On the same bacterium, without doing the molecular assays, are there methods to determine MRSA perfectly? I request for opinions and if possible reference supporting the same.
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Most certainly, depending on what you mean with perfectly. MRSA is a phenotypic classification to start with, and most easily grouped by testing for resistance to methicillin or oxacillin. Lately, cefoxitin is increasing in popularity due to better discrimination between mecA negative and positive isolates (see http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1698388/ for example)
Isolates can be further grouped by for example clindamycin resistance, which can roughly discriminate between CA-MRSA (clindamycin sensitive) and HA-MRSA (resistant). Note that this procedure may very well misclassify the MRSA strain, as the correlation is not 100% (see http://www.cvm.missouri.edu/vrs/2008posters/Hause.pdf for example). For accurate classification, genotypic classification is highly recommended.
Susceptibility testing may be done with either macrodilution, Etest or disc diffusion (or any other commercial method like VITEK). In our laboratory we use exclusively Etest for this purpose, and compare the MIC value with EUCAST breakpoints to determine susceptibility. Disc diffusion is more economical however, and equally precise, with the disadvantage of being limited to a R/I/S score and no true MIC value.
I hope I managed to answer your question satisfactorily,
Best regards,
Christer Malmberg
Recently we published the Dutch guideline on the laboratory detection of MRSA. Hopefully this will provide you some answers. See attachment.
Article Dutch guideline on the laboratory detection of methicillin-r...
Cefoxitin Disk Diffusion
An oxacillin screen plate with Mueller-Hinton agar (with 4% NaCL 6 µg/mL oxacillin) was once used for MRSA detection.-The said method can not be relied on to detect coagulase-negative staphylococci with oxacillin resistance.
A test for the surrogate marker of resistance such as cefoxitin- allows for a more accurate indication of oxacillin* resistance than the use of the said medication* itself. Reason being, cefoxitin induces greater expression of PBP2a in mecA genes present in some of the staphylococci in addition to being able to test for resistance.
The said method is also recommended by the CLSI, as indicated in the URL below
https://www.cdc.gov/mrsa/lab/index.html
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