MethicillinResistant

Methicillin resistance refers to bacteria that are resistant to methicillin and related beta-lactam antibiotics. The best known example is methicillin-resistant Staphylococcus aureus (MRSA), but resistance to methicillin is also found in other Staphylococcus species and, less commonly, in other genera. In clinical microbiology, methicillin resistance is often described by the presence of mecA or mecC genes that enable the bacteria to withstand many beta-lactam drugs.

The most common mechanism involves the mecA or mecC gene carried on the staphylococcal cassette chromosome mec (SCCmec). These genes encode an altered penicillin-binding protein, PBP2a, with low affinity for beta-lactams, allowing cell wall synthesis to continue in the presence of methicillin and related antibiotics. Detection typically uses cefoxitin susceptibility testing as a surrogate, with molecular tests identifying mecA or mecC.

Clinically, MRSA and other methicillin-resistant organisms can cause a range of infections, including skin and soft tissue infections, bloodstream infections, pneumonia, and bone or joint infections. They can be acquired in healthcare settings (healthcare-associated) or in the community (community-associated). Colonization without symptoms is common and can precede infection; transmission occurs mainly through close contact and contaminated surfaces, highlighting the importance of infection control.

Diagnosis relies on laboratory susceptibility testing and molecular methods. Treatment avoids beta-lactams for resistant strains; options include vancomycin, linezolid, daptomycin, and, in some cases, ceftaroline or newer agents, guided by species and site of infection.

Prevention focuses on infection-control practices such as hand hygiene, contact precautions in healthcare settings, environmental cleaning, and, in some contexts, decolonization strategies to reduce transmission.