A prospective study to evaluate methods of MRSA detection in patients with soft tissue and bone infection in a tertiary care centre

Abstract

Staphylococcus aureus is a major pathogen causing bacteraemia, pneumonia, skin and soft tissue infections (SSTIs), and osteomyelitis. Over the past 50 years, it has acquired resistance to antimicrobials including the penicillinase-resistant ones like methicillin. Rapid identification and susceptibility testing are mandatory to prevent further dissemination of MRSA and to provide effective antimicrobial treatment. Hence, methods used to detect MRSA should be rapid with high sensitivity and specificity.1) To compare various phenotypic methods for MRSA detection. 2) To confirm the phenotypic results with Polymerase Chain Reaction. 3) To evaluate the susceptibility of MRSA isolates to other antimicrobial agents.Eighty four MRSA isolates from soft tissue and bone samples identified by the cefoxitin (30µg) disc diffusion method were subjected to Oxacillin Screen Agar (OSA), cefoxitin E-strip, automated identification & sensitivity testing using BD Phoenix system and Polymerase Chain Reaction using the GeneXpert for mecA gene detection.Although all 84 isolates were resistant by cefoxitin disk diffusion, 83 (95.4%) isolates were positive for the mecA gene. The sensitivities of the OSA, cefoxitin E-strip and BD Phoenix system were 79.5%, 80.7%, and 100%, respectively. All the isolates were sensitive to vancomycin and linezolid. 70% of the isolates were sensitive to cotrimoxazole whereas maximum resistance of 76% was seen to ciprofloxacin.Automated identification by BD Phoenix system, if available, can be considered as the most sensitive phenotypic method for MRSA detection, while cefoxitin E-strip is the most appropriate test in a resource poor setting.