Is the Cefazolin Inoculum Effect in Staphylococcus aureus Cause for Concern?

Up to one quarter of MSSA were resistant to cefazolin when tested under high-inoculum conditions, raising worries about treatment failure in high-risk infection.

Treatment of methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia consists of anti-staphylococcal penicillins (AP) or cefazolin (CZ). Although CZ has shown better tolerability with similar treatment outcomes, some MSSA demonstrate the “inoculum effect,” where raising the inoculum above the standard concentration results in CZ resistance due to beta-lactamase hydrolysis. CZ treatment failure in such settings has been documented in endocarditis cases.

Dingle and colleagues tested clinical MSSA isolates from several U.S. and Canadian microbiology laboratories for presence of the inoculum effect. A positive inoculum effect was defined in MSSA with a CZ minimum inhibitory concentration (MIC) ≥16 mg/L under high-inoculum testing and a M…

Reviewing Author

George Sakoulas, MD

Disclosures

Consultant/Advisory BoardParatek Pharmaceuticals; Nestle Corp.; AbbVie; Shionogi

Speaker’s BureauAllergan/AbbVie; Paratek Pharmaceuticals; Nestle Corp.

RoyaltiesUpToDate

Editorial BoardsAntimicrobial Agents and Chemotherapy; Open Forum Infectious Diseases