Indole Derivatives Biosynthesis in Bifidobacterium longum subsp. infantis and the Tryptophan Substrate Availability.

The metabolic processes of Bifidobacterium longum subsp. infantis, an early coloniser of the human gut, are essential for gut health, mainly due to the production of indole derivatives from tryptophan. This study investigates the capacity of B. infantis ATCC 15697 to biosynthesise indole-3-lactate (ILA), indole-3-acetate (IAA), and indole-3-carboxaldehyde (I3CA) and the regulatory effects of substrate availability on these pathways. The tryptophan catabolic profile of B. infantis ATCC 15697 under a non-growing but metabolically active state was investigated. Through HPLC-PDA and LC-MS analyses, we confirmed for the first time the production of IAA and I3CA by B. infantis ATCC 15697. The results revealed a dose-dependent relationship between tryptophan availability and the production of indole derivatives, highlighting the nutrient-driven effect of these metabolic pathways. By integrating genomic analysis with metabolic profiles, we proposed potential pathways underlying the biosynthesis of IAA and I3CA from tryptophan. These findings enhance our understanding of the role of B. infantis ATCC 15697 in human health, with ILA, IAA, and I3CA contributing to immune modulation and gut health. We also provide a platform for using B. infantis ATCC 15697 as a biocatalyst for the biosynthesis of beneficial indole derivatives through whole-cell bioconversion, which was further demonstrated in B. infantis ATCC 25962 and ATCC 15702. Future in vivo studies will help clarify the impact of these metabolites on the gut environment and inform dietary and probiotic strategies for enhancing indole derivatives production.