Indian Journal of Medical Microbiology Medknow Publications on behalf of Indian Association of Medical Microbiology ISSN: 0255-0857 EISSN: 1998-3646 Vol. 28, Num. 1, 2010, pp. 82-83

Indian Journal of Medical Microbiology, Vol. 28, No. 1, January-March, 2010, pp. 82-83

Correspondence

Validation of multiplex PCR for simultaneous detection, identification of methicilin resistant Staphylococcus aureus

*N Jindal

Department of Microbiology, Government Medical College, Amritsar - 143 001, India

Correspondence Address: *Department of Microbiology, Government Medical College, Amritsar - 143 001, India, neerjarajender@hotmail.com

Date of Submission: 10-Mar-2009
Date of Acceptance: 24-May-2009

Code Number: mb10027

PMID: 20061779

DOI: 10.4103/0255-0857.58744

Dear Editor,

I read with interest the article 'Validation of Multiplex PCR for simultaneous detection and identification of Methicilin-resistant Staphylococcus aureus' published in the October December 2008 issue of Indian Journal of Medical Microbiology. ^[1] Although the results validate the use of Multiplex PCR for MRSA detection, some clarifications are needed.

While using phenotypic method (oxicillin disk diffusion test) for detection of Methicillin resistance in Staphylococcus aureus and Coagulase-negative Staphylococcus species (CoNS) the authors interpreted all organisms (S. aureus as well as CoNS) giving an inhibition zone equal to or lesser than 10 mm as resistant to Oxacillin and organisms with a zone equal to or greater than 12 mm as susceptible; those with an inhibition zone of 11-12 mm as intermediate. With this interpretive criteria 10 of the 44 isolates of methicillin-susceptible coagulase negative species (MSCoNS), classified as oxacillin-sensitive phenotypically, yielded positive results in PCR method and the correlation between mec-A detection by PCR with disk susceptibility test in MSCoNS was reported to be only 77%. ^[1] As per CLSI guidelines {2005}, the interpretive standard for CoNS is different from that of S. aureus. According to these guidelines CoNS strains with inhibition zone diameter of less than 17 mm are taken as resistant and those greater than 18 mm are taken as susceptible to Oxacillin. ^[2] In the 19 ^th information supplement of CSLI (2009) oxicillin disk diffusion breakpoints have been deleted for CoNS and use of only cefoxitin disk has been recommened. ^[3] If authors had applied the CLSI criteria (of 19 ^th /15 ^th supplements) to detect methicillin resistance in 44 MSCoNS isolates of this study, some of these isolates which showed mecA gene in PCR might also had shown methicillin resistance in disk susceptibility test. This would have increased the correlation between mec A detection by PCR with disk susceptibility test in MSCoNS.

Therefore, strict adherence to CLSI guidelines, based on sound technical background, is important to follow in the clinical laboratory. This would help choose the right therapy and prevent the spread of methicillin-resistant CoNS.

References

1.	Rallapalli S, Verghese S, Verma RS. Validation of Multipkex PCR strategy for simultaneous detection and identification of Methicillin resistant Staphylococcus aureus. Indian J Med Microbiol 2008;26:361-4.  Back to cited text no. 1  [PUBMED]
2.	Clinical and Laboratory Standards Institute Performance standards for antimicrobial susceptibility testing; 15 th informational supplement. CLSI document: Wayne Pennsylvania: 2005. p. M100-S15.  Back to cited text no. 2
3.	Clinical and Laboratory Standards Institute. Performance standards for antimicrobial Susceptibility testing; 19 th informational supplement. CLSI document: Wayne Pennsylvania: 2009.  Back to cited text no. 3

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