Comparison of Cefoxitin Disk Diffusion Test, Automated System and Chromogenic Medium for Detection of Methicillin Resistance in Staphylococcus aureus Isolates

• Published in: Mikrobiyoloji bülteni Vol. 47; no. 1; p. 11
• Main Authors: Uzun, Berrin, Karataş Şener, Aslı Gamze, Güngör, Serdar, Afşar, Ilhan, Yüksel Ergin, Ozlem, Demirci, Mustafa
• Format: Journal Article
• Language: Turkish
• Published: Turkey 01.01.2013
• Subjects: Anti-Bacterial Agents - pharmacology; Cefoxitin - pharmacology; Humans; Methicillin Resistance - genetics; Oxacillin - pharmacology; Staphylococcus aureus - drug effects
• ISSN: 0374-9096
• DOI: 10.5578/mb.4304

The mecA gene is responsible for the development of methicillin resistance in staphylococci however accurate detection of methicillin resistance is not feasible evermore because of heterogenous expression of mecA gene. Although mecA gene determination by polymerase chain reaction (PCR) is considered as the gold standard method, molecular tests are not easily applied in all routine laboratories. Thus, for the rapid and accurate diagnosis of MRSA strains, easy and practical phenotypic tests are still required. This study was aimed to compare the performance of mecA gene analysis by gel bases multiplex PCR with dual primer (Seeplex, Seegene Inc, Korea), cefoxitin disc diffusion method (30 µg, Oxoid, UK), automated system (Phoenix 100, Becton Dickinson, USA) and chromogenic medium CHROMagar MRSA (CHROMagar Microbiology, Salubris, Turkey) for the detection of methicillin resistance in staphylococci. It was found that 60 of the 98 Staphylococcus aureus strains carried the mecA gene. Methicillin resistance was observed by cefoxitin disc diffusion test in 59 isolates, by automated system in 61 isolates, and by CHROMagar MRSA in 65 isolates. When mecA gene analysis was considered as the reference method, the sensitivity, specificity, positive and negative predictive values of the tests that were used for the detection of methicillin resistance were found as 98.3%, 100%, 100% and 97.4% for cefoxitin disc diffusion (CDD) method; 100%, 97.4%, 98.4% and 100% for automated system; 96.7%, 81.6%, 89.2% and 93.9% for chromogenic medium CHROMagar MRSA, respectively. The highest sensitivity and negative predictive values were obtained by the automated system, and the highest specificity and positive predictive values were obtained by the CDD test. Although the sensitivity of chromogenic medium was found to be similar with the CDD test at the end of 48 hours, the specificity of chromogenic medium was lower than the other tests at the end of each incubation period. Likewise, positive and negative predictive values of the chromogenic medium were determined low compared to other tests. In laboratories that cannot perform molecular analysis, the determination of methicillin resistance should be done by the CDD test which is known to be a better inducer of the mecA gene expression of staphylococci. Determination of minimum inhibitory concentration (MIC) with automated systems can be the second choice especially in laboratories with intensive work loads. As a result chromogenic media can be particularly used for screening in laboratories that have a heavy workload and insufficient personnel number. However, due to its low specificity and the possibility of false positive results, it was recommended that positive strains should be confirmed by other methods such as disc diffusion or microdilution.