A Tryptophan-Deficient Diet Induces Gut Microbiota Dysbiosis and Increases Systemic Inflammation in Aged Mice

Abstract

The gut microflora is a vital component of the gastrointestinal (GI) system that regulates local and systemic immunity, inflammatory response, the digestive system, and overall health. Older people commonly suffer from inadequate nutrition or poor diets, which could potentially alter the gut microbiota. The essential amino acid (AA) tryptophan (TRP) is a vital diet component that plays a critical role in physiological stress responses, neuropsychiatric health, oxidative systems, inflammatory responses, and GI health. The present study investigates the relationship between varied TRP diets, the gut microbiome, and inflammatory responses in an aged mouse model. We fed aged mice either a TRP-deficient (0.1%), TRP-recommended (0.2%), or high-TRP (1.25%) diet for eight weeks and observed changes in the gut bacterial environment and the inflammatory responses via cytokine analysis (IL-1a, IL-6, IL-17A, and IL-27). The mice on the TRP-deficient diets showed changes in their bacterial abundance of Coriobacteriia class, Acetatifactor genus, Lachnospiraceae family, Enterococcus faecalis species, Clostridium sp genus, and Oscillibacter genus. Further, these mice showed significant increases in IL-6, IL-17A, and IL-1a and decreased IL-27 levels. These data suggest a direct association between dietary TRP content, the gut microbiota microenvironment, and inflammatory responses in aged mice models.

Keywords: dysbiosis; gut; microbiota; systemic inflammation; tryptophan.

Figure 1

Alpha diversity measurements for aged mice fed with TRP-deficient and TRP-rich diets. α-diversity indices at the (A) genus and (B) phylum levels. Statistical differences between the group’s control diet (n = 7), TRP-deficient diet (n = 7), and TRP-rich diet (n = 7) were determined by Kruskal Wallis one-way ANOVA for doses of TRP. Data are expressed as mean ± SE and all comparisons at all indices were non-significant.

Figure 2

Non-metric multidimensional scaling (NMDS) plot of fecal bacterial community structures in animals fed with control diet (n = 7), TRP-deficient diet (n = 7), and TRP-rich diet (n = 7). Data showed no significant differences at (A) the phylum levels but formed distinct clusters at (B) the genus level specific to a TRP-deficient diet; (Permanova, F-value; 2.5034, R-squared; 0.21762, p-value < 0.022).

Figure 3

(A) Phylogenetic heat tree illustrates the differences in relative bacterial abundance between groups. The data show the changes in the bacterial families in mice fed (A) TRP-deficient/low compared to TRP-normal, (B) TRP-rich/high compared to TRP-normal, and (C) TRP-rich/high compared to TRP-deficient diets. Control diet (n = 7), TRP-deficient diet (n = 7), and TRP-rich diet (n = 7). Red nodes represent more abundant bacterial families, whereas the blue nodes represent less abundant bacterial families.

Figure 4

Operational taxonomical unit (OTU) abundance at the phylum level in animals fed with TRP-normal, TRP-deficient, and TRP-rich diets. (A) Relative increase in Verrucomicrobia and Bacteroideate and decrease in Firmicutes and Deferribacteres. (B) Box and whisker plots depict the operational taxonomical units (OTUs) from different bacterial phyla presented among three groups of mice. p values are shown where the differences were found to be significantly different from each other. Cont (control 0.2% TRP, n = 7), def/low (0.1% TRP, n = 7), and high/rich (control 1.25% TRP, n = 7).

Figure 5

Change in composition of the gut microbiota at the genus level in animals fed with control diet (n = 7), TRP-deficient diet (n = 7), and TRP-rich diet (n = 7). Total of 21 genera were found to be different between at least one comparison (con vs. TRP-def/low, con vs. TRP-high, TRP-def/low vs. TRP high) The data represent OTUs of certain genera found to be different among the groups (data are expressed and mean +/− SE and l p-values were calculated using the Kruskal Wallis test).

Figure 6

Serum cytokine levels in animals fed with a control diet, a TRP-deficient/low diet, and a TRP-rich diet. Serum was collected at the end of the experiment (week 8), followed by ELISA for immunoreactive (A) IL-6, (B) IL-17a (C) IL-1a, and (D) IL-27. Results are means ± SD (n = 7–9/per group). Data were analyzed by ANOVA followed by Bonferroni post hoc test or t-test (* p < 0.05, # p < 0.01).