An Evaluation of the Three Different Laboratory Methods of Detection of Methicillin Resistant Staphylococcus Aureus

MRSA is any strain of Staphylococcus aureus that has developed resistance to beta-lactamase antibiotics which include Penicillin and Cephalosporins. With the increasing prevalence of MRSA infections, it has become important to have an ideal method of detection. other than the molecular/MIC so that it can be carried out even in the remote areasAIMTo compare the three conventional methods namely oxacillin screen agar, cefoxitin disk diffusion and oxacillin disk diffusion for the detection of MRSA, with the gold standard MIC determination. MATERIALS AND METHODSThis study was conducted in Pondicherry institute of medical sciences, under the department of microbiology between April 2011 and October 2011.75 strains of Staphylococcus aureus were included in the study, confirmed by slide and tube coagulase tests. Oxacillin screen agarBy direct colony suspension method, any growth considered as resistant to methicillin. Oxacillin disk diffusionAfter performing the test, a Zone of inhibitionless than 13, the strain is resistant. Cefoxitin disk diffusionA 30 microg disk was used zone of inhibition less than 20 is resistant. Minimum inhibitory concentration determination Agar dilution method was used to determine the Minimum Inhibitory Concentration (MIC). Organisms with MIC of 4 and above were considered as MRSA and those with MIC below 4 were considered MSSA. ResultsThe results as obtained by all the three methods were observed to be the same. Out of the 75 strains of Staphylococcus aureus, all the methods detected 19 MSSA (25 %) and 56 (75 %) MRSA. All the three methods showed 100 percent sensitivity and specificity. ConclusionSince all the tests showed 100 percent sensitivity and specificity we recommend that any two of the methods be used combined for accuracy and reliability.