Evaluation of Three Phenotypic Tests vs. Duplex (coa & mecA) PCR for Detection of Methicillin-Resistance Staphylococcus aureus (MRSA) Strains from Patients with Nosocomial Infections (NI) in a Mexican Hospital

Abstract

Background: Staphylococcus aureus (SA) is a leading cause of hospital acquired infections worldwide. Epidemiological features have also changed in Mexican hospitals and methicillin-resistance Staphylococcus aureus (MRSA) infections appear to be an emerging phenomenon.   Aims: The aim of the present study was to detect the amplification of both coa and mecA genes by duplex PCR in MRSA isolates from Mexican patients with NI and the comparison of these results versus three phenotypic MRSA-detection methods. Place and Duration of Study: Department of Public Health, Faculty of Medicine, National University of Mexico (UNAM) in collaboration with a General Hospital “Gonzalo Castañeda Escobar” (HGC), Health Care & Social Services Institute for Workers (ISSSTE), between August 04-December 05. Methodology: Among SA (n=100) strains isolated from patients with NI, 40% were MRSA. All SA strains were tested by various methods: cefoxitin (Cfx) disk-diffusion assay (DD), automated MicroScan System (MS), Oxacillin (Ox) (range 0.5 - 4.0 µg/ml) minimum inhibitory concentration (MIC), PBP2a latex agglutination test (LA) (Oxoid) and the molecular amplification by PCR of the coa and mecA genes (PCR). SA strains were previously identified by MS, and both the tube coagulase (TCT) and mannitol (MT) tests. Results: From 100 SA strains, 35 were confirmed as MRSA isolates by detection of mecA gen by PCR in addition to only 5 strains verified phenotypically but coa gen negative. These 40 MRSA strains and 53 mecA gen negative isolates as well as 5 strains phenotypically characterized (MSSA) from NI, were assayed with all 3 laboratory tests. Only 2 clinical strains were negative to both genes: coa & mecA, but positive to all phenotypic tests. Using mecA as a gold standard (GS), category agreement for the 3 tests was: A method with higher sensitivity (SN) and specificity (SP) (SN/SP) was Cfx-DD (100/88) than other 2 methods Ox-MS (90/73) and LA (63/90). Using coa gene as a GS, category agreement for MS, and both TCT & MT were: 93/100, 95/98, and 91/98, respectively. Conclusions: Molecular detection by PCR-duplex of coa & mecA genes, is a very useful and powerful method (93%) to precisely discriminate typical MRSA strains in the laboratory. A Cfx-DD test performed much better for detection of MRSA strains than those using OX-MS and LA. Both assays, Cfx-DD and PCR-duplex, represent a simple, rapid, reliable approaches for the detection of methicillin resistant staphylococci and could be applied to all national hospitals, to implement fast and adequate anti-MRSA therapy.