Contrasting In Vitro Activity of Nitroxoline Against Multidrug-Resistant Escherichia coli and Klebsiella pneumoniae Isolates from Outpatients

Background/Objectives: Antimicrobial resistance is a major public health concern, particularly among multidrug-resistant (MDR) Enterobacterales. Nitroxoline is an older antibiotic with a unique mechanism of action that has not been widely used in recent decades. This study aimed to evaluate the in vitro activity of nitroxoline against Extended-Spectrum Beta-Lactamase (ESBL)-producing Escherichia coli (E. coli) and MDR Klebsiella pneumoniae (K. pneumoniae) isolated from urine samples of outpatients in Split-Dalmatia County, Croatia. Methods: Nitroxoline susceptibility was assessed using disk diffusion for E. coli (n = 26) and broth microdilution for K. pneumoniae (n = 33). Results: ESBL-producing E. coli isolates showed high susceptibility to nitroxoline, with 25 (96.2%) classified as susceptible. In contrast, K. pneumoniae isolates exhibited high minimum inhibitory concentration (MIC) values (range 16–128 mg/L; median 32 mg/L), indicating limited activity. Conclusions: Nitroxoline demonstrated strong in vitro activity against ESBL-producing E. coli but limited activity against MDR K. pneumoniae. Interpretation of the findings for K. pneumoniae is constrained by the lack of established clinical breakpoints.