Abstract

COMPARISON OF PHENOTYPE AND GENOTYPE METHODS FOR DETECTION OF METHICILLIN RESISTANCE IN STAPHYLOCOCCUS AUREUS FROM CLINICAL ISOLATES IN A TERTIARY CARE TEACHING HOSPITAL, SOUTH INDIA

*Ms. Jayaraman Latha¹, Dr. Subburaya Umamaheswari², Dr. Paramasivam Rajendran³

ABSTRACT

Objective: To compare the different phenotype methods of detection of methicillin resistance in Staphylococcus aureus from clinical isolates in a tertiary care centre South India. Methodology: 48 clinical isolates of S. aureus from different specimens such as blood and exudates collected from January through May 2010 were included in the study. Antibiotic susceptibility tests for various antibiotics were carried out. All the isolates were identified as MRSA by PCR for mecA gene. Conventional methods like cefoxitin disc diffusion assay, MIC by Etest for oxacillin; Phenotype rapid methods for MRSA detection like MRSA-Screen slide latex agglutination test (Denka-Seiken, Japan) and ORSAB (Oxoid Ltd., UK) were carried out to compare their sensitivity, specificity, accuracy and cost-effectiveness.Results: Out of the n=48 clincal isolates included in the study, cefoxitin disc diffusion assay recorded n=47 (98%) as resistant to methicillin and n=1 (2.1%) strain as sensitive which was mecA-positive. This strain was heterogeneously resistant to oxacilin by E-test recording a MIC of 0.38μg/ml (double drug strip), showed slight (1+s) agglutination by MRSA-Screen and ORSAB showed mild growth. MIC by E-test for oxacillin was resistant for all the n= 48 strains with a optimal sensitivity of 100%. PBP2′ detection by MRSAScreen, showed n=45 (93.75%) strains as positive with n=3 (6.25%) false negative results. These n=3 (6.25%) strains were positive for mecA, resistant by cefoxitin disc diffusion test but heterogeneously resistant to oxacillin with MICs <32μg/ml. n=1 (2.1%) strain had a borderline MIC of 4μg/ml. In ORSAB n=2 (4.17%) out of the three strains showed mild growth on repeat and n=1 (2.1%) showed growth after 48 hrs. ORSAB recorded n=46 strains as resistant with a sensitivity of 95.8%.Conclusion: In the present study we found that though rapid methods like MRSA-Screen, ORSAB take less time, conventional methods like cefoxitin disc diffusion and MIC by E-test for oxacillin are more sensitive and accurate. Therefore in the absence of molecular echniques, conventional methods continue to be the gold-standard for the detection of methicillin resistance in Staphylococcus aureus.

Keywords: Staphylococcus aureus, PCR, Cefoxitin disc diffusion, E-test, MRSA-Screen, ORSAB.