Investigation of the cefazolin inoculum effect in blood culture-isolated methicillin-susceptible Staphylococcus aureus strains: A Japanese multicenter study

Background: Cefazolin inoculum effect (CInE) is a microbiological phenomenon where the MIC of cefazolin against methicillin-susceptible Staphylococcus aureus (MSSA) strains increases with higher bacterial volumes.
Method: We retrospectively investigated the prevalence and characteristics of the CInE among MSSA strains isolated from blood cultures at three Japanese hospitals. The collected isolates were screened for blaZ using PCR, and the cefazolin minimum inhibitory concentration (MIC) for the blaZ-positive MSSA isolates was measured at standard and high inoculum volumes. CInE-positive MSSA strains were defined as those with a cefazolin MIC ≥16 μg/mL at 107 CFU/mL and ≤8 μg/mL at 105 CFU/mL. In these blaZ-positive strains, we performed blaZ typing and tested a modified nitrocefin-based rapid examination to detect the CInE.
Results: We collected 329 MSSA strains isolated from blood cultures. Of these, 96 (29.2%) were positive for the blaZ gene, with the following genotypes: type A (15, 15.6%), type B (3, 3.1%), type C (77, 80.2%), type D (0, 0.0%), and non-type (1, 1.0%). Among 96 blaZ-positive MSSA isolates, 11 exhibited the CInE, all of which harbored blaZ type A. The rapid nitrocefin test detected CInE positivity with high sensitivity (100%), specificity (94.1%), and diagnostic accuracy (94.8%).
Conclusion: This study highlighted the low prevalence of CInE-presenting MSSA isolates in Japan. When the cefazolin MIC is ≥1 μg/mL or the penicillin G MIC is ≥0.25 μg/mL, the rapid nitrocefin test may be useful for considering the CInE in patients with high bacterial volume MSSA infections.