Bile salts and fat absorption

Self-reinoculation with fecal flora changes microbiota density and composition leading to an altered bile-acid profile in the mouse small intestine

BACKGROUND

The upper gastrointestinal tract plays a prominent role in human physiology as the primary site for enzymatic digestion and nutrient absorption, immune sampling, and drug uptake. Alterations to the small intestine microbiome have been implicated in various human diseases, such as non-alcoholic steatohepatitis and inflammatory bowel conditions. Yet, the physiological and functional roles of the small intestine microbiota in humans remain poorly characterized because of the complexities associated with its sampling. Rodent models are used extensively in microbiome research and enable the spatial, temporal, compositional, and functional interrogation of the gastrointestinal microbiota and its effects on the host physiology and disease phenotype. Classical, culture-based studies have documented that fecal microbial self-reinoculation (via coprophagy) affects the composition and abundance of microbes in the murine proximal gastrointestinal tract. This pervasive self-reinoculation behavior could be a particularly relevant study factor when investigating small intestine microbiota. Modern microbiome studies either do not take self-reinoculation into account, or assume that approaches such as single housing mice or housing on wire mesh floors eliminate it. These assumptions have not been rigorously tested with modern tools. Here, we used quantitative 16S rRNA gene amplicon sequencing, quantitative microbial functional gene content inference, and metabolomic analyses of bile acids to evaluate the effects of self-reinoculation on microbial loads, composition, and function in the murine upper gastrointestinal tract.

RESULTS

In coprophagic mice, continuous self-exposure to the fecal flora had substantial quantitative and qualitative effects on the upper gastrointestinal microbiome. These differences in microbial abundance and community composition were associated with an altered profile of the small intestine bile acid pool, and, importantly, could not be inferred from analyzing large intestine or stool samples. Overall, the patterns observed in the small intestine of non-coprophagic mice (reduced total microbial load, low abundance of anaerobic microbiota, and bile acids predominantly in the conjugated form) resemble those typically seen in the human small intestine.

CONCLUSIONS

Future studies need to take self-reinoculation into account when using mouse models to evaluate gastrointestinal microbial colonization and function in relation to xenobiotic transformation and pharmacokinetics or in the context of physiological states and diseases linked to small intestine microbiome and to small intestine dysbiosis. Video abstract.

Dioxin-Produced Alteration in the Profiles of Fecal and Urinary Metabolomes: A Change in Bile Acids and Its Relevance to Toxicity

This study investigated dioxin-induced changes in metabolomes in pubertal rat excrement. The administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or restricting dietary intake (pair-fed group) markedly altered the metabolomic profile including lipids, hormones, and vitamins in the urine and feces. TCDD caused an increase in the fecal chenodeoxycholic acid and taurocholic acid content and in urinary adrenaline and 17β-estradiol, while the urinary melatonin level was reduced by TCDD. These changes were not observed in the pair-fed group. In accordance with the elevated level of fecal bile acids, TCDD reduced the intestinal expression of the apical sodium-dependent bile salt transporter, which plays a role in resorbing bile acids from the bile duct. In addition, CYP7A1, a rate-limiting enzyme for bile acid biosynthesis, was attenuated by TCDD treatment, although TCDD induced hepatic CYP8B1, an enzyme essential for cholic acid synthesis. Supplying cholic acid or chenodeoxycholic acid to TCDD-exposed rats tended to restore the TCDD-produced reduction in serum triglycerides, whereas no similar trend was observed in wasting syndrome and lipid accumulation in the liver. These results suggest that: 1) TCDD alters the circulating levels of bile acids and hormones via a mechanism distinct from an attenuation in dietary intake, although the majority of TCDD-induced changes in nutrient contents in the excrement is due to a reduction in food intake; and 2) TCDD facilitates the excretion of bile acids and disrupts their biosynthesis, resulting in the disturbance of lipid homeostasis.

Evaluation of coagulation in dogs with partial or complete extrahepatic biliary tract obstruction by means of thromboelastography

OBJECTIVE

To characterize in vitro coagulation status in a cohort of dogs with extrahepatic biliary tract obstruction (EHBO) and to evaluate these patients for hypercoagulability by means of thromboelastography.

DESIGN

Prospective cohort study. Animals-10 dogs with EHBO and 19 healthy control dogs.

PROCEDURES

Partial or complete EHBO was confirmed via exploratory celiotomy. Venous blood samples were collected for evaluation of prothrombin time (PT) and activated partial thromboplastin time (APTT); fibrinogen and D-dimer concentrations; protein C and antithrombin activities; and factor VII, VIII, and XI coagulant activities in plasma as well as thromboelastography in whole blood. Thromboelastography variables were measured from the thromboelastography tracing, and a coagulation index was calculated. Thromboelastography results were compared with those of healthy control dogs previously evaluated by the same laboratory.

RESULTS

Hypercoagulability was diagnosed in all dogs with EHBO on the basis of a high coagulation index. Thromboelastography variables, including maximal amplitude, α-angle, and coagulation index, were significantly higher, and K (clot formation time) and R (reaction time) were significantly lower in these dogs than in control dogs. All dogs with EHBO had PT and APTT within respective reference ranges. Plasma D-dimer and fibrinogen concentrations were above reference ranges in 8 and 7 dogs, respectively, and protein C and antithrombin activities were below reference ranges in 3 and 1 dogs, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

In vitro hypercoagulability was commonly detected in dogs with naturally occurring EHBO. The traditional view of EHBO as a disease that causes hypocoagulability may need to be reconsidered.

Influence of the diet on the composition of faecal bile acids in rats

1. [24-14C]cholic acid was given orally to rats receiving three different diets, a semi-synthetic diet (D7) with and without 20% cellulose, and a commercial, pelleted diet. The labelled metabolites excreted in the faeces in the 8 days following administration of the labelled cholic acid were chromatographically separated, and tentatively identified by means of thin-layer chromatography.

2. In all three dietary groups the faecal bile acids mainly consisted of unconjugated and 7α-dehydroxylated metabolites. In rats receiving pellets or diet D7with cellulcse, there was a higher percentage of monohydroxy-monoketo-cholanoic acids than in those receiving diet D7alone. Dihydroxy-cholanoic acids, present in approximately the same ratio as were the 3α- and 3β-hydroxy isomers, were excreted by all the rats in the three dietary groups.