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Abstract
Background
This study compared three non-molecular methods for the detection of methicillin-resistance directly from blood cultures containing Staphylococcus aureus: penicillin-binding protein (PBP) 2a latex agglutination (LA), PBP2a immunochromatographic assay (ICA) and MRSA chromogenic medium (CM).
Methods
Fifty methicillin-resistant S. aureus (MRSA) and 50 methicillin-susceptible S. aureus (MSSA) were seeded into blood-culture bottles. When isolates returned a positive signal, 5 mL of culture was added to serum separator tubes and centrifuged at 1,300 g for 10 min. The pellets were then used as the inoculum for the PBP2a LA, MRSA-CM and PBP2a ICA. The pure colony was used for PBP2a LA test, additionally.
Results
The respective sensitivities and specificities were 98 and 100% for PBP2a ICA, and 100 and 100% for MRSA-CM in direct detection of MRSA from positive blood culture. The results of PBP2a LA test using pure colony were entirely compatible with those by mecA gene PCR but the PBP2a LA test using the pellets directly isolated from positive blood culture showed sometimes ambiguous agglutination; its sensitivity and specificity were 78 and 100%, if ambiguous results were scored as negative, and were 90 and 92%, if ambiguous results were scored as positive, respectively.
Conclusion
For direct detection of MRSA in positive blood culture, MRSA-CM and PBP2a ICA provided excellent results. The PBP2a LA test using pure colony also gave excellent results but the PBP2a LA test by the direct method using pellet of positive blood culture was slightly less sensitive than the other two methods.
Figures and Tables
Table 1
Results of PBP2a latex agglutination, PBP2a immunochromatographic assay and MRSA-chromogenic medium for identifying MRSA directly from positive blood cultures
*When ambiguous agglutination were scored as negative or positive, the result of PBP2a LA were represented by number without or in bracket, respectively. †Used the broth after 4-hr incubation in enrichment broth. See methods. ‡Showed the positive results in testing after 8-hr incubation. Abbreviations: PBP2a, penicillin binding protein 2a; LA, latex agglutination; ICA, immunochromatographic assay; MRSA, methicillin-resistant Staphylococcus aureus; CM, chromogenic medium; P, positive; N, negative.
References
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