COMPARISON OF DIFFERENT PHENOTYPIC METHODS OF DETECTION OF METHICILLIN RESISTANCE IN Staphylococcus aureus WITH THE MOLECULAR DETECTION OF mec-A GENE.
OBJECTIVE
To evaluate accuracy, cost-effectiveness and ease to perform different phenotypic methods i.e. Cefoxitin 30 microg disc, Oxacillin 1microg disc and Oxacillin agar screening plate (6microg/ml ) for early and accurate identification of MRSA by comparing with the detection of mec-A gene in our clinical isolates.


METHODS
A comparative study.


METHODS
Clinical samples submitted in the Clinical Microbiology Laboratory at Aga Khan University Hospital, Karachi, from 1st August to 31st October 2006.


METHODS
Out of 200 clinical samples, conventional Polymerase chain reaction (PCR) was done on 62 pure biochemically identified S. aureus isolates for mec-A gene detection. Phenotypic methods for detecting methicillin sensitivity (Cefoxitin 30 microg disc, Oxacillin 1 microg disc and Oxacillin agar screening plate) were also used according to the recommended incubation time, duration and temperature on the same isolates.


RESULTS
Out of 62 isolates of S. aureus, mec-A gene were detected (MRSA) in 32, whereas 30 were mec-A gene negative (MSSA). Cefoxitin disc and agar screening plate correctly identify all MRSA isolates with the sensitivity and specificity of 100%. Single isolate was false, positively detected as sensitive with Oxacillin 1microg disc, due to which, the sensitivity and negative predictive value of this method were reduced to 96.9% and 96.8% respectively, while positive predictive value and specificity remained 100%.


CONCLUSIONS
Comparing different phenotypic methods for MRSA screening in routine microbiology laboratory, Cefoxitin disc and Oxacillin agar screening has better sensitivity and specificity comparative to Oxacillin disc. However, Cefoxitin disc can be preferred especially for small laboratories because it is easy to perform, do not require special technique and media preparation is consequently more cost-effective.
Zeeshan
Mohammad
M
Department of Pathology and Microbiology, The Aga Khan University Hospital, Karachi, Pakistan.
Jabeen
Kausar
K
Khan
Erum
E
Irfan
Seema
S
Ibrahim
Saira
S
Parween
Zeba
Z
Zafar
Afia
A