RAPID DETECTION OF METHICILLIN RESISTANT STAPHYLOCOCCI FROM THE NASAL SCREENING SWABS BY MANNITOL SALT AGAR WITH CEFOXITIN AND OXACILLIN DISKS

Methicillin Resistance Staphylococci

Abstract

Objective: To assess the reliability of Manitol salt agar (MSA) for directly identifying Methicillin Resistant Staphylococcus aureus (MRSA) and Methicillin Resistant Coagulase negative Staphylococci, (MRCoNS) in nasal swabs for screening purposes using cefoxitin and oxacillin disks.

Study Design: Descriptive and Quasi-experimental

Place and duration of Study: The study was done in the two surgical units of Combined Military Hospital, Rawalpindi and all the samples were processed at the Department of Microbiology, Armed Forces Institute of Pathology, Rawalpindi during July 2007.

Material and Methods:  A total of eighty four duplicate swabs were taken from the anterior nares of various staff members of the two surgical units and were directly inoculated on Mannitol salt agar with Cefoxitin disc 30µg (MSAFOX) and oxacillin disc 1µg (MSAOX). All the samples were simultaneously inoculated on blood and MacConkey agar for conventional testing, using standard conditions, and confirmed as MRSA or MRCoNS by oxacillin disk diffusion technique. The staphylococcal isolates were later confirmed as MRSA/ MRCoNS by polymerase chain reation (PCR) for mecA gene analysis.

Results: There were 45 staphylococci which revealed mec A gene (40 MRCoNS and 5 MRSA) by PCR. Both the disks with MSA effectively identified the methicillin resistance. MSA with cefoxitin could identify 40 methicillin resistant staphylococci (35 MRCoNS and 5 MRSA) where as MSA with oxacillin could identify 39 methicillin resistant staphylococci (34 MRCoNS and 4 MRSA). There was no significant difference between the two disks in sensitivity, specificity, positive and negative predictive values and overall efficacy of the procedures.  

Conclusion: MSA with cefoxitin 30µg and oxacillin 1µg appear to be highly accurate, easy to perform and beneficial for quick and reliable detection of methicillin resistant staphylococci from the nasal carriers in a routine microbiology laboratory.