Figure 3. Bacterial colonization determinants.

Several factors affect the localization of bacteria within the gastrointestinal tract, including the ability to utilize different glycans and to resist antimicrobial peptides (AMPs). A) Sus-like systems in Bacteroides species allow the utilization of complex polysaccharides from the diet or the host. The figure illustrates a generalized schematic of a Sus-like system. Homologues of SusD and other outer membrane lipid-anchored enzymes bind and cleave the glycans (such as starch) into smaller oligosaccharides that are then imported by the SusC-like outer membrane transporter. Interaction with the cognate glycan often leads to transmembrane signaling to activate gene regulatory mechanisms, such as a two-component system or a transmembrane anti-sigma factor which releases and activates a sigma factor. Downstream transcriptional regulation allows Bacteroides species to respond to local availability of glycans. B) Cationic AMPs in the small intestine, which also pass into the colon via the fecal stream, disrupt bacterial outer membranes by interacting with negative charges on their surface. By removing phosphate groups from lipid A of lipopolysaccharide (LPS), pathogens and commensals alike – such as Helicobacter pylori, Salmonella spp., and various Firmicutes and Bacteroidetes –reduce the negative charge on their membranes and evade attack by cationic AMPs.