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I have encountered a blood culture isolate of staph. aureus having the following properties: 1, it gives a zone size of OX1 from 11-14 & Cefoxitin zone size as 27mm on Muller Hinton agar; 2, it gives Etest MIC of OX as 1.5 (sensitive) and OX1 disc as 0mm zone size (resistant)on 2%NaCl Muller Hinton agar plate. The isolate is found to be MecA gene PCR negative but the isolate on 2%NaCl Muller Hinton agar is PBP2 latex positive. I want to ask if there is an induction of methicillin resistance; if there is discrepancy between MecA gene PCR result and PBP2 latex testing; what should I report this isolate: MRSA or MSSA?
Thanks a lot!
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There is clearly a discrepancy between the PBP2a latex test and the mecA PCR test which needs to be resolved. Also, you note a discrepancy between oxacillin disk diffusion testing when the isolate is tested on Mueller-Hinton with and without salt. I think the following tests are the minimum tests required to resolve these discrepancies: 1) repeat of the mecA PCR, and 2) oxacillin MIC by broth microdilution. If the mecA PCR is negative on repeat, then the PBP2a latex is likely a false-positive. The oxacillin MIC by broth microdilution is helpful because this isolate may be salt dependent. In the broth microdilution test, salt is added to the Mueller-Hinton broth, so if the oxacillin resistance is salt-dependent then the isolate would be oxacillin resistant by broth microdilution. If the isolate is mecA negative but oxacillin resistant by broth microdilution, then it is likely that the isolate probably has a mechanism of oxacillin resistance other than mecA. Such an isolate would still be reported as oxacillin resistant. Mechanisms of oxacillin resistance other than mecA are rare but do occur. Usually, there is low-level oxacillin resistance. These mechanisms could be hyper beta-lactamase production or alteration of a native PBP, or other mechanisms that has not been characterized.
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