Biphasic bactericidal activity of nitroxoline against Acinetobacter baumannii assessed by Raman-DIP

The increasing antimicrobial resistance (AMR) of Acinetobacter baumannii presents a challenge to clinical management and underscores its role as a critical pathogen in refractory urinary tract infections (UTIs). Nitroxoline has gained renewed interest as a potential therapeutic option. Raman deuterium stable isotope probing (Raman-DIP), which allows for the analysis of bacterial metabolic activity, has shown promise as a tool for assessing antimicrobial efficacy. The aim of this study was to evaluate the bactericidal activity of nitroxoline against A. baumannii and the potential of Raman-DIP to assess this activity. Thirty-four A. baumannii isolates were collected from patients with UTI. Minimum inhibitory concentrations (MICs), minimum bactericidal concentrations (MBCs), optimal bactericidal concentrations (OBCs), and time-killing curves were determined. Raman-DIP evaluated nitroxoline's effects on bacterial metabolism and survival, using the C-D ratio (C-D to the sum of the C - H and C - D band intensities) as a metabolic activity metric. Nitroxoline demonstrated potent antimicrobial activity against A. baumannii, with MIC_50/90 and MBC_50/90 values of 2/2 and 2/4 mg/L, respectively. Notably, it showed biphasic bactericidal characteristics with an OBC_50/90 value of 4/8 mg/L. Raman-DIP revealed a decrease in the C-D ratio with an increase in nitroxoline concentration, indicating reduced metabolic activity. An inverse correlation was observed between bacterial survival and the C-D ratio at concentrations above the OBC. These findings underscore the necessity to optimize dosing regimens for enhancing the efficacy of nitroxoline. Raman-DIP may serve as an effective tool for investigating the effect of nitroxoline on bacterial metabolism, thereby informing its clinical applications.