A Long-Chain Dextran Produced by Weissella cibaria Boosts the Diversity of Health-Related Gut Microbes Ex Vivo.

Long-chain dextrans are α-glucans that can be produced by lactic acid bacteria. NextDextTM, a specific long-chain dextran with a high degree of polymerisation, produced using Weissella cibaria, was recently shown to exert prebiotic potential in vitro. In this study, the ex vivo SIFR® technology, recently validated to provide predictive insights into gut microbiome modulation down to the species level, was used to investigate the effects of this long-chain dextran on the gut microbiota of six human adults that altogether covered different enterotypes. A novel community modulation score (CMS) was introduced based on the strength of quantitative 16S rRNA gene sequencing and the highly controlled ex vivo conditions. This CMS overcomes the limitations of traditional α-diversity indices and its application in the current study revealed that dextran is a potent booster of microbial diversity compared to the reference prebiotic inulin (IN). Long-chain dextran not only exerted bifidogenic effects but also consistently promoted Bacteroides spp., Parabacteroides distasonis and butyrate-producing species like Faecalibacterium prausnitzii and Anaerobutyricum hallii. Further, long-chain dextran treatment resulted in lower gas production compared to IN, suggesting that long-chain dextran could be better tolerated. The additional increase in Bacteroides for dextran compared to IN is likely related to the higher propionate:acetate ratio, attributing potential to long-chain dextran for improving metabolic health and weight management. Moreover, the stimulation of butyrate by dextran suggests its potential for improving gut barrier function and inflammation. Overall, this study provides a novel tool for assessing gut microbial diversity ex vivo and positions long-chain dextran as a substrate that has unique microbial diversity enhancing properties.

Systematic optimization of fermentation conditions for in vitro fermentations with fecal inocula.

In vitro fermentation strategies with fecal inocula are considered cost-effective methods to gain mechanistic insights into fecal microbiota community dynamics. However, all in vitro approaches have their limitations due to inherent differences with respect to the in vivo situation mimicked, introducing possible biases into the results obtained. Here, we aimed to systematically optimize in vitro fermentation conditions to minimize drift from the initial inoculum, limit growth of opportunistic colonizers, and maximize the effect of added fiber products (here pectin) when compared to basal medium fermentations. We evaluated the impact of varying starting cell density and medium nutrient concentration on these three outcomes, as well as the effect of inoculation with fresh vs. stored fecal samples. By combining GC-MS metabolite profiling and 16 s rRNA gene-based amplicon sequencing, we established that starting cell densities below 1010 cells/ml opened up growth opportunities for members the Enterobacteriaceae family. This effect was exacerbated when using fecal samples that were stored frozen at -80°C. Overgrowth of Enterobacteriaceae resulted in lowered alpha-diversity and larger community drift, possibly confounding results obtained from fermentations in such conditions. Higher medium nutrient concentrations were identified as an additional factor contributing to inoculum community preservation, although the use of a less nutrient dense medium increased the impact of fiber product addition on the obtained metabolite profiles. Overall, our microbiome observations indicated that starting cell densities of 1010 cells/ml limited opportunities for exponential growth, suppressing in vitro community biases, whilst metabolome incubations should preferably be carried out in a diluted medium to maximize the impact of fermentable substrates.

Low-no-calorie sweeteners exert marked compound-specific impact on the human gut microbiota ex vivo.

Low-no-calorie sweeteners (LNCS) are used as sugar substitutes as part of strategies to reduce the risk of chronic diseases related to high sugar intake (e.g. type 2 diabetes (T2D)). This study investigated how a range of sweeteners [tagatose (TA)/maltitol (MA)/sorbitol (SO)/stevia (ST)/sucralose (SU)/acesulfame K (ACK)] impact the gut microbiota of T2D subjects and healthy human adults using the ex vivo SIFR® technology (n = 12). The cohort covered clinically relevant interpersonal and T2D-related differences. ACK/SU remained intact while not impacting microbial composition and metabolite production. In contrast, TA/SO and ST/MA were respectively readily and gradually fermented. ST and particularly TA/SO/MA increased bacterial density and SCFA production product-specifically: SO increased acetate (∼Bifidobacterium adolescentis), whilst MA/ST increased propionate (∼Parabacteroides distasonis). TA exerted low specificity as it increased butyrate for healthy subjects, yet propionate for T2D subjects. Overall, LNCS exerted highly compound-specific effects stressing that results obtained for one LNCS cannot be generalised to other LNCS.

How Microbial Food Fermentation Supports a Tolerant Gut.

The gastrointestinal tract harbors a complex resident microbial ecosystem, comprising over 500 species, spanning commensals, mutualist, opportunistic, and professional pathogens thriving on undigested food components originating from the diet and endogenous secretions. Despite this high concentration of food and bacterial antigens, a healthy gut has a near absent level of inflammation, a status called intestinal immune homeostasis. This immune homeostasis is built and maintained in the presence, and interestingly, with cooperation of the microbiota. The microbiota ferments undigested food components into a wide variety of metabolites, some of which interact with the intestinal immune system. In particular short-chain fatty acids, aryl hydrocarbon receptor ligands, and bile acid metabolites have been involved in the induction of intestinal immune homeostasis. The production of these metabolites is influenced by the microbial load and community structure, as well as the availability of substrates and the gut environment which are directly or indirectly modulated by food intake. In this manuscript, the factors that influence the production of these metabolites and their interaction with the immune cells that play key roles in maintaining intestinal immune homeostasis in the healthy gut are reviewed.