains with low oxacillin MICs (<0.5 µg/ml) were susceptible by disk diffusion.
• mecA-positive S. epidermidis strains with higher oxacillin MICs (≥0.5 µg/ml) were resistant by disk diffusion.

Conclusions:

• The mecA genotype is a reliable indicator of methicillin resistance in S. epidermidis.
• Current NCCLS MIC interpretive criteria may underestimate methicillin resistance in S. epidermidis.
• Further refinement of susceptibility testing guidelines for S. epidermidis is warranted.