|M.Sc Student||Tarazi Riess Hila|
|Subject||Evaluating the Impact of Physiologically Digested|
Carrageenan on the Human Gut Microbiome
Composition and Short Chain Fatty Acid
|Department||Department of Biotechnology and Food Engineering||Supervisor||Professor Uri Lesmes|
|Full Thesis text|
Carrageenan (CGN), a family of sulphated polysaccharides isolated from red seaweeds, is a common food additive. In recent years there has been a significant increase in CGN consumption, while at the same time, concerns about CGNs' safety have been raised for public debate. Various studies suggest adverse effects of CGN on gastrointestinal and overall consumer health. CGN is found to be resistant to enzymatic hydrolysis in the stomach and small intestine. Therefore, CGN can probably be breaken down by gut microbiota fermentation. The human gut microbiota is an ecosystem of microorganisms that live in symbiosis in our body and perform various essential functions, including: nutrient and drug metabolism, immune stimulation and function asa barrier against enteric pathogens and synthesis of essential nutrients such as short chain fatty acids (SCFAs).
In this study, we investigated in-vitro the possible effects of three common CGNs, kappa (κ)-, iota (i)- and lambda (λ)-CGN on the human gut microbiota composition and SCFAs production.
First, we tried to understand the effects of CGN physiological digestion on its physicochemical properties. In order to mimic physiological digestion of adult, a static in-vitro model that simulates the gastrointestinal tract was used. The physiologically digested carrageenans (pdCGNs) were characterized compared to commercial CGN, which have not been digested, in terms of particle size, surface charge, antioxidant capacity and rheology. The results obtained show changes in various properties of CGN after physiological digestion, and provide evidence for the importance of pdCGN use in the rest of this study. In order to study the effects of CGN on the gut microbiota, in-vitro batch system that mimics the conditions in the colon was established.
The three types of pdCGN, and the prebiotic fiber fructooligosaccharide (FOS) as a positive control, were fermented with human fecal bacteria from 10 healthy volunteers. Samples collected during the fermentation were subjected to metagenomic sequencing and analysis to determine profile of SCFAs. The sequencing results were analyzed using bioinformatics pipelines and information on the composition of the gut microbiota was obtained. At the phylum level, there was a significant decrease in the relative abundance of Proteobacteria in i-CGN compared to FOS (p<0.015). Proteobacteria is known to be associated with disease-causing bacteria. In addition, there was a decrease in the relative abundance of Bacteroidetes in κ- and λ-CGN compared to FOS (p<0.05). This decrease led to an increase in Firmicutes/Bacteroidetes ratio, which has been implicated in onset of obesity. Furthermore, it was found that the distribution of the bacterial phyla in CGN fermentation is uneven, in a way that evenness of distribution decreases with the increase in the number of sulfate groups of the fermented CGN. In terms of SCFAs production, fermentation of κ- and λ-CGN seems to have led to a decrease in acetate and propionate production compared to FOS.
Overall, this study provides fundamental evidence on the possible effects of CGN on the human gut microbiota. Further studies are needed to gain a deeper understanding of these effects in more realistic settings and food stuffs.