Metabolomics reveal dynamic host responses in lipid, amino acid, and energy metabolism after acute exposure of gut microbiota in germ‐free rats

Abstract

Background Integrating germ-free and gnotobiotic models have advanced the understanding of how gut microbiome impacts host physiology. High-throughput omics technologies further provide microbial and metabolite targets for translational purposes. Yet, limited knowledge is known about the immediate effects of natural microbial colonization on host lipid, amino acid, and energy metabolism. Methods To address this, germ-free Sprague Dawley rats were acclimated with the gut microbiota by cohousing them with microbiota-intact rats (1:1 ratio) for 10 days. Characterization of the microbiota composition in the feces was quantified by 16S ribosomal RNA gene sequencing. Cecal metabolites related to primary microbial and secondary host-bacterial co-metabolisms (e.g., tricarboxylic acid cycle, anaerobic fermentation, amino acids) were analyzed by H1-NMR based metabolomics. Targeted metabolomic analysis of serum eicosanoids and leukotrienes were examined by liquid chromatography-mass spectrometry. Gas chromatography was used for analysis of hepatic total fatty acids. Results Natural colonization of the gut microbiota by coprophagy was sufficient to reconstitute the gut microbiota in conventionalized germ-free (GFC) rats. Among the colonizing bacteria, Akkermansia spp. was one of the top abundant genera in GFC rats and thus, could be one of the main contributors to the elevation of inosine and its metabolites, hypoxanthine and xanthine, in the cecal contents. Additionally, catabolism of choline to trimethylamine and dimethylamine was diminished, but there were no changes in creatine levels. GFC rats also exhibited more aromatic (e.g., tyrosine) and acidic (e.g., glutamate) amino acids, but less carboxamide-chained (e.g., glutamine, asparagine) amino acids. Increase of lactate levels suggested a favorability for anaerobic fermentation even though elevated succinate and fumarate indicated functional tricarboxylic acid cycle activity. These changes in energy and amino acid metabolism were paralleled in the serum of GFC rats. Analyzing hepatic lipid fractions revealed an increase of both ω6 and ω3 fatty acids in GFC rats, including their eicosanoid and leukotriene derivatives in circulation (e.g., leukotriene 4; 15S-hydroxy-5Z,8Z,11Z13E,17X-eicosapentaneoic acid). Conclusion This study demonstrates that acute exposure to the gut microbiota causes dynamic metabolic changes in the gut-liver axis.