Gut flora plays a key role in the maturation of intestinal mucosal immune systems such as the expression of class II MHC antigens on intestinal epithelial cells, and the cell expansion and functional maturation of both IgA-producing B lymphocytes in the lamina propria and TCR αβ-expressing intraepithelial lymphocytes in the intestinal epithelium in mice. In normal mice, the mucosal immune responses evoked through colonization of gut flora attained levels found in mice reared under normal gut flora-bearing conditions. However, in SAMP1/Yit mice, recently established as a murine model of Crohn's disease, transmural ileitis and cecetis developed following the introduction of commensal gut flora from normal mice, although no intestinal inflammation was observed under germfree conditions. These results suggested that commensal gut flora play critical roles in the development of Crohn's disease-like intestinal inflammation in SAMP1/Yit mice. In this review, we focus on the specific interactions between the gut flora and mucosal immune systems that induce physiological or unphysiological mucosal immune responses.

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Interest in consumption of probiotic and prebiotics (indigestible oligosaccharides) to improve human gastrointestinal health is increasing. Consumption of beneficial probiotic bacteria combined with oligosaccharides may provide enhanced gastrointestinal benefits and improvements in internal health. The objective of this study was to evaluate the efectiveness of administering Bifidobacterium longum 1941 or B. longum BB536 and inulin to healthy, adult volunteers over 2-wk to observe changes in gastrointestinal indices (bacterial counts in stool, stool defecation frequency and consistency, and in organic acids, β-glucuronidase and β-glucosidase enzyme concentration, pH and moisture). A randomised, double-blind and placebo-controlled parallel group coparison was carried out. Subjects were randomly assigned to receive 25 mg of freeze-dried bacterial preparation containing≥1×10¹⁰ cfu/g of either B. longum 1941 and 475 mg inulin (n=10), B. longum BB536 and 475 mg inulin (n=10) or a placebo containing 475 mg inulin (n=10). Efficacy was based on comparison of initial values of gastrointestinal indices with final values. No significant difference between the baseline and the final reading among the three treatment groups was observed on bacterial counts, defecation frequency, stool consistency, pH, enzyme and organic acid concentrations or moisture percentage of stools. However, levels of butyric acid increased after subjects consumed probiotic capsules. No subjects reported worsening in gastrointestinal health after consumption of probiotic capsules. These results indicate that the administration of B. longum 1941 and B. longum BB536 did not significantly alter the intestinal environment, defecation frequency and faecal characteristics of healthy, human subjects. These results were possibly due to the short duration of the study and the participation of healthy, adult populations consuming probiotic bacteria and prebiotics.

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The aim of this study was to select a Lactobacillus with probiotic abilities suitable for in vivo studies in farmed freshwater fish. Fifty-five Lactobacillus isolated from the intestines of freshwater fish were screened for inhibition of fish and human pathogenic bacteria in vitro; and some selected strains by absence of production of biogenic amines and resistance to gastric and intestinal fluids (GIF) in a simulation model. A strain was studied in co-cultures with a pathogen in fish feed extract. Selected strains were tentatively identified as Lactobacillus plantarum 44a, whose a mechanism of inhibition was based on acid production, and L. brevis 18f which was detected as a high H₂O₂ producer, because its supernatant adjusted to pH 6 strongly inhibited Aeromonas hydrophila; this activity was not observed when supernatant was treated with catalase. In the exposure of cells to GIF, L. plantarum 44a survived better than the other strains to pH 2, 2.5 and 3 and pepsin. L. brevis 18f had a very low survival in GIF. L. plantarum 44a co-cultured with A. hydrophila in fish feed extract with an initial ratio≈10³ : 10⁷ and 10⁷ : 10³ respectively, started killing the pathogen when pH was around 5.5. L. plantarum 44a has potential applications as probiotic in freshwater fish. L. brevis 18f as a H₂O₂ producer may have application as a possible fish pathogen antagonist in the upper gastrointestinal tract, the skin, the gills and eggs where oxygen is available.

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Germfree and conventionalized rats were fed a basal or 2% phytate diet. On mucosal phytase activity, the intestinal microflora has no marked effect. Dietary phytate decreased phytase activity in mucosal homogenates, but this effect was not clear in the course of purification. In the germfree intestinal mucosa, the electrophoretic pattern showed two peaks of phytase activity which were different in divalent metal ion requirements. One of these purified phytases did not have any alkaline phosphatase activity.

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Bifidobacteria are generally regarded as safe. However, to date no investigation has been performed of the potential risk factors of bifidobacteria. The presence of known virulence factors clinical blood isolates, dairy and faecal isolates of bifodobacteria was investigated. No significant differences could be observed between clinical and faecal isolates, the results confirm the general opinion that bifidobacteria are safe.

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