Illuminating Vital Surface Molecules of Symbionts in Health and Disease

The immunomodulatory surface molecules of commensal and pathogenic bacteria are critical to the microbe’s survival and the host’s response.(1,2) Recent studies have highlighted the unique and important responses elicited by commensal-derived surface macromolecules;(3–5) however, the technology available to track these molecules in host cells and tissues remains primitive. We report here an interdisciplinary approach that uses metabolic labeling combined with bioorthogonal click chemistry (i.e., reactions performed in living organisms)(6) to specifically tag up to three prominent surface immunomodulatory macromolecules – peptidoglycan (PGN), lipopolysaccharide (LPS), and capsular polysaccharide (CPS) – either simultaneously or individually in live anaerobic commensal bacteria. Importantly, the PGN labeling enables for the first time the specific labeling of live endogenous, anaerobic bacteria within the mammalian host. This approach has allowed us to image and track the path of labeled surface molecules from live, luminal bacteria into specific intestinal immune cells in the living murine host during health and disease. The chemical labeling of three specific macromolecules within a live organism offers the potential for in-depth visualization of host-pathogen interactions.