BackgroundBifidobacteria are key commensals in human gut, and their abundance is associated with the health of their hosts. Although they are dominant in infant gut, their number becomes lower in adult gut. The changes of the diet are considered to be main reason for this difference. Large amounts of whole-genomic sequence data of bifidobacteria make it possible to elucidate the genetic interpretation of their adaptation to the nutrient environment. Among the nutrients in human gut, starch is a highly fermentable substrate and can exert beneficial effects by increasing bifidobacteria and/or being fermented to short chain fatty acids.ResultsIn order to determine the potential substrate preference of bifidobacteria, we compared the glycoside hydrolase (GH) profiles of a pooled-bifidobacterial genome (PBG) with a representative microbiome (RM) of the human gut. In bifidobacterial genomes, only 15% of GHs contained signal peptides, suggesting their weakness in utilization of complex carbohydrate, such as plant cell wall polysaccharides. However, compared with other intestinal bacteria, bifidobacteiral genomes encoded more GH genes for degrading starch and starch hydrolysates, indicating that they have genetic advantages in utilizing these substrates. Bifidobacterium longum subsp. longum BBMN68 isolated from centenarian’s faeces was used as a model strain to further investigate the carbohydrate utilization. The pathway for degrading starch and starch hydrolysates was the only complete pathway for complex carbohydrates in human gut. It is noteworthy that all of the GH genes for degrading starch and starch hydrolysates in the BBMN68 genome were conserved in all studied bifidobacterial strains. The in silico analyses of BBMN68 were further confirmed by growth experiments, proteomic and real-time quantitative PCR (RT-PCR) analyses.ConclusionsOur results demonstrated that starch and starch hydrolysates were the most universal and favorable carbon sources for bifidobacteria. The low amount of these carbon sources in adult intestine was speculated to contribute to the low relative abundance of bifidobacteria.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0362-3) contains supplementary material, which is available to authorized users.

Probiotics have been reported to be associated with the alleviation of constipation. The aim of this study was to detect and determine the effect of Bifidobacterium animalis subsp. lactis MN-Gup (MN-Gup) on the alleviation of constipation in BALB/c mice and humans, and to elucidate the mechanisms underlying its effect by measuring changes in the concentration of short-chain fatty acids and the composition of microbes in human faeces. BALB/c mice were given MN-Gup by gavage for 14 days. On the 8th day of this treatment, constipation was induced by the application of diphenoxylate via gavage. The results showed that MN-Gup significantly decreased the first black stool defecation time, and significantly increased black faecal wet weight, black faecal number and the gastric-intestinal transit rate (P<0.05), thereby relieving constipation. In humans, a randomised, double-blind, placebo-controlled trial was performed to investigate the effect of MN-Gup in adults with functional constipation. After 4 weeks of intervention with placebo or MN-Gup yogurt, constipation-related symptoms (including defecation frequency, stool consistency, straining and incomplete feeling during defecation) in the constipated subjects were significantly improved in the two groups, but not different between the groups at the end of the intervention. The concentration of acetate increased significantly in the MN-Gup group compared to the placebo group and before ingestion. Significant changes in the composition of gut microbiota were found after intake of MN-Gup yogurt when compared to placebo. The relative abundances of acetate-producing Bifidobacterium, Ruminoccaceae_UCG-002 and Ruminoccaceae_UCG-005 were significantly increased after intake of MN-Gup yogurt. These results showed that MN-Gup could relieve constipation related to increased acetate-producing Bifidobacterium, Ruminoccaceae_UCG-002 and Ruminoccaceae_UCG-005.

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3-Methylbutanal is one of the primary substances that contribute to the nutty flavor in cheese. Lactococcus strains have been shown to have higher aminotransferase and α-keto acid decarboxylase activities compared with other microbes, indicating that they might form a higher amount of 3-methylbutanal by decarboxylation. Several dairy lactococcal strains have been successfully applied as adjunct cultures to increase the 3-methylbutanal content of cheese. Moreover, compared with dairy cultures, the nondairy lactococci are generally metabolically more diverse with more active AA-converting enzymes. Therefore, it might be appropriate to use nondairy lactococcal strains as adjunct cultures to enrich the 3-methylbutanal content of cheese. This study thereby aimed to select a nondairy Lactococcus strain that is highly productive of 3-methylbutanal, identify its biosynthetic pathway, and apply it to cheese manufacture. Twenty wild nondairy lactococci isolated from 5 kinds of Chinese traditional fermented products were identified using 16S rRNA sequence analysis and were found to belong to Lactococcus lactis ssp. lactis. The nondairy strains were then screened in vitro for their production of 3-methylbutanal and whether they met the criteria to become an adjunct culture for cheese. The L. lactis ssp. lactis F9, isolated from sour bamboo shoot, was selected because of its higher 3-methylbutanal production, suitable autolysis rate, and lower acid production. The enzymes involved in the catabolic pathway of leucine were then evaluated. Both α-keto acid decarboxylase (6.96 μmol/g per minute) and α-keto acid dehydrogenase (30.06 μmol/g per minute) activities were detected in nondairy L. lactis F9. Cheddar cheeses made with different F9 levels were ripened at 13°C and analyzed after 90 d by a combination of instrumental and sensory methods. The results showed that adding nondairy L. lactis F9 significantly increased 3-methylbutanal content and enhanced the nutty flavor of the cheese without impairing its textural properties. Thus, nondairy L. lactis F9 efficiently enhanced the biosynthesis of 3-methylbutanal in vitro and in manufactured cheese.