Roles of Indole and Its Derivative in Modulating E. coli–Candida albicans Biofilm Formation

Candida albicansis the causal agent of invasive candidiasis, which might be lethal in immunocompromised patients. Biofilm formation is considered a key virulence factor ofC. albicansand is associated with its elevated resistance to antifungals.C. albicansand bacteria likeE. coliare frequently found to form mixed biofilms on biotic or abiotic surfaces, rendering them more refractory to existing antifungals. To investigate howE. coliendogenous indole interplaying with exogenous IAA exerts modulatory effects on dual-species biofilm withC. albicans, anE. colistrain deficient in the indole biosynthetic genetnaAwas constructed, and the enzyme TnaA inhibitor was administered to block the indole production inE. colimonoculture and/orE. coli–C. albicansdual culture. Phenotypic assay revealed that indole deficiency attenuatedE. colimono-species biofilm by 12% (tnaA∆ versus WTE. coli), and the lack of indole in theE. colicell-free culture filtrate abolished the ability to promoteC. albicansbiofilms, evidenced by the fact that the treatment with WTE. coliculture supernatants exhibited a 1.7-fold promotive effect, while treatment withtnaA∆ displayed no significant difference from the broth control towardsC. albicansbiofilms. Furthermore, impairedE. coliindole production might disruptE. coli–C. albicansbiofilm, as examined by confocal laser scanning microscopy (CLSM). Moreover, indole-3-acetic acid (IAA) was found to exhibit more potent biofilm-modulatory activity than indole by CLSM imaging with dual biofilms of WTE. coli–C. albicans, in contrast to those ofE. coli tnaA∆–C. albicanspost-supplemented with exogenous IAA. This study provides evidence for indole as a signaling molecule mediating bacterial–fungal communication during mixed-biofilm formation. Indole and its derivatives, particularly in combination with existing antifungals, have potential in the development of anti-biofilm strategies to eradicate refractory fungal infections.