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CORRESPONDENCE
Year : 2012  |  Volume : 30  |  Issue : 2  |  Page : 252-253
 

Comparative evaluation of Latex agglutination method with other phenotypic methods for detection of Methicillin-resistant Staphylococcus aureus


Microbiology Department, Sri Guru Ram Das Institute of Medical Sciences and Research, Vallah, Sri Amritsar, Punjab, India

Date of Submission10-Jan-2012
Date of Acceptance07-Mar-2012
Date of Web Publication28-May-2012

Correspondence Address:
L Oberoi
Microbiology Department, Sri Guru Ram Das Institute of Medical Sciences and Research, Vallah, Sri Amritsar, Punjab
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0255-0857.96727

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How to cite this article:
Kaur R, Oberoi L, Aggarwal A. Comparative evaluation of Latex agglutination method with other phenotypic methods for detection of Methicillin-resistant Staphylococcus aureus. Indian J Med Microbiol 2012;30:252-3

How to cite this URL:
Kaur R, Oberoi L, Aggarwal A. Comparative evaluation of Latex agglutination method with other phenotypic methods for detection of Methicillin-resistant Staphylococcus aureus. Indian J Med Microbiol [serial online] 2012 [cited 2019 Jun 19];30:252-3. Available from: http://www.ijmm.org/text.asp?2012/30/2/252/96727


Dear Editor,

Despite the introduction of effective antimicrobial agents and improvements in hygiene, Staphylococci have persisted as important hospital and community pathogens. The treatment of infections caused by these organisms has become problematic due to the development of methicillin resistance. Methicillin-resistant Staphylococcus aureus Scientific Name Search  (MRSA) strains harbour the mecA gene which encodes a modified penicillin-binding protein (PBP2a) having low affinity for methicillin and all β-lactam antibiotics. Resistance to this antibiotic implies resistance to all β-lactam antibiotics leaving few therapeutic options to treat such severe infections. So, rapid and accurate identification of MRSA is required to immediately start the appropriate antimicrobial therapy and to avoid the spread of these strains. [1]

Several studies have reported a number of phenotypic methods for detection of methicillin resistance in S. aureus. [2],[3] But these methods are inadequate as the expression of resistance is subject to environmental and conditional expression of a PBP2a antigen. Discrepancies in detection have lead to an adverse effect on patient management, thereby highlighting the importance of accuracy in detection. Our study was undertaken to evaluate the efficacy of the Latex agglutination test (Slidex MRSA), to detect MRSA and to compare it with the other phenotypic methods.

The study included 97 strains of Staphylococcus aureus isolated from 400 clinical samples in SGRD hospital, a rural tertiary care hospital, from June 2009 to June 2011. All strains were identified and characterized as per standard recommendations. [4] The isolates were tested for methicillin resistance by disc diffusion (DD) tests using oxacillin (1 μg) and cefoxitin (30 μg), oxacillin screen agar (HiMedia) and Chrom agar (HiMedia, HiChrome Me Re Sa Agar, M1674). Minimum Inhibitory Concentration (MIC) was determined by E test using oxacillin strips; Latex agglutination method (Slidex MRSA, BioMerieux) was done to detect the presence of mecA gene product PBP2a. The resistance was interpreted using CLSI criteria. [5] Known positive control MRSA (ATCC 43300) was included in each set.

Out of 97 S. aureus strains, 53 (56.64%) were identified as methicillin resistant including 03 with intermediate resistance and 44 (45.36%) as methicillin sensitive by oxacillin DD method. Using the cefoxitin DD method, 46 (47.42%) were identified as MRSA and 51 (52.58%) as MSSA. Seven isolates showed discrepancy between oxacillin and cefoxitin DD method. Results of oxacillin screen agar were in concordance with the cefoxitin DD method detecting 45 (46.39%) MRSA. Chrom agar detected 45 MRSA including 11 false positives, which were identified as MSSA by other methods. Out of 46 isolates resistant to both cefoxitin and oxacillin DD method, 41 isolates had an MIC between 8 and 16 μg/ml, while the rest 05 strains, which were resistant with cefoxitin disc diffusion, had an MIC between 2 and 4 μg/ml. All 46 MRSA strains detected by the cefoxitin DD method showed positive reaction for the presence of PBP2a by Latex agglutination [Table 1].
Table 1: Detection of MRSA by different methods

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A number of methods, as recommended by CLSI, are being used for the detection of MRSA. These methods except for PCR and Latex agglutination methods are prone to errors due to heterogeneous nature of methicillin resistance and dependence on environmental conditions. In this study, we attempted to evaluate six phenotypic methods for the detection of MRSA. The oxacillin DD method was found to be inferior to the cefoxitin DD method with sensitivity and specificity of 93.18% and 77.36%, while the cefoxitin DD method was found to be highly sensitive (100%) and specific (96.23%) when compared with the latex agglutination test. Similar results were quoted by several other studies. [2],[3]

The seven additional strains which were resistant by oxacillin DD, but sensitive by cefoxitin DD had MIC values <2 mcg/ml. These strains probably are BORSA (Borderline resistant strains) that hyper produce beta lactamase and while they appear oxacillin resistant, they do not possess the usual genetic mechanism for such resistance. This was corroborated by the fact that all the isolates that were resistant to oxacillin but sensitive to cefoxitin were negative for PBP2a detection by Latex agglutination.

Sensitivity and specificity of oxacillin screen agar were 97.73% and 96.23%, respectively. The results of oxacillin screen agar were comparable with that of the cefoxitin DD method. Similar findings were reported by a study done by Tiwari et al. [2] Chrom agar was found to be less sensitive (77.27%) and less specific (79.25%). Results of E test for MIC (oxacillin) were not so accurate detecting only 41 MRSA (42.27%) giving a sensitivity of 90.91% and specificity 98.11%. Strains possessing the mecA gene are either heterogeneous or homogeneous in their expression. Heteroresistant strains may show lower expression resulting in MICs that appear susceptible [Table 2].
Table 2: Comparative evaluation of various phenotypic methods used for MRSA detection

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The accurate and early determination of methicillin resistance is of key importance in the prognosis of infections caused by S. aureus. Although the phenotypic methods for detecting MRSA are cost effective, they are dependent upon environmental conditions. Hence, detecting the mecA gene or its product is recognized as a gold standard for detection of MRSA. However, the use of PCR assays is limited to reference labs, the Latex agglutination method though expensive is rapid and independent of environmental variations and can be the best predictor to detect MRSA. The cefoxitin disc diffusion method shows maximum comparability of results with Latex agglutination than other methods tested. So, these two methods when combined together can be the best option to detect MRSA in resource constraint clinical settings.

 
 ~ References Top

1.Barber M. Methicillin Resistant Staphylococci. J Clin Pathol 1961;14:385-93.  Back to cited text no. 1
    
2.Tiwari HK, Sapkota D, Das AK, Sen MR. Assessment of different methods to detect Methicillin Resistant Staphylococcus aureus. Southeast Asian J Trop Med Public Health 2009;40:801-6.  Back to cited text no. 2
    
3.Velasco D, Mar Tomas MD, Cartelle M, Beceiro A, Perez A, Molina F, et al. Evaluation of different methods for detecting methicillin (oxacillin) resistance in Staphylococcus aurues. J Antimicrob Chemother 2005;55:379-82.  Back to cited text no. 3
    
4.Collee JG, Miles RS, Watt B. tests for identification of bacteria. In: Mackie and Mc Cartney Practical Medical Microbiology. 14 th ed. Philadelphia: Churchill Livingstone; 1996. p. 131-49.  Back to cited text no. 4
    
5.Clinical Laboratory Standards Institute (CLSI). Performance standards for antimicrobial susceptibility testing; Eighteenth Informational Supplement. Zone Diameter Interpretive Standards and Equivalent Minimal Inhibitory Concentration (MIC) Breakpoints for Staphylococcus spp. 2008 M100-S18;28:48.  Back to cited text no. 5
    



 
 
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