mecApositive

mecApositive is a descriptor used in clinical microbiology to indicate that a bacterial isolate carries the mecA gene. Most commonly, mecApositive refers to Staphylococcus species—especially Staphylococcus aureus—but it can also describe coagulase-negative staphylococci. The mecA gene is located on the staphylococcal cassette chromosome mec (SCCmec) and encodes penicillin-binding protein 2a (PBP2a), a variant with low affinity for beta-lactam antibiotics. As a result, mecApositive strains are typically resistant to methicillin and to most beta-lactams, a phenotype used to define MRSA when the isolate is S. aureus.

Detection and typing involves molecular and phenotypic methods. PCR targeting mecA is a common molecular approach, often complemented by phenotypic tests such as cefoxitin disk diffusion or oxacillin MIC to signal resistance. PBP2a immunoassays can confirm the presence of the resistance protein. Typing of SCCmec elements (types I–VIII) helps describe epidemiology and infection-control implications, distinguishing hospital-associated from community-associated strains. In some regions, mecA homologs such as mecC can also confer resistance and may require expanded testing to avoid false negatives.

Clinical considerations focus on antibiotic choice and infection control. mecApositive status indicates resistance to most beta-lactams and guides therapy toward non–beta-lactam agents, guided by susceptibility data and guidelines. Infections caused by mecApositive Staphylococcus aureus require appropriate antimicrobial therapy and measures to limit transmission. The term also applies to mecApositive coagulase-negative staphylococci, which are clinically significant in device-related infections and in immunocompromised patients.