Detection of methicillin/oxacillin resistance in staphylococci

Methicillin/oxacillin-resistant staphylococci are heterogeneous in their expression of resistance to beta-lactam agents and the test conditions have a major effect on the expression and therefore the detection of resistance. Conflicting recommendations regarding the most reliable method for routine use are partly related to differences between strains and there may be a variable interaction between the factors affecting the expression of resistance, including the agent tested, the medium, the NaCl concentration, the inoculum, temperature and period of incubation and the reading of endpoints. 'Borderline' resistant strains may have altered PBPs or be penicillinase hyperproducers, and these can be difficult to distinguish from resistant strains that carry the mecA gene. Recommended methods for MIC and disc diffusion testing are described, although it is unlikely that any single method will detect all resistant strains. Some rapid and/or automated methods are also available, including latex agglutination techniques for the detection of PBP2a. The gold standard method for the detection of resistance mediated by mecA is PCR, which is most commonly used as a reference method at present.