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Abstract

Microbes in the intestines of mammals degrade dietary glycans for energy
and growth. The pathways required for polysaccharide utilization are
functionally diverse; moreover, they are unequally dispersed between
bacterial genomes. Hence, assigning metabolic phenotypes to genotypes
remains a challenge in microbiome research. Here we demonstrate that
glycan uptake in gut bacteria can be visualized with fluorescent glycan
conjugates (FGCs) using epifluorescence microscopy. Yeast alpha-mannan
and rhamnogalacturonan-II, two structurally distinct glycans from the
cell walls of yeast and plants, respectively, were fluorescently labeled
and fed to Bacteroides thetaiotaomicron VPI-5482. Wild-type cells
rapidly consumed the FGCs and became fluorescent; whereas, strains that
had deleted pathways for glycan degradation and transport were
non-fluorescent. Uptake of FGCs, therefore, is direct evidence of
genetic function and provides a direct method to assess specific glycan
metabolism in intestinal bacteria at the single cell level.