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.

Supplementation with Bifidobacterium bifidum G9‐1 (BBG9‐1) has been established to enhance the production of butyrate, a short‐chain fatty acid (SCFA) known for its beneficial effects in alleviating constipation. We hypothesized that BBG9‐1 alters gut microbiota such that bacteria that produce butyric acid from lactate and acetate become more abundant. In this study, we sought to determine whether BBG9‐1 promotes the growth of butyrate‐producing bacteria and thereby enhances butyrate production. BBG9‐1 was cocultured with different butyrate‐producing bacteria to compare differences in the SCFA production of cocultures and monocultures. We indeed detected significant increases in the production of SCFAs in cocultures compared to monocultures. Moreover, lactate and butyrate production increased in a time‐dependent manner in the BBG9‐1 and Faecalibacterium prausnitzii ID 6052 coculture. In addition, acetate production in cocultures initially increased until 16 h, followed by a decline between 20 and 24 h, and a subsequent significant increase at 48 h. Comparatively, lactate and acetate production in the BBG9‐1 and Anaerostipes caccae JCM 13470^T coculture peaked at 16 h and declined thereafter, and butyrate production increased in a time‐dependent manner. In contrast, lactate, acetate, and butyrate production in the BBG9‐1 and Roseburia hominis JCM 17582^T coculture increased in a time‐dependent manner. These findings indicate that butyrate‐producing bacteria increase butyrate production by utilizing BBG9‐1‐produced lactate and acetate. Thus, the butyrate‐mediated physiological activity of BBG9‐1 could be attributed to an indirect enhancement of butyrate production.

Probiotics can help prevent antibiotic‐associated diarrhea; however, those lacking antimicrobial resistance may be ineffective during antimicrobial treatment. Here, we aimed to examine the effects of antimicrobials on the probiotic strain Bifidobacterium bifidum G9‐1 (BBG9‐1). Minimum inhibitory concentrations were determined in vitro by culturing B. bifidum G9‐1 with antimicrobials and assessing its viability. For in vivo analysis, germ‐free and specific pathogen‐free mice were administered B. bifidum G9‐1 along with antimicrobials. Gut microbiota composition and viable B. bifidum G9‐1 abundance were determined. Bifidobacterium bifidum G9‐1 was highly sensitive to antimicrobials in vitro. However, in a complex bacterial environment mimicking the gut environment, the abundance of viable B. bifidum G9‐1 was significantly high despite antimicrobial exposure. Dominant bacterial populations were more affected by antimicrobials than nondominant populations, with B. bifidum G9‐1 exhibiting increased viability in the presence of diverse bacterial species. In vivo, combined administration of antimicrobials and B. bifidum G9‐1 significantly reduced B. bifidum G9‐1 abundance in germ‐free mice but not in specific pathogen‐free mice, where the gut microbiota composition shifted following administration of B. bifidum G9‐1. The presence of diverse live bacteria in the intestine promotes the survival of B. bifidum G9‐1 and its beneficial effects, even in the presence of antimicrobials. This finding suggests that B. bifidum G9‐1, despite lacking intrinsic antimicrobial resistance, can survive and reach the large intestine, maintaining its probiotic function. Therefore, B. bifidum G9‐1 can potentially be used for antibiotic‐associated diarrhea prevention.