Prevalence of β-Lactamase Genes and the Inoculum Effect with Cefazolin Among Bloodstream Isolates of Methicillin-Sensitive Staphylococcus Aureus

Background: Some methicillin-susceptible Staphylococcus aureus (MSSA) strains produce β-lactamases that efficiently hydrolyze cefazolin at high bacterial inoculums.  This phenomenon, known as the inoculum effect, has been associated with cefazolin failure in case reports of MSSA infections. 

Objective: To define the prevalence of β-lactamases and the inoculum effect among MSSA bloodstream infections. 

Methods: Consecutive MSSA bloodstream isolates were collected from participating microbiology laboratories in metropolitan Atlanta. The blaZ gene was identified by Southern blot hybridization or polymerase chain reaction.  Amplified blaZ gene segments were sequenced to distinguish the 4 β-lactamase types (A-D).  Broth microdilution was performed at a standard (10⁵ cfu/mL) and high (10⁷ cfu/mL) inoculum.  The inoculum effect was defined as a MIC of ≥ 16 µg/mL at 10⁷ cfu/mL.

Results: Eighty unique MSSA bloodstream infections were identified between 1/1/2010 and 7/23/2010 from 5 hospitals.  The mean patient age was 41 years (range 3 weeks–83 years); 63% were male, and 49% were African American.  Co-morbidities were present in 46%, and 29% had central venous catheters at the time of the index blood culture.  46% developed deep-seated MSSA infections. 

The blaZ gene was present in 61/80 strains (76%), and 56/61 (92%) have been sequenced: 38% produced type A β-lactamase, 23% type B, 38% type C, and 2% type D.  The MIC₉₀ at standard inoculum was 1.0 mg/mL for nafcillin and 2.0 for cefazolin.  At high inoculum, the MIC₉₀ was 1.0 mg/mL for nafcillin and 4.0 for cefazolin. 

Six (8%) strains demonstrated the inoculum effect with cefazolin.  All strains expressing the inoculum effect also carried the type A β-lactamase gene.  Overall, 6/21 (29%) isolates producing type A demonstrated the inoculum effect.  Neither age, gender, race, nor deep-seated infection was associated with the inoculum effect.

Conclusions: Over two-thirds of MSSA bloodstream isolates collected in Atlanta carried the blaZ gene; most often types A and C. However, the inoculum effect with cefozolin was present in only 8% of the MSSA isolates, all of which carried the type A β-lactamase gene.  The efficacy of cefazolin for serious MSSA infections caused by isolates exhibiting the inoculum effect requires further study.