Prevalence of Cefazolin Inoculum Effect in Methicillin-Susceptible Staphylococcus aureus Isolates from Healthcare Systems

Abstract

Introduction: The principal treatment for methicillin-susceptible Staphylococcus aureus (MSSA) infections comprises of first-generation cephalosporins, specifically Cefazolin (CFZ). Despite advantages such as better tolerability, convenient dosing, and reduced nephrotoxicity, treatment with cefazolin poses a significant concern due to the existence of the cefazolin inoculum effect (CzIE). CzIE is defined as reduced effectiveness of cefazolin against bacterial isolates when the bacterial inoculum is large, leading to a decrease in the antibiotic’s ability to inhibit bacterial growth. The general criteria for CzIE is defined as cefazolin minimum inhibitory concentration (MIC) values of ≥ 16 µg/mL at the high inoculum and ≤ 8µg/mL at the standard inoculum. The presence of the CzIE in MSSA isolates is linked to the beta-lactamase gene (BlaZ) expression and is associated with clinical treatment failure. Methods: In this investigation, we evaluated the CzIE on a cohort of S. aureus isolates derived from cystic fibrosis, pediatric, and bone and joint infections. MIC tests were performed using micro-broth dilution assays based on CLSI (Clinical & Laboratory Standards Institute) guidelines. Briefly, antibiotic was serially diluted in a broth medium across a 96-well microtiter plate, and bacterial inoculum was added to each sample well. After incubating the plates at 37°C for 24 hours, the MIC was determined as the lowest concentration of antibiotic that completely inhibited visible bacterial growth. Results: A cohort of 97 isolates were tested with cefoxitin to identify potential MSSA isolates. Isolates exhibiting MIC values ≤ 4µg/mL were classified as MSSA and were taken further for CFZ antibiotic susceptibility testing at low (10^4) and high (10^8) inoculum concentrations. We identified 32 MSSA isolates, among which only 3 demonstrated the CzIE (high inoculum MIC ≥ 16 µg/mL; low inoculum MIC ≤ 8 µg/mL). The MSSA isolates that did not exhibit CzIE were characterized as CzIE-negative. The CzIE-positive isolates will undergo further testing for the β-lactamase enzyme (BlaZ) expression evaluation via the Nitrocefin chromogenic assay. Additionally, the BlaZ gene cascade will be identified through genome sequencing. Conclusion: This study highlights the need for a more extensive collection of MSSA isolates, given the considerable variation in CzIE frequency across different geographical regions.No embargoAcademic Major: Pharmaceutical Science