Enrichment of bile salt hydrolase-producing bacteria mediated by tetracycline resistance genes is associated with intestinal barrier damage in Rana chensinensis tadpoles.

Tetracycline (TET) is a pervasive contaminant in aquatic environments, yet how it reshapes gut microbiota composition and function to influence bile acid (BA) profiles and intestinal health remains poorly understood. In this study, Rana chensinensis tadpoles at Gosner stage 26 (Gs26) were exposed to environmentally relevant concentrations of tetracycline hydrochloride (10 and 100 μg/L) until metamorphic climax Gs38 and examined using a multi-pronged approach integrating histological analysis, intestinal targeted BA metabolomics, and fecal metagenomic sequencing. Our results showed that TET exposure disrupted intestinal barrier integrity in a dose-dependent manner, as evidenced by reduced enterocyte height, widened intercellular spaces, and irregular nuclear morphology. Metagenomic profiling revealed that TET treatment significantly enriched tetracycline resistance genes (e.g., tet(Q), tet(T), tetA(46), tetA(60)), which was accompanied by an increased abundance of bile salt hydrolase (BSH)-producing bacteria, including Bacteroides, Parabacteroides, and Vibrio. This microbial shift was accompanied by enhanced BA deconjugation, as reflected by a significantly increased ratio of unconjugated to conjugated BAs (p < 0.01). Notably, the enhanced deconjugation activity was paralleled by a marked accumulation of the hydrophobic and cytotoxic BA, chenodeoxycholic acid (CDCA) (p < 0.001), which was accompanied by a 73.9% reduction in total BA levels - a pattern that may reflect Farnesoid X Receptor (FXR)-mediated negative feedback regulation of hepatic BA synthesis, although this pathway was not directly examined. Furthermore, elevated CDCA levels were associated with intestinal histopathological damage. Collectively, these findings suggest a potential mechanistic cascade in which TET-induced enrichment of antibiotic resistance genes is associated with the expansion of BSH-active microbiota, together with disrupted BA homeostasis and compromised intestinal barrier function in amphibians. Causal relationships within this cascade await functional validation. Our study highlights the hidden ecological risks of antibiotic contamination in aquatic ecosystems and underscores the need for further molecular investigations into the signaling pathways involved.