Comparative evaluation different laboratory methods for detection of methicillin resistant staphylococcus aureus

Abstract

Background: 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 areas. Aim: To 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 Methods: This study was conducted at Department of Microbiology, at Patna Medical College and Hospital, Patna from March 2020 to September 2020. 75 strains of Staphylococcus aureus  were included in the study, confirmed by slide and tube coagulase tests. Oxacillin screen agar: By direct colony suspension method, any growth considered as resistant to methicillin. Oxacillin disk diffusion:After performing the test, a Zone of inhibition< 13, the strain is resistant. Cefoxitin disk diffusion:A 30 microg disk was used zone of inhibition < 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. Results: The 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. Conclusion: Since all the tests showed 100 percent sensitivity and specificity we recommend that any two of the methods be used combined for accuracy and reliability.

Keywords: MRSA; MIC; Oxacillin; Cefoxitin.