The antimycotic 5-fluorocytosine is a virulence inhibitor of uropathogenic <i>Escherichia coli</i> and eradicates biofilm-embedded bacteria synergizing with β-lactams

Biofilm can enhance antibiotic tolerance in bacteria, making treatment of biofilm-associated infections in clinical settings a significant challenge. 5-Fluorocytosine (5-FC), an FDA-approved drug mostly used as an antifungal, can hinder biofilm formation and production of virulence factors in Gram-negative bacteria. In this study, we tested 5-FC on nine uropathogenic <i>Escherichia coli</i> (UPEC) strains plus a fecal isolate. Our data indicated that 5-FC reduced curli fiber gene expression and inhibited virulence factors in UPEC strains. Unlike what was observed in other microorganisms, 5-FC antivirulence and antibiofilm properties were unaffected by either growth temperature or the medium pH, which might prove critical in urinary tract infection (UTI) treatment. Additionally, 5-FC impaired the expression of various UPEC virulence factors, including secreted toxins and type I and P fimbriae, thus leading to decreased UPEC adherence to bladder epithelial cells and improved survival of host cells. Finally, we found that a combination of 5-FC with β-lactams, but not other classes of antibiotics, significantly lowered the viability of bacteria in preformed biofilms. Despite a small set of pathogenic <i>E. coli</i> strains and an <i>in vitro</i> infection model, our findings strongly suggest that 5-FC might be a possible candidate as an antivirulence agent, particularly in a synergistic approach with β-lactam antibiotics.