Intestinal mucus forms a protective barrier secreted by goblet cells to prevent contact of lumenal contents, including the microbiota, with the epithelium. A complex glycoprotein, mucin is composed of a protein backbone decorated with a large diversity of O-linked glycosylation (O-glycans), containing ~100 unique linkages. Multiple members of the gut microbiota are known to degrade different components of host mucin, such as Ruminococcus torques which degrades intact Muc2 and Bacteroides thetaiotaomicron which degrades released mucin O-glycans. I have demonstrated the ability of R. torques to grow on rectal glycoprotein (rGP) derived from porcine rectal mucus. Further, I have observed degradation of rGP during growth of R. torques with Periodic acid-Schiff and Alcian blue stained gels. Exposure experiments of R. torques culture or supernatant to rGP have revealed that degradation is dependent on anaerobic conditions and the previous growth substrate. I have also observed that R. torques degrades bovine submaxillary mucin (BSM), a simpler mucin glycoprotein with shorter O-glycan chains, which occurs under all oxygen and previous growth substrate conditions tested. Both rGP and BSM degradation by R. torques is inhibited in the presence of EDTA, suggesting involvement of metal-dependent glycoside hydrolases or metalloproteases. While R. torques degrades more intact mucin substrates and may release components of these for cross-feeding to other species, B. thetaiotaomicron can only access O-glycans released from these substrates. We have previously identified three endo-glycoside hydrolase 18 (GH18) enzymes encoded by Bt, which release short oligosaccharide products from mucin O-glycans and related keratan sulfate. Two of these endo-GH18 enzymes are encoded in polysaccharide utilization loci (PULs) regulated by hybrid two-component systems, so I seek to identify the signal sugars that bind the sensor domain and result in upregulation of these PULs. I have found that keratan sulfate exposure induces expression of both PULs, even in a mutant B. thetaiotaomicron DanSME strain lacking functional sulfatases targeting mucins, suggesting that the signal sugar generated is sulfated. Further, one PUL is induced by both keratan sulfate and tetra-N-acetyllactosamine, a similar structure to keratan sulfate but lacking sulfation, while the other PUL is only induced by keratan sulfate. These results demonstrate that structural differences in signal sugar features such as presence of sulfate groups drive specificity of regulation in these PULs. Future studies will include activity-guided proteomics to identify enzymes critical to mucin glycoprotein degradation by Rt, co-culture experiments with Rt and Bt to assess whether Rt releases substrates usable by Bt during growth on rGP, and probing Bt endo-GH18 expression in response to mucin glycans with other structural variations. Understanding the mechanisms and regulation of mucin degradation by different species will allow us to identify key enzymes or steps to target for inhibition to prevent mucus erosion in disease.