ID Corner - Methicillin-sensitive Staphylococcus aureus (MSSA) Bacteremia Pearls

Question #1: What does “methicillin-sensitive” mean? It means that the isolate is susceptible to beta-lactam antibiotics with antistaphylococcal activity such as oxacillin, nafcillin, cefazolin and others. In contrast, methicillin-resistant Staphylococcus aureus (MRSA) is resistant to most beta-lactams due to a modification of the penicillinbinding proteins that results in lower avidity. Methicillin is actually not commercially available in the United States but when resistance was described in 1961, methicillin was used to treat S. aureus infections. So the terminology remains in use because of its historic role.