An easy-to-use, rapid and inexpensive method to determine methicillin resistance in Staphylococcus aureus

Abstract

Objective: We aimed to report a new turbidimetric method to identify methicillin resistance in S. aureus strains just two hours after identification of the microorganism, and to analyze diagnostic and discrimination abilities of this new method.Methods: A total of 319 S. aureus isolates were included. Identification of bacteria was done by the colony morphology, and conventional biochemical methods. The methicillin resistance of the S. aureus strains was studied as indicated in Clinical Laboratory Standards Institute 2009. The turbidimetric method we developed is based on different growth rates of S. aureus in two media, with or without oxacillin. The growth rates of MRSA and MSSA are similar in normal media, however the MRSA grows significantly faster in the media containing oxacillin. Therefore, after 2 hours of incubation, the difference of turbidity produced by bacteria is less in MRSA, and more in MSSA. The absorbance of the microplates were measured before incubation, and at 2nd and 3rd hours of incubation. The “absorbance rate” was calculated for each bacteria and the bacteria were classified as MRSA or MSSA based on the absorbance rate. Results: All MRSA and MSSA strains were correctly discriminated via our turbidimetric method, when an absorbance rate of 1.900 was taken as cut-off value. The new method could diagnose MRSA with 100% specificity and 100% sensitivity in just two hours. Conclusion: The turbidimetric method is a rapid, easy and cheap method that does not require any specific equipment. It can be easily performed in every microbiology laboratory.