Indole decreases the virulence of enterotoxigenic Escherichia coli.

Indole signaling is being explored as one of the promising targets for anti-virulence therapy against antibiotic-resistant pathogens. Indole has been reported to influence key virulence-related phenotypes, particularly motility and biofilm formation, in various bacterial pathogens. This study aimed to investigate the impact of indole on enterotoxigenic Escherichia coli (ETEC). The results revealed that indole decreased both swimming motility and biofilm formation in ETEC across concentrations ranging from 0 to 1000 µM, with a maximum reduction of 75% in swimming motility and 47% in biofilm formation at the highest concentration. RT-qPCR analysis revealed that motility-related genes (i.e., flagellin fliC and flagellar transcription activator flhC) were downregulated by 5-fold and 2.5-fold, respectively. Additionally, the stress response sigma factor gene rpoS and the heat-stable enterotoxin gene estP were downregulated by 2-fold and 1.7-fold, respectively. Competitive ELISA showed that indole decreased secreted heat-stable enterotoxin levels by more than 10-fold, which is consistent with the decreased estP mRNA levels. Finally, indole decreased the virulence of ETEC towards the invertebrate infection model Galleria mellonella, resulting in a 1.8-fold increased survival after 5 days. These findings indicate that indole signaling controls ETEC virulence, making it an interesting target for anti-virulence therapy.