Evaluation of Antimicrobial Activity of Gallic Acid, Quercetin-3-D-Glucuronide, and Apigenin Against Gram-Negative Uropathogens: A Novel Approach to Urinary Tract Infection Therapy

Urinary tract infections (UTIs), particularly catheter-associated UTIs (CAUTIs), represent a significant clinical problem due to the predominance of Gram-negative uropathogens, their ability to form biofilms, and the increasing prevalence of antimicrobial resistance, which together reduce the effectiveness of conventional antibiotic therapy. This study aimed to evaluate the antimicrobial activity of selected natural plant-derived compounds against clinical Gram-negative uropathogens isolated from CAUTIs. The antibacterial effects of gallic acid, quercetin-3-D-glucuronide, and apigenin were assessed against Escherichia coli, Pseudomonas aeruginosa, and Proteus mirabilis, including both ciprofloxacin-susceptible and -resistant strains. Antimicrobial activity was determined using the broth microdilution method, followed by quantitative assessment of bacterial viability based on colony-forming unit (CFU) enumeration. Gallic acid exhibited the strongest concentration-dependent inhibitory activity, reducing bacterial viability by up to 2–3 log10 CFU across all tested species. Quercetin-3-D-glucuronide demonstrated moderate antibacterial effects with a predominantly bacteriostatic profile, resulting in a partial but consistent reduction in CFU counts. In contrast, apigenin showed only weak effects on bacterial viability under the applied experimental conditions. None of the tested compounds achieved complete bacterial eradication. These findings indicate that gallic acid and quercetin-3-D-glucuronide possess inhibitory activity against Gram-negative uropathogens, including antibiotic-resistant strains, supporting their potential use as adjunctive agents targeting bacterial persistence in UTIs rather than as standalone antimicrobials. In the present study, the viability of planktonic bacterial cells was assessed; however, future studies should focus on evaluating the direct impact of the tested compounds on biofilm structure and biofilm formation dynamics.