Efficacy of cefoxitin disk diffusion in predicting mecA gene mediated oxacillin resistance in coagulase negative staphylococci

ABSTRACT

Accurate detection of mecA gene mediated resistance to oxacillin is necessary for appropriate antimicrobial chemotherapy for staphylococcal infections. To determine the efficacy of cefoxitin disk diffusion tests in detecting mecA gene mediated oxacillin resistance in coagulase negative Staphylococci. This descriptive study was carried out after the approval of Bioethical Committee of the Faculty of Applied Medical Sciences, Umm Al Qura University, Makkah, Saudi Arabia. Clinical isolates of coagulase negative Staphylococci from blood culture of neonates suspected to be suffering from septicemia from January 2012 to December 2013 at Maternity and Children hospital Makkah, Saudi Arabia were studied. In this study 241 coagulase negative Staphylococci clinical isolates from Maternity and Children hospital, Makkah were included. Identification of coagulase negative Staphylococci species and their antibiotic susceptibility testing i.e., minimum inhibitory concentration was performed using Microscan Walk Away system. Concurrently, all isolates were subjected to disk diffusion tests [oxacillinmicrog disc, cefoxitin 30micro g disc] following Clinical and Laboratory Standards Institute guidelines. A total of 241 coagulase negative Staphylococci strains isolated from neonatal blood cultures included S.epidermidis 99[41.1%], S.haemolyticus 71[29.4%], S.hominis 38[15.7%], S.xylosus 14[5.9%], S.capitis 9[3.9%] and others 10[4.2%]. The results of phenotypic detection of mecA gene by cefoxitin and oxacillin disk diffusion were compared with their MIC results. The cefoxitin disk diffusion predicted mecA gene in 89.6% isolates and oxacillin disk diffusion in 83.8%. The cefoxitin minimum inhibitory concentration results matched totally with cefoxitin disk diffusion results but oxacillin minimum inhibitory concentration results did not match totally with its disk diffusion results. Minimum inhibitory concentration results of oxacillin were same as that of the cefoxitin minimum inhibitory concentration and disk diffusion results. Cefoxitin disk diffusion has an edge over oxacillin disk diffusion in predicting mecA mediated oxacillin resistance in coagulase negative Staphylococci. Those using disk diffusion as their routine susceptibility testing method for Staphylococci may replace lug oxacillin disc with 30microg cefoxitin disc for routine testing but should report oxacillin in its place