Dietary L-Tryptophan Modulates the Structural and Functional Composition of the Intestinal Microbiome in Weaned Piglets

1The intestinal microbiota is now recognized to have broad biological effects on the health and growth of both humans and animals. The intestinal microbiota confers several physiological effects to the host by metabolizing dietary nutrients, producing SCFAs from indigestible carbohydrates, synthesis of amino acids and vitamins, regulating metabolism, and affecting the maturation of the intestinal immunity. These events are fundamental to the integrity of intestinal mucosal barrier function and health of the host. Importantly, the diversity and composition of the microbial community are influenced by diet, age, stress, lifestyle, and various environmental factors, which in turn would affect, directly or indirectly, nutrient metabolism, immune responses, and intestinal homeostasis, thus constituting a cross-talk between intestinal microbiota and host. In support of the critical role of the intestinal microbiota, dysbiosis (defined as changes in the intestinal bacterial ecosystem) is associated with intestinal disorders, such as ulcerative colitis, Crohn’s disease, metabolic disease, and malnutrition. Thus, maintenance and restoration of host intestinal microbiota homeostasis has shown beneficial effects in clinical patients or experimental animals.

2Dietary protein intake can alter the gut microbiota, thus contributing to structural and functional changes of the host intestine. However, there is a scarcity of information regarding impacts of specific amino acids on the gut flora composition and intestinal health in animals. Besides serving as a substrate for protein synthesis, Trp is metabolized to a variety of biologically active compounds, such as serotonin, tryptamine, indoles, kynurenines, and nicotinamide adenine dinucleotide (NAD+), which in turn regulate inflammation, immune response, and neurological functions. Intestinal commensal bacteria can actively metabolize Trp to produce IAA and indole, two critical metabolites that enhance both intestinal mucosal barrier integrity and immune function via activation of the aryl hydrocarbon receptor (AhR) or related signaling pathways. These findings highlight a crosstalk between gut bacteria and host intestinal homeostasis in which Trp is a functional amino acid that regulates host physiology and metabolism. reported that formula feeding to piglets between day 2 and 21 of age resulted in reduced microbial diversity as well as alterations in Trp metabolism and immune response in the colon. In addition, in vitro studies have shown that Trp enhances the abundances of tight junction proteins and modulates protein turnover in intestinal porcine epithelial cells. Based on the foregoing, the present study was conducted to test the hypothesis that dietary Trp supplementation can alter intestinal microbial composition and diversity, improve intestinal mucosal barrier function, and activate AhR signaling in the large intestine of weaned piglets.

3A total of 126 piglets (Landrace × Yorkshire, initial body weight 7.6 ± 0.04 kg) were randomly assigned into one of three groups (0%, 0.2%, and 0.4% supplemental Trp groups). Each treatment group consisted of six pens with seven piglets per pen. The corn-soybean based diets were formulated to meet nutritional requirements of piglets throughout phase I (7–11 kg BW) (Supplementary Table S1) and phase II (11–25 kg BW) (Supplementary Table S2), which contained 0.25% and 0.21% Trp, respectively. L-Alanine was used to formulate isonitrogenous diets as previously described. The amounts of supplemental Trp used in our study were based on a previous study showing that dietary supplementation with 0.23% Trp enhanced feed intake and growth performance in weaned piglets. In the present study, all piglets were weaned at 24 days of age and the first day of weaned was recorded as day 0 of the experimental period. At the end of the 28-day trial, 24 piglets (eight from each treatment) were sacrificed by exsanguination. The tissues and the large-intestinal contents were collected and placed in liquid nitrogen and then were stored at -80°C for later analysis.

4The total genomic DNA of cecal bacteria was extracted by using a DNA Kit (Qiagen, Hilden, Germany) according to the manufacturer’s instructions. The integrity of DNA was assessed by agarose gel electrophoresis, and then the genomic DNA was used as a template for PCR amplification. The 16S RNA V3–V4 gene region was amplified by using the primers F341 and R806. PCR amplification was carried out in a 25 μl reaction system and the condition of PCR amplification was: initial pre-denaturation at 94°C for 4 min, denaturation at 98°C for 10 s, renaturation at 58°C for 30 s, elongation at 72°C for 2 min, 30 cycles, and then the last elongation step at 72°C for 10 min. The 16S rRNA gene was sequenced on the Illumina HiSeq PE250 sequencing platform at the Realbio Genomics Institute (Shanghai, China) according to the manufacturer’s instructions.

5The sequences raw data obtained from the Illumina HiSeq platform were quality-filtered and demultiplexed by using the Quantitative Insights into Microbial Ecology (QIIME) version 1.8.0-dev, as described by the previous study. Illumina raw data were treated to remove low quality reads. The reads with an average quality score of no less than 25 and with a length of 400–440 nt were retained by SOAPaligner (v 2.21). USEARCH version 7.1 software was used for reads cluster and cutoff (based on 97% similar identity) for OTUs, and chimeric sequences were identified and removed using UCHIME. Alpha diversity (chao1, observed species, Shannon index, and Simpson index) was assessed by MOTHUR v.1.35.0. Beta diversity was calculated based on unweighted uniFrac distances by QIIME. An unweighted unifrac PCoA based on OTUs was performed to provide the overview of the microbial diversity and composition in the cecum of pigs fed different doses of tryptophan. Analysis of molecular variance (AMOVA) was performed to compare the difference between different treatment groups. Besides, PCA was used to determine the similarity between the composition of samples under the same condition. LEfSe analysis was performed at the genus level to determine the microbes that had a significant effect on the division of the samples, and the more intuitive heatmap analysis (Kruskal–Wallis test) was used to indicate similarities and differences in community composition of the samples. PICRUSt was applied to predict the functions of microbiota communities based on the 16S rRNA gene library composition.

6All the sequencing data were submitted to the National Center for Biotechnology Information GenBank Sequence Read Archive database under accession number SRP 150032.

7Contents of the cecum and colon were prepared for SCFAs determination as previously described. Briefly, 1.0 g contents of hindgut were homogenized with 4 ml double distilled water and centrifuged at 12,000 rpm for 10 min at 4°C. Then the supernatant fluid (3 ml) was diluted by 0.6 ml 25% (w/v) metaphosphoric acid solution, the mixture was vortexed and centrifuged at 12,000 rpm for 10 min at 4°C after 30 min incubation on ice. The collected supernatant was filtered through a 0.45 μm polysulfone filter and injected into a HP 6890 Series Gas Chromatograph (Hewlett Packard, PA, CA, United States) for the determination of SCFAs.

8Trp-derived metabolites in cecal and colonic contents were extracted with cold 50% methanol (methanol: H2O = 1:1) according to the previous study with modifications. Briefly, 100 mg samples were weighed and mixed with 700 μl cold 50% methanol. The mixture was homogenized and centrifuged at 12,000 rpm for 10 min at 4°C. The supernatant was transferred to new tubes, mixed with 600 μl cold 50% methanol, and then centrifuged at 12,000 rpm for 10 min at 4°C. The supernatant fluid was mixed with equal volume of working solution (0.12% benzoic acid: 50% methanol = 1:1). The mixture was mixed thoroughly and centrifuged at 12,000 rpm for 10 min at 4°C. The supernatant fluid was filtered through a 0.45 μm polysulfone filter and injected into a Waters e2695 high-performance liquid chromatograph system (Waters, United States) with in-line pre-column derivatization with OPA.

9Colonic tissues were homogenized in liquid nitrogen for protein extraction and the protein abundance was determined with the use of the Western blotting technique as previously described. Equal amounts of protein (40 μg) were separated on SDS-PAGE gels, and then proteins were transferred onto PVDF membranes (Millipore, Billerica, MA, United States). The membranes were incubated with 5% skimmed-milk solution in Tris-buffered saline containing 0.05% Tween-20 (TBST) for 30 min at 25°C, and then were incubated with one of the indicated primary antibodies overnight at 4°C. After that, the membranes were washed with TBST three times and then were incubated with horseradish peroxidase (HRP)-conjugated secondary antibody for 1 h. The protein bands were developed by an enhanced chemiluminescence kit (Applygen Technologies Inc., Beijing, China) using the ImageQuant LAS 4000 mini system (GE Healthcare). Quantification of band density was determined by using the Quantity One software (Bio-Rad Laboratories).

10Small pieces of colonic tissues were homogenized in liquid nitrogen. About 30–50 mg of the resulting tissue powders was used for total RNA extraction using the Trizol reagent (Takara, Takara Biomedical Technology in Beijing, China), followed by reverse transcription using the High Capacity cDNA Archive kit (Takara) according to the manufacturer’s protocols. The whole process was carried out under the RNase free condition, and the obtained cDNA and remaining RNA were stored at -80°C for later analysis. SYBR Premix Ex Taq II (TaKaRa) and the ABI-Prism 7500 Sequence Detection System (Applied Biosystems) were used for Real-time PCR. The mRNA level of GAPDH was used as the internal control and the 2-ΔΔCT method was used to determine the fold changes in mRNA levels of different genes in each sample. The primer sequences used for the mRNA determined were listed in Supplementary Table S3.

11Experimental data including SCFAs, metabolites, the relative grayscale protein and the relative gene expression, were analyzed by one-way ANOVA and the Duncan multiple comparison test with the use of SPSS statistical software (SPSS, Inc., Chicago, IL, United States). P < 0.05 was taken to indicate statistical significance. For the analysis of 16S rRNA gene sequencing data, data were normalized by copy number. In order to compare differences among control and Trp groups, Kruskal–Wallis ANOVA performed on ranks were used for non-parametric data and one-way ANOVA with Bonferroni post hoc test were used for parametric data. P < 0.05 was taken to indicate statistical significance.

12Compared with the control group, dietary supplementation with 0.2% and 0.4% Trp increased (P < 0.05) the ADFI of piglets by 9.9% (387 g/d vs. 352 g/d) and 8.5% (382 g/d vs. 352 g/d) from day 0 to day 14. During the whole experimental period, both 0.2% and 0.4% Trp supplementation increased (P < 0.05) ADG of piglets by 11.8% (398 g/d vs. 356 g/d) and 6.2% (378 g/d vs. 356 g/d), respectively. However, the FCR was not affected by Trp supplementation (Supplementary Table S4).

13The microbiota composition in the cecum of weaned piglets was analyzed by the Illumina HiSeq sequencing system. After size filtering, quality control, and chimera checking, a total of 35,050 ± 714, 32,982 ± 1,558, and 34,983 ± 763 reads were observed in the 0%, 0.2%, and 0.4% Trp supplementation groups, respectively. OTUs were obtained at a sequence-similarity level of 97%. A Venn diagram was used to reveal the shared and unique microbiota present in the control and/or Trp supplementation groups. As shown in Figure 1, 407 OTUs common to all of the three groups were identified, accounted for 80.4% of all sequences. A varied amount of unique OTUs, 32, 35, and 32 for 0, 0.2, and 0.4% Trp group were observed (Figure 1A), respectively. In addition, there are 42, 29, and 41 OTUs shared by 0% and 0.2% Trp groups, 0% and 0.4% Trp groups, and 0.2% and 0.4% Trp groups, respectively.

14In agreement with previous studies, Bacteroidetes and Firmicutes were two of the most abundant phyla at the phylum level in all groups of pigs (Supplementary Figure S1). The abundance of Bacteroidetes was enhanced by Trp (from 63% in the control group to 71.2% and 69.1% in 0.2% and 0.4% Trp groups, respectively) as compared to the control group. In contrast, the abundance percentage of Firmicutes was 35.4%, 27.3%, and 29.3% in 0%, 0.2%, and 0.4% Trp groups, respectively, indicating a reduced abundance of Firmicutes as compared with control piglets (Supplementary Figure S1).

15At the genus level, compared with the control, dietary supplementation with 0.2% tryptophan increased the relative abundance of Prevotella (61.7% vs. 51.5%), Roseburia (5.31% vs. 3.31%), and Succinivibrio (0.64% vs. 0.11%). Of note, dietary supplementation with 0.4% tryptophan reduced the relative abundance of Clostridium sensu stricto (1.89% vs. 3.95%), Clostridium XI (0.31% vs. 1.33%) and Lactobacillus (0.56% vs. 1.95%) (Figure 1B and Supplementary Table S5). In addition, an 11.8% increase in number of OTUs in the 0.2% Trp group (331 vs. 296 in the control) was observed (Supplementary Table S5).

16Using the abundances of bacterial OTUs across samples, we explored the global effects of dietary Trp on bacterial diversity in the cecum. As shown, the quantity of observed species increased as the sequencing depth increased. The ends of the rarefaction curves tapered off with increasing numbers of sequencing per sample (Supplementary Figure S2), indicating an adequate sequencing depth to investigate the dominant bacterial populations. Chao1, observed species, Shannon index, and Simpson index values were used as parameters of the alpha diversity, which can be used as an indication of the richness of the bacterial community in the intestinal microbiota. Statistical analysis on the alpha diversity at the genus level was conducted (Supplementary Table S6). We found that Chao1 index (Supplementary Figure S2a) and observed species (Supplementary Figure S2b) in the 0.2% Trp group were increased relative to that of the control and 0.4% Trp groups (P < 0.05). Although the Shannon index (Supplementary Figure S2c) and the Simpson index (Supplementary Figure S2d) in the 0.2% Trp group were numerically higher than the control and 0.4% Trp groups, there was no statistically significant difference.

17Principal coordinate analyses were used to estimate beta diversity among the three treatment groups. The PCoA plot of the unweighted unifrac distances showed that the 0.2% Trp group formed a distinct cluster and separated markedly from the control and 0.4% Trp groups (P < 0.05) along the first principal coordinates (Figure 2A). Further analysis using the PCA demonstrated that the OTUs of the microbial communities were clustered into 3 groups by Trp supplementation at the genus level (Figure 2B).

18According to the results of LEfSe analysis, a total of 28 OTUs at the phylum (1 OTU), class (4 OTUs), order (5 OTUs), family (6 OTUs), and genus levels (12 OTUs) were identified to have significant differences among groups. Among the significantly different OTUs, at the genus level, Anaerotruncus and Treponema were the two most abundant bacteria in the 0.2% Trp group, as well that for Succinivibrio and Tissierella in 0.4% Trp group (Figure 3A). To compare the relative abundance of the 12 OTUs in all the sequenced samples, the heatmap was used to show the similarities and differences in community composition of the samples (Figure 3B). The abundance of Treponema in the 0.2% Trp group was 0.0027 which is 25 and 3.7 times higher than that in the control or 0.4% Trp group, respectively. Furthermore, the decreased abundance of Clostridium XI and the increased abundance of Succinivibrio by dietary Trp supplementation was observed (Figure 3B).

19To investigate the contribution of gut microbiota to host metabolism, the correlation prediction analysis of metabolic pathways was conducted. A total of 11 pathways based on environmental, cellular process, metabolism, and genetic information processing were affected significantly by dietary tryptophan. We found that the significantly higher indole alkaloid biosynthesis, Trp metabolism, and mTOR signaling pathway were affected by Trp (Supplementary Figure S3).

20To explore whether the change in the intestinal microbiota affects the production of SCFAs following Trp supplementation, the SCFAs of hindgut contents were determined. An elevated concentration of isobutyrate and isovalerate in colonic contents were observed in the 0.2% and 0.4% Trp groups (P < 0.05). Also, the amount of propionate in colonic contents was increased in the 0.2% Trp group. However, the amounts of acetate, butyrate, and valerate in colonic contents did not differ among the three groups (Figures 4A,B). Moreover, an elevation in the production of IAA was observed in the 0.2% and 0.4% Trp groups relative to the control group. Intriguingly, we found that the production of indole in the 0.2% Trp group was increased (P < 0.05) in both the cecum and the colon. The concentrations of tryptamine in colonic contents of the 0.2% and 0.4% Trp groups were higher as compared with that of the control group (P < 0.05). In contrast, no difference in the production of 5-HIAA and IPA were observed in the hindgut contents (Figures 4C,D). All these data suggested that dietary Trp promoted the generation of SCFAs and Trp metabolites (IAA and indole).

21Increased production of IAA and indole has been reported as an endogenous ligand responsible for AhR activation. Therefore, quantitative real-time PCR was performed to determine the mRNA expression of AhR and downstream targets, including CYP1A1, CYP1B1, and various immune cytokines in cecum and colon tissues. As illustrated, the mRNA levels of AhR and CYP1A1 in the cecum and colon epithelium were enhanced (P < 0.05) in comparison to the control group. In contrast, CYP1B1 was found to be induced in the colon, but not in the cecum tissues (Figures 5A,B). Dietary Trp supplementation reduced the mRNA levels for pro-inflammatory factors (TNF-α and IL-8) in cecum tissues, but reduced IL-8 gene expression in the colon (Figures 5C,D). Collectively, Trp supplementation induced AhR activation in both the cecum and the colon, enhanced mRNA levels of CYP1A1 in the hindgut of piglets, and reduced mRNA levels for TNF-α and IL-8 in the cecum.

22To determine if the alterations in the composition and diversity of microbiota influenced intestinal mucosal barrier integrity, Western blot analysis was performed to investigate the protein abundances for tight junction proteins in colonic tissues of weaned piglets. The results showed that dietary supplementation with 0.2% Trp enhanced the abundances of proteins for ZO-1 and occludin (P < 0.05), compared with the control group (Figure 6).

23A complex and vast microbial community is colonized in the mammalian hindgut which plays an important role in nutritional, physiological, and immunological processes, and profoundly influences intestinal health. A large amount of evidence indicates that diet is a critical factor that modulates the gut microbial composition and diversity, regulates immunity response and metabolism, thus exerting a beneficial effect to the host. Of interest, Trp has attracted growing attention due to its functional role in regulating host physiology, metabolism, and immunity. However, it remains largely unknown how the intestinal microbiota is modified by Trp supplementation. In the present study, weaning piglets, an animal model for nutrition and metabolism, were supplemented with 0.2% or 0.4% Trp throughout days 35–63 of age. The positive effects of dietary Trp on improving feed intake and growth performance were confirmed in our study. This indicates that the current NRC requirements of growing pigs for Trp have been grossly underestimated. Besides, with the continuous improvement in the technology of mass production of Trp, the cost of Trp for feed composition will be further reduced in the near future.

24To evaluate the effect of Trp on intestinal microbiota, cecal microbiota was extracted for 16S rRNA analysis. In consistence with previous studies, we found that Prevotella was the dominant bacteria in cecal microbiota among three dietary groups at the genus level, followed by the Alloprevotella, Clostridium XlVa, and Roseburia. Trp supplementation was associated with reduced abundances of Clostridium sensu stricto, Clostridium XI, Ruminococcus, and Enterobacter, as well as increased abundances of Prevotella, Roseburia, and Succinivibrio. Clostridium species are a potential pathogenic bacteria which has been reported to cause intestinal disorders. The reduction of these bacteria indicated an inhibitory effect of Trp or its metabolites on potential intestinal pathogens. Prevotella and Roseburia belong to Bacteroides and Firmicutes, respectively, which have been reported to produce SCFAs, critical molecules with the ability to regulate intestinal homeostasis in humans and animals. Accordingly, Trp supplementation increased the concentrations of SCFAs in the large intestine of weaned pigs. Moreover, we found that the concentrations of isobutyrate and isovalerate, BCFAs, produced mainly through microbial deamination of branched-chain amino acids (valine and leucine), were substantially increased by Trp supplementation, indicating an important role for Trp in stimulating the metabolism of branched-chain amino acids in the hindgut. Further studies are needed to isolate and identify bacteria involved in this process. Noteworthy, although the amount of Trp metabolites produced from the bacterial catabolism of Trp was altered in the large intestine by dietary Trp supplementation, the Trp source for the bacterial fermentation is not clear. And other factors, such as alternation in chemical composition of the ileal digest by Trp, should be also taken into consideration. Therefore, the alternation of the microbiota in the pig large intestine by dietary Trp supplementation can be considered as indirect effect. However, the altered microecology of the large intestine by dietary Trp supplementation will affect intestinal physiology.

25Several types of intestinal bacteria, such as Bacteroides thetaiotaomicron, Paracolobactrum coliforme, Achromobacter liquefaciens, Micrococcus aerogenes, Escherichia coli, Clostridium difficile, Clostridium sticklandii, Clostridium lituseburense, Clostridium subterminale, and Clostridium putrefaciens have been reported to metabolize Trp into IAA, indole, and other metabolites. Consistently, these metabolites have been reported to be positively correlated with dietary Trp intake in human subjects and with improved mucosal barrier function. In our study, the content of IAA in the cecum and colon were elevated by Trp supplementation. Among the bacteria with the ability to produce IAA from Trp, we found that the abundances of Clostridium XlVa, Clostridium IV, and Bacteroides were increased, while the abundances of Clostridium XI and Clostridium sensu stricto genera were reduced in Trp-supplemented pigs, as compared with controls. The reason for this inconsistence might be due to differences in the capability of bacteria to metabolize Trp and its relative contribution to IAA production among the gut microflora. Also, the existence of other bacteria that can produce IAA from Trp cannot be excluded. For example, bacteria in the genera of Anaerotruncus, Desulfovibrio, and Treponema have been reported to metabolize Trp by tryptophanase and produce indole alkaloid, ammonia, and pyruvate. In line with the previous study, we observed increases in indole production and the abundance of Trp-producing bacteria in Trp-supplemented pigs. It should be noted that the concentration of indole in the colonic contents of pigs receiving 0.4% Trp supplementation was not altered, which might be due to the limitation of the amount or activity of tryptophanase. Considering that intestinal porcine epithelial cells do not degrade Trp, our results indicate that the production of indole and IAA was predominantly due to Trp-induced alterations in the intestinal microbiota. Besides, we speculate that the production of Trp metabolites by the large-intestinal bacteria may follow a substrate-to-product relationship rather than the changes of the composition and abundance of the Trp-metabolizing bacteria. Moreover, the effects of dietary supplementation of Trp on the composition/abundance of the microbiota in the large intestine either directly or indirectly will be more important as the changed bacteria species together with their metabolites will have important effects on the physiology of the large intestine.

26AhR is a cytoplasmic ligand-induced receptor which can be activated by xenobiotics and is associated with their detoxification by activating downstream CYP1A1 targets in various tissues. Recent studies demonstrated that certain Trp metabolites can function as endogenous AhR ligands in the gastrointestinal tract, thus regulating immunity response and intestinal homeostasis. In agreement with these findings, we found that dietary Trp supplementation activated both AhR and the downstream CYP1A1 gene, which might be mediated by bacterial metabolites of Trp. In contrast, the gene expression of CYP1B1 was modestly induced by Trp, suggesting a selective effect of AhR activation on intestinal epithelial cells. This result was consistent with the previous study showing that indole, IAA, tryptamine, and 3-indoxyl sulfate can activate AhR signaling in human CaCo-2 cells. Furthermore, activation of the AhR signaling participates in resiliency and anti-inflammatory responses. This is consistent with our finding that the intestinal mRNA levels for TNF-α and IL-8 were reduced in Trp-supplemented piglets.

27Because of the need to provide Trp beyond the current NRC requirements for young pigs, it is important to define a role for this amino acid in intestinal mucosal integrity. Of particular interest, bacterial Trp metabolites have been reported to activate AhR in mucosal tissues and be positively correlated with enhanced protein abundance of ZO-1 and occludin, two critical tight junction proteins involved in intestinal permeability. This is in agreement with our previous study with intestinal porcine epithelial cells and the novel findings of our present in vivo experiments involving weaned pigs. It is noteworthy that the alterations in the composition and diversity of the cecum microbiota by Trp supplementation may be indirect. Besides, Trp itself could enhance the mucosal barrier integrity according to our previous in vitro study. Moreover, Trp metabolism by microbiota in the intestine is complex, and results from the present study indicated that Trp metabolites (including indole and 3-indoleacetic acid) or metabolites (SCFAs) from carbohydrates fermentation were associated with intestinal mucosal barrier integrity.

28Based on the foregoing, we suggest that Trp administration itself maybe one of the most important factor mediated in the regulation of mucosal barrier integrity. Despite data presented here indicating the role of the microbiota-Trp metabolites-AhR axis, other signaling pathways might be also implicated in the beneficial effects of Trp on intestinal epithelial integrity. For example, farnesoid x receptor (FXR) or pregnane x receptor (PXR) can participate in the regulation of mucosal homeostasis in mice and in the function of cultured hepatocytes. Although we could detect only five major metabolites of Trp in the contents of the piglet large intestine, potential roles of other quantitatively minor products of Trp in gut function cannot be excluded. Besides, in this study, we only analyzed microbiota changes in the large intestine. As dietary Trp was first absorbed and metabolized in the small intestine, therefore, further study to uncover the alteration of microbiota community in the small intestine by dietary Trp is needed.

29Using the piglet as an animal model, we demonstrated that dietary Trp supplementation for 4 weeks altered the intestinal microbiota and enhanced the abundance of intestinal tight-junction proteins. The beneficial effect of Trp was associated with an increased abundance of microflora and consequent activation of AhR signaling. Modification of the intestinal microbiota by Trp or its metabolites might be a novel strategy to improve intestinal health in humans and animals.

30All animal treatment and experimental procedures were approved by the China Agricultural University Animal Care Committee.

31GW and ZW designed the research. HL, ZD, and JL conducted the research. HL, NL, YJ, JC, YZ, JL, and GW analyzed the data. HL, ZD, YY, ZW, and GW wrote the paper. ZD, ZW, and GW had responsibility for the final content. All authors read and approved the final manuscript.

32JL was employed by Henan Yinfa Animal Husbandry Co. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.