Efficacy of 7-benzyloxyindole and other halogenated indoles to inhibit Candida albicans biofilm and hyphal formation.

Certain pathogenic bacteria and yeast form biofilms on biotic and abiotic surfaces including medical devices and implants. Hence, the development of antibiofilm coating materials becomes relevant. The virulence of those colonizing pathogens can be reduced by inhibiting biofilm formation rather than killing pathogens using excessive amounts of antimicrobials, which is touted as one of the main reasons for the development of drug resistance. Candida albicans is an opportunistic fungal pathogen, and the transition of yeast cells to hyphal cells is believed to be a crucial virulence factor. Previous studies have shown that indole and its derivatives possess antivirulence properties against various bacterial pathogens. In this study, we used various indole derivatives to investigate biofilm-inhibiting activity against C. albicans. Our study revealed that 7-benzyloxyindole, 4-fluoroindole and 5-iodoindole effectively inhibited biofilm formation compared to the antifungal agent fluconazole. Particularly, 7-benzyloxyindole at 0.02 mM (4.5 μg ml ) significantly reduced C. albicans biofilm formation, but had no effect on planktonic cells, and this finding was confirmed by a 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) assay and three-dimensional confocal laser scanning microscopy. Scanning electron microscopy analyses revealed that 7-benzyloxyindole effectively inhibited hyphal formation, which explains biofilm inhibition. Transcriptomic analysis showed that 7-benzyloxyindole downregulated the expressions of several hypha/biofilm-related genes (ALS3, ECE1, HWP1 and RBT1). A C. albicans-infected Caenorhabditis elegans model system was used to confirm the antivirulence efficacy of 7-benzyloxyindole.