Gut ecosystems are competitive environments with a few trillion bacteria eager to capitalize from prebiotic fibres so targeting bacteria with specific fibres has wide-ranging implications for the treatment of diseases, such as those linked to inflammation and the colon.
According to the study team, this is the first-time fibre specificity has been related to “similarity and intensity of response in distinct human gut microbiota” and they assert that the data offers “a new approach to design prebiotic or synergistic symbiotic-based interventions …. with the opportunity to be effective in a large number of subjects.”
The in vitro investigation into the impact of fibre specificity on microbiota structure indicates that fibres with high-specificity (and specifically insoluble β-glucan) can potentially bypass the competitive jostling for fibre utilization to increase target bacteria and promote significant shifts in gut microbial.
“Our results imply that high-specificity fibres generate more dramatic gut microbial shifts than low-specificity ones, which has potential implications not only for the microbial ecology itself but also for aspects of clinical relevance.”
Dietary (prebiotic) fibres are carbohydrate polymers and oligomers made up of one or more types of sugar units (such as xylose, fructose, glucose, galactose, arabinose, rhamnose) and promote colonic and systemic health, however they provoke different and unpredictable responses to gut microbiota.
The chemical and physical properties of fibre define how well gut bacteria will respond to supplementation and depend on the composition of an individual’s baseline gut ecosystem, which is affected by factors such as diet and genetics, the scientists explain.
“The overall gut microbiota composition establishes the competitive environment of the target bacteria for nutrient acquisition and utilization and was shown to be an important driving force to interindividual variability regarding fibre responses.”
The team hypothesized that structurally complex fibres (classified as high-specificity fibres) could be selected to limit target bacteria numbers and reduce competitive pressures for the substrate. This would promote an increase in target bacteria (independent of the overall background microbial community) and encourage similar microbial shifts in subjects with distinctly different microbiota environments.
To test their theory, fibres were classified as low (fructooligosaccharides), low-to-intermediate (type 2 resistant starch), intermediate (pectin), and high (insoluble β-1,3-glucan) specificity, and microbiota fermentation evaluated using faecal inoculum from 10 healthy subjects presenting different ratios of gut microbiota.
Analyses determined individual shifts in target bacteria which confirm divergent fibre responses occur when utilising both low-specificity dietary fibres. Fibres of intermediate and high specificity elicited similar responses across subjects in support of targeted bacteria and high-specificity dietary fibres. This promoted dramatic changes in microbial community structure, compared to low-specificity fibres.
Findings show that high-specificity fibres induce similar fibre as long as the target bacteria are present. No discriminant features were found for the low and low to intermediate specificity fibres, confirming a lack of specificity of these fibres to specific bacteria in participants.
The authors commented: “The high-specificity insoluble β-glucan promoted a large increase of the target bacteria (from 0.3 to 16.5% average for Anaerostipes sp. and 2.5 to 17.9% average for Bacteroides uniformis).”
These were associated with increases in ratios of related metabolites (butyrate and propionate, respectively) in every microbial community in which these bacteria were present, which help support many of the body’s essential functions, they explained.
The study has potential limitations in the absence of testing for any other intermediate-specificity states not evaluated here and the researchers suggest a more precise method to determine degree of fibre specificity should be developed.
“This might include machine learning to link fibre structure and bacterial gene expression for fibre hierarchical classification.”
In vivo tests to confirm specificity outcomes are also recommended since in vitro models lack factors, such as the presence of other energy sources for bacteria, which could affect results, they said.
Source: American Society of Microbiology
Published May/June 2021 doi.org/10.1128/mBio.01028-21
“Dietary Fiber Hierarchical Specificity: the Missing Link for Predictable and Strong Shifts in Gut Bacterial Communities”
Authors: Thaisa M. Cantu-Jungles, Nuseybe Bulut, Eponine Chambry, Andrea Ruthes, Marcello Iacomini, Ali Keshavarzian, Timothy A. Johnson, Bruce R. Hamaker