Linear and branched <i>β-</i>Glucans degrading enzymes from versatile <i>Bacteroides uniformis</i> JCM 13288^T and their roles in cooperation with gut bacteria

Abstract

<i>β-</i>glucans are the dietary nutrients present in oats, barley, algae, and mushrooms. The macromolecules are well known for their immune-modulatory activity; however, how the human gut bacteria digest them is vaguely understood. In this study, <i>Bacteroides uniformis</i> JCM 13288 ^T was found to grow on laminarin, pustulan, and porphyran. We sequenced the genome of the strain, which was about 5.05 megabase pairs and contained 4868 protein-coding genes. On the basis of growth patterns of the bacterium, two putative polysaccharide utilization loci for <i>β-</i>glucans were identified from the genome, and associated four putative genes were cloned, expressed, purified, and characterized. Three glycoside hydrolases (GHs) that were endo-acting enzymes (<i>Bu</i>GH16, <i>Bu</i>GH30, and <i>Bu</i>GH158), and one which was an exo-acting (<i>Bu</i>GH3) enzyme. The <i>Bu</i>GH3, <i>Bu</i>GH16, and <i>Bu</i>GH158 can cleave linear exo/endo- <i>β-</i> 1-3 linkages while <i>Bu</i>GH30 can digest endo- <i>β-</i> 1-6 linkages. <i>Bu</i>GH30 and <i>Bu</i>GH158 were further explored for their roles in digesting <i>β-</i> glucans and generation of oligosaccharides, respectively. The <i>Bu</i>GH30 predominately found to cleave long chain <i>β-</i> 1-6 linked glucans, and obtained final product was gentiobiose. The <i>Bu</i>GH158 used for producing oligosaccharides varying from degree of polymerization 2 to 7 from soluble curdlan. We demonstrated that these oligosaccharides can be utilized by gut bacteria, which either did not grow or poorly grew on laminarin. Thus, <i>B. uniformis</i> JCM 13288 ^T is not only capable of utilizing <i>β-</i>glucans but also shares these glycans with human gut bacteria for potentially maintaining the gut microbial homeostasis.