An amateur gut microbial configuration formed in giant panda for striving to digest cellulose in bamboo: Systematic evidence from intestinal digestive enzymes, functional genes and microbial structures

Abstract

The giant panda has been considered to maximize nutritional intake including protein and soluble carbohydrates in bamboo, but it has spent almost entire life with the high-cellulose diet. Whether giant panda is still helpless about digesting bamboo cellulose or not is always contentious among many researchers around the world. The work has systematically clarified this issue from the perspectives of digestive enzymes, functional genes, and microbial structures in giant panda gut. The intestinal cellulase activities of panda increase with bamboo consumption, performing that the endoglucanase activity of adults reaches 10-fold that of pandas first consuming bamboo. More abundance and types of microbial endoglucanase genes occur in bamboo-diet giant panda gut, and the corresponding GH5 gene cluster is still efficiently transcribed. Gut microbes possessing cellulose-degrading genes, belong to the phylum Firmicutes and some Bacteroidetes, but their structural and functional configurations are insufficient to completely degrade cellulose. Therefore, giant panda is striving to digest cellulose in bamboo, but this adaptation is incomplete. This is probably related to the short straight carnivore-like gut structure of the giant panda, preventing the colonization of some efficient functional but anaerobic-preferred flora.

Keywords: cellulases activity; cellulose digestion; dietary adaptation; giant panda; gut microbiome.

FIGURE 1

The activities of different digestive enzymes in giant panda during its development. (A) Endoglucanse activity; (B) exoglucanase activity; (C) total amylase activity; (D) crude protease activity. Each box has reflected the range of digestive activities of individual pandas (n > 3) at the specific developmental stage.

FIGURE 2

The activities of endoglucanase and exoglucanase in giant panda at different developmental stages under different pH and temperature conditions. (A,B) Represented respectively the activities of endoglucanase and exoglucanase of the giant pandas on milk-fed diet, dietary-shift process and bamboo-fed diet, under different pH at 4.4∼5.2 (T = 40°C); (C,D) represented respectively the activities of endoglucanase and exoglucanase of giant pandas at different dietary stages under different temperature at 40∼50°C (pH 4.8). Each point or column has reflected the range of digestive activities of individuals (n > 3) at the specific developmental stage and culture condition.

FIGURE 3

The gene frequency and abundance of cellulase for giant panda during the dietary adaptation process and the transcription efficiency of GHs in gut flora. (A–D) Represented the gene frequency of α-amylase (EC 3.2.1.1), endoglucanase (EC 3.2.1.4), endoxylanase (EC 3.2.1.8), and β-D-xylosidase (EC 3.2.1.37), respectively, of the giant panda at cub, dietary-shift, sub-adult and adult stages; (E) showed the heat map of GHs genes corresponding to cellulose degradation in the giant panda with different dietary compositions, calculated with the relative abundance in genome; (F) represented the percentage of endoglucanase (EC 3.2.1.4) gene abundance in the giant panda on milk-fed diet and bamboo-fed diet, and (G) represented the counts of gene transcripts of all classes of carbohydrate-active enzymes and their main families of gut microbiome in the bamboo-fed giant panda.

FIGURE 4

The diversity and structure of gut flora and the relative abundance of microbial populations possessing endoglucanase genes in giant panda at different developmental stages along with the adaptation to bamboo diet. (A,B) Represented the α analysis of Simpson’s and Shannon’s Index in growth of the giant panda, respectively, and also have shown the significant difference (0.01 < P ≤ 0.05, marked with *; 0.001 < P ≤ 0.01, marked with **; P ≤ 0.001, marked with ***) of these indexes among different stages; (C) β diversity analysis based on Bray-Curtis method has quantified the differences in the species abundance distributions among the samples at different developmental stages; (D) has shown the relative abundance of microbial populations possessing endoglucanase (EC 3.2.1.4) genes in the giant panda at different dietary stages; (E,F) were heat maps that have shown the succession of bacterial and fungal composition respectively in the giant panda.