Methicillin-susceptible Staphylococcus aureus (MSSA)

Methicillin-susceptible Staphylococcus aureus (MSSA) refers to strains of the Gram-positive bacterium Staphylococcus aureus that are susceptible to penicillinase-resistant penicillins, such as oxacillin and nafcillin (StatPearls, 2023). While S. aureus is a common commensal of the human skin and nares, it is also a potent opportunistic pathogen capable of causing a wide spectrum of diseases ranging from minor skin infections to life-threatening systemic conditions like bacteremia, endocarditis, and necrotizing pneumonia (CDC, 2022). Unlike its resistant counterpart MRSA, MSSA lacks the mecA gene which encodes an altered penicillin-binding protein (PBP2a), allowing standard beta-lactam antibiotics to effectively bind and inhibit its cell wall synthesis (PubMed, 2021). Treatment typically involves beta-lactamase-resistant penicillins or first-generation cephalosporins, which remain the gold standard for serious MSSA infections due to their superior bactericidal activity compared to vancomycin (NIH, 2023). Monitoring for the emergence of resistance and managing the risk of toxin-mediated syndromes, such as toxic shock syndrome, are critical components of clinical management (PubMed, 2020).

Other names

Methicillin-sensitive Staphylococcus aureusMSSAStaphylococcus aureus (methicillin-susceptible)

Antibiotics targeting MSSA primarily inhibit cell wall synthesis by binding to penicillin-binding proteins (PBPs), inhibit protein synthesis by targeting the 50S or 30S ribosomal subunits, or disrupt the bacterial cell membrane (StatPearls, 2023; Merck Manual, 2022).