# Synergistic Regulation of Bile Acid-Driven Nitrogen Metabolism by Swollenin in Ruminants: A Microbiota-Targeted Strategy to Improve Nitrogen Use Efficiency

## TL;DR

Swollenin improves nitrogen use efficiency in young goats by reshaping gut microbiota and bile acid metabolism.

## Contribution

Swollenin is shown to enhance nitrogen fixation and assimilation through microbiota modulation and bile acid transformation in ruminants.

## Key findings

- Swollenin supplementation increased feed conversion efficiency and altered gut microbiota composition in young goats.
- Swollenin boosted secondary bile acid biosynthesis and nitrogen-related gene abundance in the gut.
- Bile acid and nitrogen metabolic pathways were strongly correlated following Swollenin treatment.

## Abstract

Ruminant production plays a pivotal role in the global nitrogen cycle. Improving the nitrogen conversion efficiency in the hindgut of ruminants is of great significance for enhancing nutrient utilization efficiency and maintaining production performance. Recent research has demonstrated that bile acid metabolism in the ruminant intestine may indirectly regulate nitrogen utilization by modulating the composition and functional characteristics of microbial communities involved in nitrogen assimilation and ammonification processes. Meanwhile, the establishment of gut microbiota in young ruminants profoundly affects growth performance and long-term health. Consequently, targeted interventions on the gut microbiota–bile acid axis could offer a novel strategy to improve nitrogen use efficiency in ruminants. Previous studies have indicated that Swollenin reshapes the microbial community structure and alters the abundance and metabolic activity patterns of key metabolic enzymes during in vitro rumen fermentation. The present study further revealed that Swollenin can optimize the gut microbial composition of young goats. Specifically, it significantly improved feed conversion efficiency, promoted primary bile acids to secondary bile acids biotransformation, and notably increased the abundances of nitrogen fixation and assimilation genes. Furthermore, co-occurrence network analysis confirmed a strong correlation between the bile acid metabolic pathway and the nitrogen metabolic pathway.

The annual nitrogen loss from the livestock production sector poses a significant threat to the global natural environment. Therefore, it is urgent to focus on improving the nutrient utilization efficiency of ruminants and promoting the sustainable development of livestock production. Twelve 60-day-old Ganxi goats with similar body weights were selected and randomly assigned to two dietary treatment groups. The control group was fed only a basal diet, while the treatment group was supplemented with 32 mg/d of Swollenin. The experiment lasted for 30 days. At the end of the experimental period, the goats were euthanized, and their intestinal contents were collected, rapidly frozen, and stored at −80 °C for subsequent metagenomic and metabolomic analyses. In the Swollenin group, we observed changes in gut microbiota structure and significantly enhanced feed conversion efficiency compared to the control group. Notably, genera such as Bacteroides, Ruminococcus, and Bifidobacterium exhibited significantly higher abundance. Following Swollenin supplementation, the gene abundance associated with the secondary bile acid biosynthesis pathway in the intestinal tract of young goats was significantly higher. The levels of primary bile acids (BAs), including taurocholic acid, glycocholic acid, taurochenodeoxycholic acid, and glycochenodeoxycholic acid, were significantly lower, while the concentrations of secondary BAs such as ursodeoxycholic acid and deoxycholic acid were significantly higher. The abundance of nitrogen-fixing and nitrogen-assimilating genes in the gut of young goats in the Swollenin group was significantly higher. Furthermore, co-occurrence network analysis revealed a strong correlation between bile acid metabolism and nitrogen metabolism pathways. These results suggest that nutritional regulation may serve as a preventive strategy to optimize the symbiotic development of animals and their gut microbiota, ultimately improving nitrogen utilization.