A Horizontally Transferred Alginate Metabolism Gene Cluster in the Human Gut Genus <i>Bacteroides</i>

Alginate, a heteropolysaccharide composed of α-L-guluronic acid (G) and β-D-mannuronic acid (M), comprises poly-G, poly-M, and mixed poly-MG regions. Alginate lyases, classified within the polysaccharide lyase (PL) family, degrade alginate into unsaturated saccharides via β-elimination. Due to the abundance of alginate in brown algae, various marine bacteria produce alginate lyases for its assimilation. Recently, alginate lyases have also been identified in gut bacteria such as those of the genus <i>Bacteroides</i>. In this study, we purified an alginate lyase from enrichment culture supernatants containing alginate, using a human fecal sample, and isolated <i>B. xylanisolvens</i> strain MK6803, which can grow on alginate as a sole carbon source-unlike the type strain <i>B. xylanisolvens</i> XB1A. Draft genome sequencing of strain MK6803 revealed an alginate-metabolizing gene cluster encoding three alginate lyases belonging to PL6_1, PL17_2, and PL38, along with a putative oxidoreductase. This gene cluster was shared with <i>B. ovatus</i> CP926 and <i>B. xylanisolvens</i> CL11T00C41, but not with the type strain XB1A. <i>Bacteroides</i> species lacking this gene cluster exhibited no alginate assimilation, even if they possessed genes encoding one or more of the three alginate lyases. This suggests that the presence of the putative oxidoreductase, alongside the lyases, is essential for alginate assimilation in <i>Bacteroides</i> species. Phylogenetic analysis indicated horizontal gene transfer within the genus <i>Bacteroides</i>. These findings highlight the role of alginate metabolism in the adaptation of human gut microbiota.