Biochemistry of complex glycan depolymerisation by the human gut microbiota

Abstract

The human gut microbiota (HGM) makes an important contribution to health and disease. It is a complex microbial community of trillions of microbes with a majority of its members represented within two phyla, the Bacteroidetes and Firmicutes, although it also contains species of Actinobacteria and Proteobacteria. Reflecting its importance, the HGM is sometimes referred to as an 'organ' as it performs functions analogous to systemic tissues within the human host. The major nutrients available to the HGM are host and dietary complex carbohydrates. To utilise these nutrient sources, the HGM has developed elaborate, variable and sophisticated systems for the sensing, capture and utilisation of these glycans. Understanding nutrient acquisition by the HGM can thus provide mechanistic insights into the dynamics of this ecosystem, and how it impacts human health. Dietary nutrient sources include a wide variety of simple and complex plant and animal-derived glycans most of which are not degraded by enzymes in the digestive tract of the host. Here we review how various adaptive mechanisms that operate across the major phyla of the HGM contribute to glycan utilisation, focusing on the most complex carbohydrates presented to this ecosystem.

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“…Prebiotics are recognised for their ability to manipulate host microbiota to the benefit of the host (Gibson et al ., ). Currently fructans (Ahmad & Khalid, ; Bajury et al ., ; Joshi et al ., ), inulin (Ahmadi et al ., ; Fu et al ., ; Heydari et al ., ) and galactans (Cremon et al ., ; Fehlbaum et al ., ; Ndeh & Gilbert, ), fit this definition of prebiotics. Prebiotic products have a strong science background from positive clinical trials but have yet to obtain regulator‐approved health claims from the European Food Safety Authority (EFSA) in Europe.…”

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“…Additionally, compared with other phyla, Bacteroidetes interact less with the nutrition resource ( Figure 3-C), indicating that the members of this phylum rely on the availability of a smaller variety of nutrients. Bacteroidetes representatives, e.g., Bacteroides and Prevotella spp., efficiently use host glycans and/or dietderived fibers 39 , which has been accounted for in the AGORA resource through extensive literature-based curation 6 . Plant polysaccharide and proteoglycan contents of foodstuffs are currently not reported in nutritional databases, such as the USDA 34 , thus explaining the low overlap of fibers and glycans usable by the Bacteroidetes from the Nutrition resource.…”