Gram-positive bacteria are held at a distance in the colon mucus by the lectin-like protein ZG16.

Abstract

The distal colon functions as a bioreactor and harbors an enormous amount of bacteria in a mutualistic relationship with the host. The microbiota have to be kept at a safe distance to prevent inflammation, something that is achieved by a dense inner mucus layer that lines the epithelial cells. The large polymeric nets made up by the heavily O-glycosylated MUC2 mucin forms this physical barrier. Proteomic analyses of mucus have identified the lectin-like protein ZG16 (zymogen granulae protein 16) as an abundant mucus component. To elucidate the function of ZG16, we generated recombinant ZG16 and studied Zg16^-/- mice. ZG16 bound to and aggregated Gram-positive bacteria via binding to the bacterial cell wall peptidoglycan. Zg16^-/- mice have a distal colon mucus layer with normal thickness, but with bacteria closer to the epithelium. Using distal colon explants mounted in a horizontal perfusion chamber we demonstrated that treatment of bacteria with recombinant ZG16 hindered bacterial penetration into the mucus. The inner colon mucus of Zg16^-/- animals had a higher load of Gram-positive bacteria and showed bacteria with higher motility in the mucus close to the host epithelium compared with cohoused littermate Zg16^+/+ The more penetrable Zg16^-/- mucus allowed Gram-positive bacteria to translocate to systemic tissues. Viable bacteria were found in spleen and were associated with increased abdominal fat pad mass in Zg16^-/- animals. The function of ZG16 reveals a mechanism for keeping bacteria further away from the host colon epithelium.