Microbial Tryptophan Metabolism Activates Host Lysosomal Activity to Facilitate Lipid Breakdown and Ameliorate Hepatic Steatosis

Lysosomes are central to lipid metabolism, yet how gut microbiota-derived metabolites regulate lysosomal function to influence host lipid homeostasis remains unknown. Here, we identify an evolutionarily conserved mechanism in which bacterial tryptophan metabolism activates lysosomal activity to promote lipid breakdown. By developing a lysosomal-responsive lipid reporter in C. elegans to screen for bacterial metabolic states that modulate host lipid storage, we discover that E. coli tryptophan catabolism via tryptophanase TnaA induces lysosomal lipid chaperone LBP-8, driving lipid mobilization. Moreover, tryptophan metabolites enhanced lysosomal acidification and degradation capacity, while genetic disruption of lysosomal regulators reversed these effects. Strikingly, bacterial tryptophan metabolism further promoted mitochondrial {beta}-oxidation through lysosomal lipase activity. This pathway was conserved in mammalian hepatocytes, where E. coli-derived tryptophan metabolites enhance lysosomal function and reduce lipid accumulation. In high-fat diet mice, restoring gut bacterial tryptophan metabolism alleviated hepatic steatosis. Our work uncovers microbiota-regulated lysosomal activation as a critical axis in lipid homeostasis, highlighting its potential as a therapeutic target for metabolic disorders linked to lysosomal dysfunction. SignificanceWe uncover a conserved mechanism by which microbial tryptophan metabolism enhances lysosomal function to maintain host lipid homeostasis. Specifically, we demonstrate that bacterial tryptophan catabolism--via the enzyme TnaA--promotes lysosomal acidification, proteolytic capacity, and structural remodeling in C. elegans, driving lipid breakdown through the lysosomal chaperone LBP-8. This activation boosts mitochondrial {beta}-oxidation and reduces lipid storage. Importantly, the same pathway operates in mammalian hepatocytes and in a high-fat diet mouse model, where restoring bacterial tryptophan metabolism markedly alleviates hepatic steatosis. Our findings bridge microbial metabolism and lysosomal dynamics, offering fresh insights into host-microbe crosstalk and metabolic regulation. HighlightsTnaA-mediated bacterial tryptophan catabolism promotes lipid mobilization via lysosomal chaperone. Bacterial tryptophan metabolites boost lysosomal function, lipid breakdown, and mitochondrial {beta}-oxidation. Conserved microbiota-lysosome-lipid axis from worms to mammalian liver. Restoring gut bacterial tryptophan metabolism alleviates hepatic steatosis in high-fat diet mice.