In vitro fermentation of Cookeina speciosa glucans stimulates the growth of the butyrogenic Clostridium cluster XIVa in a targeted way

Highlights

• Two β-glucans from Cookeina speciosa were obtained and chemically characterized.

• They were submitted to an in vitro human fecal fermentation model.

• Both glucans were highly butyrogenic and propiogenic, with low gas production.

• Glucans promoted bacterial shifts distinct from fructooligosaccharides.

• Specific increases in genera from the Clostridium cluster XIVa were observed.

Abstract

Dietary fiber chemical and physical structures may be critical to the comprehension of how they may modulate gut bacterial composition. We purified insoluble polymers from Cookeina speciosa, and investigated its fermentation profile in an in vitro human fecal fermentation model. Two glucans, characterized as a (1 → 3),(1 → 6)-linked and a (1→3)-linked β-D-glucans were obtained. Both glucans were highly butyrogenic and propiogenic, with low gas production, during in vitro fecal fermentation and led to distinct bacterial shifts if compared to fructooligosaccharides. Specific increases in Bacteroides uniformis and genera from the Clostridium cluster XIVa, such as butyrogenic Anaerostipes and Roseburia were observed. The (1 → 3)-linked β-D-glucan presented a faster fermentation profile compared to the branched (1 → 3),(1 → 6)-linked β-D-glucan. Our findings support the view that depending on its fine chemical structure, and likely its insoluble nature, these dietary fibers can be utilized to direct a targeted promotion of the intestinal microbiota to butyrogenic Clostridium cluster XIVa bacteria.

Introduction

The human gut harbors a complex community of microorganisms comprising around 3.8 × ·10¹³ bacterial cells, with the largest concentration of bacteria occurring in the large intestine (Sender, Fuchs, & Milo, 2016). The majority of microorganisms belong to Bacteroidetes and Firmicutes phyla corresponding to over 90% of total gut bacteria (Qin et al., 2010). The human gut microbiota is known to perform diverse physiological functions, from protective functions to metabolic regulation, mostly related to the production of short chain fatty acids (SCFA) such as acetate, butyrate and propionate, during fermentation of indigestible carbohydrates (Prakash, Rodes, Coussa-Charley, & Tomaro-Duchesneau, 2011). SCFA profiles produced by members of the two dominant phyla differ, with a tendency for butyrate production by members of the Firmicutes, whilst propionate production tends to be dominated by Bacteroidetes (den Besten et al., 2013). It is generally accepted that butyrate serves as the main oxidative fuel for normal colonic epithelium, has immunomodulatory and anti-inflammatory effects, and plays an important role in the maintenance of colonic health (Canani et al., 2011; Lin & Zhang, 2017). In this scenario, butyrate production is widely distributed among gut commensal Clostridia in the Firmicutes phylum, particularly in the Clostridium clusters XIVa and IV (Rivière, Selak, Lantin, Leroy, & De Vuyst, 2016).

The dietary modulation of the composition and/or activity of the gut microbiota with food ingredients towards a more butyrogenic colon environment has been highlighted as a potential target for the treatment and/or prevention of obesity, insulin resistance, hypercholesterolemia, metabolic diseases and colorectal cancer (Canani et al., 2011; Neyrinck et al., 2012). Accordingly, the fermentation of a food ingredient by the gut microbiota is dependent on its physicochemical structure (Hamaker & Tuncil, 2014). Thus far, most studies have focused on the potential of soluble fibers in modulating the microbial environment, especially inulin-type fructooligosaccharides (FOS). However, bacterial cells from Clostridium cluster XIVa were found strongly attached to insoluble substrates such as bran and mucins in an in vitro fermentation model, presenting a specific bacterial profile at the species level depending on the kind of substrate tested (Leitch, Walker, Duncan, Holtrop, & Flint, 2007; Van den Abbeele et al., 2013). Similarly, insoluble chitin–β-1,3 glucan complexes were shown to modulate Clostridium cluster XIVa gut bacteria (Roseburia spp.), while the same was not observed for the soluble FOS (Neyrinck et al., 2012). Relevant to the current study, the potential of isolated insoluble β-D-glucans in modulating commensal gut bacteria is yet to be determined.

With the belief that the fine chemical structure is critical to the comprehension of how dietary fibers can modulate gut bacterial composition, and knowledge that insoluble fibers may be preferentially colonized by the Clostridium cluster XIVa, we aimed to purify and characterize insoluble β-D-glucans from the cell walls of the edible mushroom-forming ascomycete Cookeina speciosa, and investigate its fermentation profile in an in vitro fermentation model. Our interest was to understand bacterial preferences and specific targeting of the Clostridium cluster XIVa species through RT-qPCR and DNA sequencing in fermented samples.