Exogenous indole promotes florfenicol tolerance in <i>Edwardsiella tarda</i>

Bacterial metabolism is important for antibiotic resistance and tolerance. However, the impact of indole on bacterial metabolism and antibiotic efficacy has not been fully elucidated. In this study, we investigated the effect and specific mechanism of exogenous indole on the antibiotic susceptibility of <i>Edwardsiella tarda</i>, a common pathogen in freshwater and marine fish farming. We found that exogenous indole promoted <i>E. tarda</i> tolerance to the antibiotic florfenicol, and reprogrammed the <i>E. tarda</i> metabolome. A total of 108 metabolites were detected, including 66 differential metabolites that regulate various metabolic pathways, such as the tricarboxylic acid (TCA) cycle and nucleotide metabolism. Exogenous indole disrupted the TCA cycle in <i>E. tarda</i> by increasing the intracellular NADH contents and activating the respiratory chain to increase the reactive oxygen species levels, thereby increasing the intracellular Fe²⁺ content to activate the Fenton reaction, which in turn promotes the oxidative stress response. Furthermore, indole inhibited antibiotic entry into the cell and activated efflux pumps to reduce the intracellular antibiotic content, ultimately promoting antibiotic tolerance. In vivo, exogenous indole compromised the ability of florfenicol to protect fish survival and eliminate pathogenic bacteria. These results shed light on the metabolic changes induced by indole and suggest future directions for addressing antibiotic tolerance and clinical infections of <i>E</i>. <i>tarda</i> in aquaculture. This study serves as a reminder of the adverse effects of combining antibiotics with metabolites in aquaculture.