methicillinsensitive

Methicillin-sensitive, in microbiology, describes microorganisms that are inhibited by methicillin, a beta-lactam antibiotic. The term is often used in reference to Staphylococcus aureus, where strains are categorized as methicillin-susceptible (MSSA) or methicillin-resistant (MRSA). Methicillin-sensitive organisms are typically susceptible to a range of beta-lactam antibiotics, including isoxazolyl penicillins such as nafcillin and oxacillin.

Mechanism and resistance

Methicillin works by binding to penicillin-binding proteins (PBPs) involved in cell wall synthesis. In methicillin-sensitive strains, these PBPs are effectively inhibited, leading to bacterial death. Methicillin resistance, most commonly in S. aureus, usually arises from acquisition of the mecA gene, which encodes an altered PBP (PBP2a) with reduced affinity for beta-lactams. This enables the bacterium to continue cell wall synthesis despite the presence of methicillin and related drugs.

Testing and interpretation

Laboratories determine methicillin susceptibility using surrogate tests such as cefoxitin or oxacillin disc diffusion, or molecular methods to detect mecA. Results classify isolates as methicillin-susceptible or methicillin-resistant. Because methicillin itself is unstable in testing conditions, cefoxitin is a preferred surrogate for detecting mecA-mediated resistance.

Clinical relevance

MSSA infections are typically treatable with beta-lactam antibiotics effective against methicillin-susceptible strains, such as nafcillin, oxacillin, or flucloxacillin, depending on local guidelines and patient factors. MRSA requires alternative therapies, including non-beta-lactam agents. Distinguishing MSSA from MRSA informs antibiotic choice and helps guide infection control measures.