Methicillin-Susceptible Staphylococcus Aureus Bacteremia: Cefazolin in Prime But Nafcillin Not Ready for an Exit

摘要

The antistaphylococcal penicillins (ASPs), such as nafcillin or oxacillin, have long been regarded as first-line treatments for methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia. These agents are effective but do not have favorable pharmacologic and toxicity profiles. Cefazoline, on the other hand, is better tolerated and easier to use but has been relegated, at least until recently, as an alternative due to concern about clinical failures linked to the cefazolin inoculum effect (CIE). The CIE is an in vitro phenomenon in which isolates are susceptible at standard inocula (10~5 CFU/mL) but become less susceptible (>4-fold increase in MIC) or resistant (MIC >16 mug/mL) at high inocula (10~7 CFU/mL). This phenomenon is usually mediated by a type A beta lactamase that hydroly-ses cefazolin but not the ASPs. Despite being described in experimental animal models and case reports of treatment failures since the 1970s, the clinical relevance of the CIE has been a matter of contention.