Although high-fat diet (HFD)-related dysbiosis is involved in the development of steatohepatitis, its pathophysiology especially in the small intestine remains unclear. We comprehensively investigated not only the liver pathology but also the microbiome profile, mucosal integrity and luminal environment in the small intestine of mice with HFD-induced obesity. C57BL/6J mice were fed either a normal diet or an HFD, and their small-intestinal contents were subjected to microbial 16S rDNA analysis. Intestinal mucosal permeability was evaluated by FITC-dextran assay. The levels of bile acids in the small-intestinal contents were measured by liquid chromatography/mass spectrometry. The expression of tight junction molecules, antimicrobial peptides, lipopolysaccharide and macrophage marker F4/80 in the small intestine and/or liver was examined by real-time RT-PCR and immunohistochemistry. The abundance of Lactobacillus was markedly increased and that of Clostridium was drastically decreased in the small intestine of mice fed the HFD. The level of conjugated taurocholic acid was significantly increased and those of deconjugated cholic acid/secondary bile acids were conversely decreased in the small-intestinal contents. The expression of occludin, antimicrobial Reg IIIβ/γ and IL-22 was significantly decreased in the small intestine of HFD-fed mice, and the intestinal permeability was significantly accelerated. Infiltration of lipopolysaccharide was significantly increased in not only the small-intestinal mucosa but also the liver of HFD-fed mice, and fat drops were apparently accumulated in the liver. Pathophysiological alteration of the luminal environment in the small intestine resulting from a HFD is closely associated with minimal inflammation involving the gut-liver axis through disturbance of small-intestinal mucosal integrity.

Although dysbiosis is likely to disturb the mucosal barrier system, the mechanism involved has remained unclear. Here, we investigated alterations of colonic mucosal permeability and tight junction (TJ) molecules in mice with antibiotic-induced dysbiosis. Mice were orally administered vancomycin or polymyxin B for 7 days, and then fecal samples were subjected to microbial 16S rRNA analysis. The colonic mucosal permeability was evaluated by chamber assay. The colonic expression of TJ molecules and cytokines was examined by real-time RT-PCR, Western blotting, and immunohistochemistry. Caco2 cells were stimulated with cytokines and their transepithelial electric resistance (TEER) was measured. Vancomycin-treated mice showed significantly lower gut microbiota diversity than controls, and the same tendency was evident in polymyxin B-treated mice. The colonic mucosal permeability was significantly elevated in both vancomycin- and polymyxin B-treated mice. The expression of claudin 4 in the colonic mucosa was decreased in both vancomycin- and polymyxin B-treated mice. Colonic expression of TNF-α and/or IFN-γ was significantly increased in mice that had been administered antibiotics. TNF-α and IFN-γ stimulation dose-dependently decreased TEER in Caco2 cells. Antibiotic-induced dysbiosis is correlated with the enhancement in colonic tissue permeability, accompanied by a reduction in claudin 4 expression and enhancement in TNF-α and/or IFN-γ expression in mice.

Although probiotics may be useful for the treatment of irritable bowel syndrome (IBS), it is unclear how probiotics play a role in colonic mucosal integrity and immunity. Here, we aimed to investigate the effect of Bifidobacterium bifidum G9-1 (BBG9-1) on colonic mucosal integrity and macrophage behavior in rats subjected to maternal separation (MS) as a model of IBS. MS pups were individually separated from their mother rats, and a proportion of the MS rats were orally administered BBG9-1. The colonic mucosal permeability was evaluated by Ussing chamber assay. The expression of tight junction proteins and cytokines and the population of CD80-positive cells was examined in the colonic tissues by real-time reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. Caco2 cells were stimulated with cytokines and the transepithelial electric resistance (TEER) was measured. MS rats showed significantly higher colonic permeability and lower claudin 4 expression in the colonic epithelium relative to controls. The number of CD80-positive macrophages was significantly increased in the colonic mucosa of MS rats, accompanied by the increase of IL-6 and IFN-γ expression. BBG9-1 treatment ameliorated the increase of M1 macrophage and IL-6/IFN-γ expression in the colonic tissue of MS rats. Simultaneously, BBG9-1 treatment improved the enhanced mucosal permeability and the decreased claudin 4 expression in the colon of MS rats. IL-6 and IFN-γ, whose expression is enhanced in the colon of MS rats, significantly decreased TEER in Caco2 cells in vitro. Probiotic BBG9-1 has a preventive effect on the acceleration of colonic permeability and M1 macrophage population in maternally separated rats.

Investigation of the influence of alive or dead dietary lactic acid bacteria, specifically Lactobacillus gasseri, on mice fed a tyrosine-rich diet was conducted in this study. Mice fed a tyrosine-rich diet exhibited increased p-cresol levels in the serum and cecum and deteriorated skin characteristics in terms of skin moisture and transepidermal water loss. These conditions were partially ameliorated by the intake of lactic acid bacteria. To gain insight into the mechanism, the intestinal microbiota of mice fed lactic acid bacteria was examined. The results revealed an increase in bacteria including genus Faecalibacterium and Lactobacillus in fecal samples from mice fed lactic acid bacteria. Muribaculaceae was most associated with the added group of alive or dead L. gasseri. These findings indicate that lactic acid bacteria inhibit the conversion of tyrosine to p-cresol in the colon, thereby ameliorating skin disorders.