Mitigating Candida albicans virulence by targeted relay of pulcherriminic acid during antagonistic biofilm formation by Bacillus subtilis

The antagonistic biofilms formed by probiotic Bacilli may significantly mitigate persistent strains of <i>Candida albicans</i>, which are often involved in severe oral, vulvovaginal or systemic infections in humans. Here, we report on a spatiotemporal antagonistic activity mediated through pulcherriminic acid (PA) production by biofilm-forming <i>B. subtilis</i>, which is subsequently transported to the extracellular environment and binds ferric iron to form red-coloured pigment pulcherrimin. We show that the pulcherrimin building-up is targeted towards the <i>C. albicans</i> macrocolony via <i>B. subtilis</i> biofilm branching and successive PA relay. Furthermore, biofilm-forming <i>B. subtilis</i> cells demonstrate robust hyphal colonization that results subsequent eradication of <i>C. albicans</i>. Besides, extracted pulcherrimin mitigates <i>C. albicans</i> biofilm formation and yeast-to-hyphae (Y-H) transition. We further find that the mode of hyphal colonization could be regulated via SpoA-SinI pathway, while pulcherrimin relay is connected to surfactin production machinery. We assume therefore that the pulcherrimin relay for iron hijacking, in parallel to the direct hyphal colonization by biofilm-forming Bacilli, may provide a promising platform for developing therapeutic concepts to overcome antibiotic persistence in pathogenic yeasts.