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Title: Development of new targeted prebiotic approaches for optimising the human intestinal microbiota
Author: Chung, Wing Sun Faith
ISNI:       0000 0004 6056 8909
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2016
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Dietary intake of specific non-digestible carbohydrates is an effective approach for manipulating the composition and activities of the human gut microbiota to benefit host health. Prebiotic candidates of particular interest are pectin, inulin and arabinoxylan oligosaccharide (AXOS). These three substrates have unique structural characteristics that seem likely to limit their utilisation to specific groups of gut bacteria. In vitro fermentor studies were used to simulate human colon fermentation, which allowed control conditions of pH and substrates supply. Results have shown the potential for these selected substrates in modifying the gut microbiota and these modifications occur at the operational taxonomic unit (OTU) level based on 16S rRNA gene sequences (species level). Single substrates, pectin and inulin, resulted in specific enrichment of certain OTUs, where different Bacteroides species OTUs were promoted between the two substrates and Eubacterium eligens from the Firmicutes phylum in particular were stimulated by pectin only. Furthermore, the gut environment pH plays a key role in determining the outcome of interspecies competition. pH was controlled at 5.5, 6.0, 6.4 and 6.9 in the study with parallel vessels in which pH was stepped up and down. In several experiments, the low pH (at 5.5) resulted in the stimulation of an important beneficial species, Faecalibacterium prausnitzii, while Bacteroides spp. abundance was low, suggesting the influence of pH on bacterial growth. Substrate complexity is likely to be a key factor influencing microbiota diversity, as shown in in vitro fermentor studies (at a constant pH of 6.1) using single substrates (pectin, inulin and AXOS) and substrate mixtures at the same final substrate concentration (3 mixtures of these substrates and 6 mixtures that included these three substrates with an addition of galactomannan, resistant starch and ß-glucan). Inulin, with a simple chemical structure was found to support a less diverse microbial community as determined by 16S rRNA gene sequencing. Pectin and AXOS in contrast have a more heterogeneous structure and composition and overall they resulted in a higher microbial diversity as determined by the Shannon diversity index which was similar to three mixture and six mixture, suggesting substrate complexity may also be a key factor influencing microbiota diversity. An in vivo study was also conducted using AXOS as a dietary supplement for twenty one healthy aging volunteers (age ranged 60-75). Sequence analysis of the 16S rRNA gene from DNA extracted from collected faecal samples revealed high inter-individual variation in microbial composition, and variation in response to the supplemented prebiotic. Prevotella and Bacteroides species abundance were found to be negatively correlated. Prevotella were stimulated during the AXOS supplement period (up to 47 % proportional abundance) in individuals that had a detectable level of Prevotella in their starting microbiota. The addition of AXOS fibre increased the total faecal short chain fatty acid concentrations and the level of propionate produced correlated with the abundance of Prevotella spp. present across the volunteers. Overall, however, inter-individual variation played a larger role in determining gut microbiota composition than did the dietary supplement. In conclusion, it seems crucial to identify the range of bacteria that may be stimulated by a given prebiotic and also to take into consideration the role of gut environmental factors such as pH. The development of prebiotics intended to benefit human health has to take account of the highly individual species profiles that may result and the high inter-individual variation between consumers that may respond differently from the intended effect.
Supervisor: Not available Sponsor: Biotechnology and Biological Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Prebiotics ; Gastrointestinal system