It is of merit to study the appropriate amount of dietary fiber to add to free-range chickens’ feed to improve their microbial diversity and gut health in times of plant fiber deprivation. Lignocellulose is a useful source of dietary fiber, and its positive effects on the growth performance and laying performance of chickens has already been proven. However, few researchers have researched the effects of adding it on the gut microbiota of chickens. In this research, we added three different levels of eubiotic lignocellulose (0%, 2%, and 4%) to the feed of caged and free-range Bian chickens from September to November, aiming to observe the effects of added dietary fiber and different rearing systems on the gut microbial diversity and gut health of chickens, as well as to determine an appropriate amount of lignocellulose. The results showed that adding dietary fiber increased the thickness of the cecum mucus layer and the abundance of Akkermansia and Faecalibacterium in caged chickens, and 4% lignocellulose was appropriate. In addition, adding lignocellulose increased the microbial diversity and the abundance of the butyrate-producing bacteria Faecalibacterium and Roseburia in fee-range chickens. The α-diversity and the length of the small intestine with 2% lignocellulose in free-range chickens were better than with 2% lignocellulose in caged chickens. Maybe it is necessary to add dietary fiber to the feed of free-range chickens when plant fibers are lacking, and 2% lignocellulose was found to be appropriate in this experiment. In addition, compared with caged chickens, the free-range chickens had a longer small intestine and a lower glucagon like peptide-1 (GLP-1) level. The significant difference of GLP-1 levels was mainly driven by energy rather than short chain fatty acids (SCFAs). There was no interaction between added dietary fiber and the rearing system on SCFAs, cecum inner mucus layer, and GLP-1.

A long-term observation of changes of the gut microbiota and its metabolites would be beneficial to improving the production performance of chickens. Given this, 1-day-old chickens were chosen in this study, with the aim of observing the development of the gut microbiota and gut microbial function using 16S rRNA gene sequencing and metabolites short-chain fatty acids (SCFAs) from 8 to 50 weeks. The results showed that the relative abundances of Firmicutes and genus Alistipes were higher and fiber-degradation bacteria were less at 8 weeks compared with 20 and 50 weeks (P < 0.05). Consistently, gut microbial function was enriched in ATP-binding cassette transporters, the energy metabolism pathway, and amino acid metabolism pathway at 8 weeks. In contrast, the abundance of Bacteroidetes and some SCFA-producing bacteria and fiber-degradation bacteria significantly increased at 20 and 50 weeks compared with 8 weeks (P < 0.05), and the two-component system, glycoside hydrolase and carbohydrate metabolism pathway, was significantly increased with age. The concentration of SCFAs in the cecum at 20 weeks was higher than at 8 weeks (P < 0.01), because the level of fiber and the number of dominant fiber-degradation bacteria and SCFA-producing bacteria were more those at 20 weeks. Notably, although operational taxonomic units (OTUs) and the gut microbial α-diversity including Chao1 and abundance-based coverage estimator (ACE) were higher at 50 than 20 weeks (P < 0.01), the concentration of SCFAs at 50 weeks was lower than at 20 weeks (P < 0.01), suggesting that an overly high level of microbial diversity may not be beneficial to the production of SCFAs.

Eubiotic lignocellulose is a new and useful dietary fiber source for chickens. However, few studies have been undertaken on the impacts of its use as a supplement in different chicken breeds. In this experiment, 108 Chinese native breed Bian hens (BH) and 108 commercial breed ISA Brown hens (IBH) were chosen. They were randomly divided into three groups, and 0, 2, or 4% eubiotic lignocellulose was added to their feed during the growing periods (9–20 weeks), respectively. We aimed to observe the impacts of adding eubiotic lignocellulose on the growth and laying performance, gut microbiota, and short-chain fatty acid (SCFA) of two breeds of hens. In this study, the addition of eubiotic lignocellulose had no significant effect on the growth performance and gut microbial diversity in the two breeds of chickens (P > 0.05). Compared with the control group, adding 4% eubiotic lignocellulose significantly increased the cecum weight, laying performance (P < 0.05), but had no significant effect on the SCFA of BH (P > 0.05); however, adding 4% significantly inhibited the intestinal development, laying performance, butyrate concentration, and SCFA content of IBH (P < 0.05). Moreover, the relative abundances of the fiber-degrading bacteria Alloprevotella and butyrate-producing bacteria Fusobacterium in the 4% group of BH were significantly higher than those in the 4% group of IBH (P < 0.05), resulting in the concentration of butyrate was significantly higher than those in it (P < 0.05). Combining these results suggests that the tolerance of BH to a high level of eubiotic lignocellulose is greater than that of IBH and adding 2-4% eubiotic lignocellulose is appropriate for BH, while 0–2% eubiotic lignocellulose is appropriate for IBH.

With the rapid development of gut microbiological research and high-throughput sequencing technology, we have gained a better understanding of the effects of the gut microbiota and its metabolites such as short-chain fatty acids (SCFAs) on the metabolism of hosts. This effect was found closely related with the consumed dietary fiber by hosts. Dietary fiber has been proven to be very important for hosts. However, hosts such as human, chickens and other monogastric animals cannot digest dietary fiber due to a lack of endogenous fiber-degrading enzymes; therefore, they must rely on gut microorganisms who own endogenous fiber-degrading enzymes such as carbohydrate-active enZymes (CAZymes) encoded by gene. Excellent fiber-degrading bacteria include members of Bacteroidetes phylum such as Bacteroides and Prevotella and members of Firmicutes phylum including Ruminococcus, Fibrobacter, Butyrivibrio, Ruminiclostridium and so on. These fiber-degrading bacteria degrade fiber into monosaccharides via different degrading mechanisms. For instance, Bacteroidetes degrade a dozen kinds of plant fiber using its unique arm-polysaccharide utilization locus (PUL). In contrast to Bacteroidetes, members of the Firmicutes use gram-positive PULs (gp PULs) to process fiber. Some members of the Firmicutes can degrade cellulose and hemicellulose through the cellulosome pathway. And then some oligosaccharides and glucose produced by dietary fiber degradation can be used as carbon and energy sources for microbial growth, thus increasing the diversity of microorganisms. Dietary fiber is the substrate of gut microorganisms. The left monosaccharides are fermented into short-chain fatty acids (SCFAs) by SCFA-producing bacteria including Bifidobacterium, Phascolarctobacterium, Faecalibacterium and so on via different pathways. SCFAs mainly include acetate, propionate and butyrate. SCFAs can further regulate the host's metabolism including energy metabolism, host appetite, liver metabolism and the glucose balance via SCFA receptors including GPR41 and GPR43 or other mechanisms. Therefore, gut microorganisms are also called our "second genome" or "forgotten organs". In this paper, we provide an overview of the interactions among dietary fiber, gut microbiota, SCFAs and host metabolism.