Incidence, Risk Factors and Clinical Outcomes of Inoculum Effect against β-lactams Including Ampicillin-Sulbactam and Cefazolin among  Methicillin-Susceptible Staphylococcus aureus(MSSA) Strains Isolated from Patients  with Bacteremia

Background Some type of staphylococcal β-lactamases could destroy also β-lactams other than penicillin, including cefazolin (Cz) and ampicillin-sulbactam (SAM) especially in the presence of higher inoculum, which called inoculum effect (InE). We aimed to investigate the incidence, risk factors, simple definiton methods and clinical implication of InE against different β-lactams including SAM and Cz in MSSA strains isolated from patients with bacteremia. Methods All patients with MSSA bacteremia between 2016 and 2018 at our hospital were included. MSSA strains isolated from blood cultures of those patients were tested via the disk diffusion and broth microdilution methods at standard (10 ⁵ /µl) and high (10 ⁷ /µl) inoculum concentrations. The presence and type of β-lactamases were determined by nitrocefin and PCR plus DNA sequencing. InE was defined as a 4-fold or greater increase in MIC values at high inoculum concentrations. The geometric mean MIC and zonal diameter were compared between the strains with and without InE or β-lactamases. Patient data were obtained retrospectively from the hospital database, and risk factors for Cz IE or SAM IE or mortality were analyzed via univariate analysis. Results A total of 52 patients with MSSA bacteremia were included in the study. Eighty-five percent of the 52 MSSA strains were β-lactamase positive, all of which were classified as type A. Thirteen (25%), 20 (38.5%), and 2 (3.8%) of the 52 MSSA strains were InE against Cz, SAM, and ceftriaxone, respectively, and no InE was observed against cefuroxime or cefotaxime. The β-lactams most affected by the high inoculum were SAM and Cz, with 2.94- and 2.20-fold increases in the MIC, respectively. Compared with MIC testing, a cefazolin zone diameter of < 28 mm was found to be 100% sensitive to both standard and high inoculum to define the CzInE. Among the β-lactams tested, only penicillin G, SAM and Cz were significantly affected by β-lactamase positivity. The mortality rate in patients infected with MSSA strains showing SAMInE and treated with SAM was significantly greater than that in those not treated with SAM (37.5% vs 68%, p = 0.044, OR 7.8, 95% CI 1.23–49.68). Conclusions Among Cz, cefuroxime, cefotaxime and ceftriaxone, SAM is the β-lactam most affected by the type A β-lactam of MSSA strains, followed by Cz, and this effect becomes more prominent with increasing inoculum. A cefazolin zone diameter of < 28 mm could be used as a screening method to define Cz InE. SAM treatment of patients infected with MSSA strains harboring SAMInE may increase mortality.