Conversion of plant polysaccharides to organic acids by a genetically accessible Bacteroidia isolate

Bacteria of the class Bacteroidia are abundant members of the gut microbiome and are well-known for their ability to degrade a variety of polysaccharides [1]. Besides, they also produce organic acids like propionate, which is an important precursor chemical in industry [2]. While the degradation of dietary fibers by Bacteroidia is extensively studied with regard to host health, their biotechnological potential has not yet been investigated. In this study, we aim to identify strains that degrade plant polymers and convert them to organic acids with regard to sustainable propionate production.

Isolates have been obtained using selective conditions for polymer using Bacteroidia. Subsequently, we compared the ability of 13 isolates and 14 type strains to produce organic acids with xylan and other polysaccharides as substrate. Isolate Dysgonomonas gadei BGG-A1 was able to use all tested substrates and, in addition, formed propionate as the main product, therefore this strain was chosen for further investigation. We checked the genome of BGG-A1 for polysaccharide-degrading enzymes, where we identified two gene loci that contain all essential genes for xylan degradation including a GH10 xylanase. The purified enzyme showed endo-xylanase activity. To test the isolate"s fermentation characteristics, it was grown in a pH-controlled batch culture with a high xylan concentration (36 g/l), where a shift in product composition was observed: Now lactate was the main product (106 mM) instead of propionate (43 mM). To increase propionate yield we aim to reduce unwanted side products by genetic engineering. As a first step, we successfully introduced an integrative plasmid into the genome, which is an important step on the way towards establishing a gene knockout system in BGG-A1.

In conclusion, we identified isolate D. gadei BGG-A1 as a propionate producing Bacteroidia strain with a large substrate spectrum. We investigated its capability to convert polysaccharides to fermentation products and could identify genes of the energy metabolism and gene clusters that are potentially involved in the degradation process. Currently we are pursuing a promising genetic approach that will enable to identify key functions of genes involved in polysaccharide conversion and may allow for engineering towards an increased propionate yield.

[1] Glowacki and Martens (2021) Bacteriol.

[2] Döring and Basen (2024) Biotechnol Biofuels Bioprod.