Project Summary Detecting various compositionally similar but structurally distinct glycans present in heterogenous mixtures has traditionally been challenging due to the limitations facing current glycomics approaches. These limitations have hindered our understanding of the availability and abundance of individual glycans in the mammalian gut environment, which is replete with diverse mixtures of microbial, mammalian, and plant-derived oligo- and polysaccharides. Moreover, the intestinal glycan composition is a primary driver of gut microbiome composition and metabolism, which represents an increasingly important human health determinant, and necessitates a deep understanding of the glycomic-microbial interface to identify important biological interactions and putatively develop glycan-derived therapeutics to target specific microbial activities. Therefore, new tools are necessary to detect and measure the relative abundance of individual glycan substrates present in the heterogenous mixtures prepared from biological sources such as mammalian intestinal contents. We have harnessed the glycan detection machinery employed by dominant members of the gut microbiota to detect, measure and isolate individual glycan substrates present in heterogenous mixtures extracted from the mammalian intestine. Herein, we demonstrate robust, specific, and scalable approaches by which engineered microbes report the presence of individual glycan substrates with incredible sensitivity. Furthermore, we demonstrate that this approach can accurately measure the abundance of individual glycans present in mixtures across wide linear ranges and that the specificity and sensitivity of these measurements can be tuned by modifying particular microbial glycan utilization genes. Finally, we demonstrate that microbially-encoded glycan-binding proteins can be used to isolate individual target glycans from mixtures for downstream compositional and structural determination. We propose to 1.) develop arrayed libraries of distinct gut microbial species, each engineered to report the presence of unique target glycans, 2.) develop a rapid glycan isolation pipeline to purify individual substrates of interest for downstream structural and functional characterization, and 3.) develop genetically modified microbial strains with enhanced sensitivity or target specificity. In addition to offering a rapid and inexpensive alternative to quantifying known glycans, we believe that further development of these tools will reveal the presence and abundance of previously undetectable glycans and dramatically enhance our understanding of the interactions between gut microbes and their mammalian hosts.