![]() coli by generating the fucose, a metabolite of host mucin ( Pacheco et al., 2012) (2) the physical barrier, which is composed of the epithelial cells and the mechanical connections between the epithelial cells. For example, Bacteroides thetaiotaomicron modulates the expression of the virulence factor ler in pathogenic E. Moreover, the commensals could inhibit pathogen virulence by suppressing the expression of the virulence gene. For example, Bifidobacterium inhibited the colonization of pathogenic Escherichia coli by decreasing the acetate concentration ( Fukuda et al., 2011). The commensals within epimural microbiome could inhibit the colonization of pathogens in the epithelium via the competition of shared nutritions, secretion of antimicrobial components (e.g., bacteriocins, microcins, and colicins), and alteration of environmental conditions required for the growth of pathogens ( Ohland and Jobin, 2015). The barriers of rumen are constituted by three parts: (1) the microbial barrier, which is composed primarily of the microbes attaching to the surface of the stratified squamous epithelium (termed the epimural microbes). The central strategy utilized by them to maintain such homeostatic relationships is to construct barriers and, therefore, to protect the ecological niche of the commensals, limit the colonization of pathogens, and clearance the invaded microbes in the intestinal epithelium ( Belkaid and Hand, 2014). During long-term evolution, various strategies have been developed by the animals and microbes to control their relationships. On the other hand, the ruminal microbes ferment plant materials into short-chain fatty acids (SCFAs) that regulate a variety of physiological functions of the rumen ( Li et al., 2016). On the one hand, it provides the space and nutrients for microbes to live within the rumen. The rumen is the most important site for digestion in ruminant animals. Our data thus indicates that diet-SCFA axis maintains the host-microbe homeostasis via promoting the diversification of epimural microbiota and maintaining the integrity of rumen epithelium in healthy animals, while via enhancing the activities of immune barrier in animal with lower rumen pH. Under these conditions, the signals on the pathways relating to the immune components increase. Concomitantly, the signals on pathways concerning the cell growth and tight junction disruption were upregulated, while the signals on pathways concerning paracellular permeability were downregulated. Within the microbial community, the relative abundance of genera Sphingobium, Acinetobacter, and Streptococcus significantly decrease. ![]() However, when the dietary concentrate shifted from 35 to 65%, the increase of acetate and reduction of pH decrease the diversity of epimural microbiota and the diversity of its gene pool. Under these conditions, the responses of immune components in the rumen epithelium decrease. Meantime, the signals on pathways concerning the mechanical connections and growth homeostasis in the rumen epithelium were upregulated. Within the microbial community, the relative abundance of genera Sphingobium, Acinetobacter, and Streptococcus increase mostly. We found that, when the dietary concentrate shifted from 10 to 35%, the increase of total SCFA is associated with the diversification of epimural microbiota and the diversity of its gene pool. By the combined methods of transcriptome sequencing, 16S rRNA gene sequencing, and metagenome shotgun sequencing, we have investigated the regulatory effects of ruminal SCFAs on the functions of rumen barriers, by determining the composition and functions of epimural microbiota and on the structure and immunity of the rumen epithelium in goats receiving a 10% (LC group), 35% (MC group), or 65% concentrate diet (HC group). So far, how the ruminal SCFAs regulate the function of rumen barriers is unclear. Ruminal short-chain fatty acids (SCFAs), which are the important signaling molecules derived from the rumen microbiota, regulate a variety of physiological functions of the rumen. The rumen barriers, constituted by the microbial, physical and immune barrier, prevent the transmission of pathogens and toxins to the host tissue in the maintenance of host-microbe homeostasis. 3Key Lab of Animal Physiology and Biochemistry, College of Veterinary Medicine, Nanjing Agricultural University, Nanjing, China.2Bioinformatics Center, Nanjing Agricultural University, Nanjing, China.1College of Life Sciences, Nanjing Agricultural University, Nanjing, China.Hong Shen 1,2†, Zhihui Xu 1,2†, Zanming Shen 3 and Zhongyan Lu 3*
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