Pseudomonas aeruginosa is a multidrug-resistant pathogen known for chronic infections, mainly due to biofilm formation. This study aimed to explore the potential repurposing of 5-fluorouracil (5-FU), an anticancer drug, to treat P. aeruginosa infections. Firstly, we investigated the inhibitory effects of 5-FU on bacterial growth using the microdilution method. Secondly, the impact of 5-FU on biofilm formation and disassembly was assessed via biofilm biomass measurements with the crystal violet staining method and confocal microscopy analyses. Lastly, the potential synergy between 5-FU and the antibiotics gentamicin and meropenem was evaluated using a checkerboard assay. Results revealed that 5-FU inhibited bacterial growth in a dose-dependent manner, with 100% inhibition observed at concentrations of 25 µg/mL and higher. Also, 70% and 100% reductions in biofilm biomass were demonstrated at concentrations of 12 and 100 µg/mL, respectively. Controversy, these higher concentrations unexpectedly increased biofilm biomass in pre-formed biofilms. Synergistic interactions were observed between 5-FU and gentamicin in both growth inhibition (FICI 0.31) and biofilm inhibition (ZIP 14.1), while no synergy was found with meropenem. These findings highlight the potential of 5-FU as an adjunctive therapy for P. aeruginosa infections, especially in combination with gentamicin. However, further research is required to address 5-FU limitations against mature biofilms.

Pseudomonas aeruginosa is an opportunistic pathogen in which motility, biofilm formation, and stress adaptation contribute to virulence. Drug repurposing represents a practical strategy for identifying compounds that influence these processes. In this study, the effects of 5-fluorouracil (5-FU) on motility, extracellular DNA (eDNA) production, and gene expression were examined in P. aeruginosa PAO1. Swimming, swarming, and twitching motility assays were performed in the presence of increasing concentrations of 5-FU. Transcriptional responses of motility, rhamnolipid, and DNA repair-associated genes were evaluated using quantitative real-time PCR. eDNA levels were quantified using fluorescence-based assays and visualized by confocal laser scanning microscopy. Exposure to 5-FU resulted in concentration-dependent reductions in swimming, swarming, and twitching motility. These changes were associated with downregulation of multiple genes involved in flagellar and type IV pili function, including motA , flhA , fliD , pilA , and pilI , and reduced expression of lasB and rhlAB . At 24 h, eDNA levels were decreased relative to untreated controls, whereas at 48 h, higher concentrations of 5-FU were associated with increased eDNA accumulation and upregulation of several DNA repair genes, including xthA , nth , recJ , sbcB , and eddB . These results indicate the multifaceted effects of 5-FU on important virulence factors of P. aeruginosa.