The human gut microbiota is a diverse community of microbes in the lower gastrointestinal tract which consumes non-starch dietary polysaccharides. Historically recent inclusion of novel food ingredients, including polysaccharides, could alter diet-microbiota effects on human health or be leveraged to engineer new niches in the microbiome. Xanthan gum is a bacterial exopolysaccharide and is a common food additive used for its thickening properties. Xanthan gum is a polymer of β(1,4)-glucose with alternating glucose units harboring a side chain of α(1,3)-linked mannose, β(1,2)-glucuronic acid, β(1,4)-mannose. The inner and outer mannose possess variable 6-O-acetylation and 4,6-O-pyruvylation, respectively. Although environmental microbes have been shown to degrade xanthan gum, this function has not been characterized in the human gut microbiota. We hypothesized that human gut microbes could consume xanthan gum and used a bacterial culturing strategy to survey the microbiota of >60 healthy adults. Surprisingly, we found that over a third of those surveyed had bacteria that grew on and depolymerized xanthan gum. Sequencing revealed a single uncultured OTU common to all positive cultures, suggesting this is a key factor in xanthan catabolism. Culture fractionation and incubation with xanthan gum suggests that microbes depolymerize the substrate to smaller oligosaccharides extracellularly, import these, then complete saccharification intracellularly. Metagenomic, transcriptomic, and proteomic analysis of an active culture has revealed a suite of enzymes that appear responsible for the full saccharification of xanthan gum. We are currently working to fully elucidate the enzymatic and cellular pathways responsible for xanthan gum catabolism.
