Objectives: The intestinal mucosa is constantly exposed to a dense and highly dynamic microbial flora and challenged by a variety of enteropathogenic bacteria. Antibacterial protection is provided, in part by Paneth cell-derived antibacterial peptides such as the a-defensins. The mechanism of peptide-mediated antibacterial control and its functional importance for gut homeostasis has recently been appreciated in patients with Crohn's ileitis. In the present study we analysed the spatial distribution of antimicrobial peptides within the small intestinal anatomical compartments such as the intestinal crypts, the overlaying mucus and the luminal content.
Methods: Preparations from the different intestinal locations as well as whole mouse small intestine were extracted and separated by reversed phase-HPLC. Antibacterial activity was determined in extracts, and the presence of antimicrobial peptides/proteins was confirmed by N-terminal sequencing, mass spectrometry analysis and immunodetection.
Results: The secreted antibacterial activity was largely confined to the mucus layer, whereas only minute activity was noted in the luminal content. The extractable activity originating from either crypt / mucus / lumen compartments respectively (given as percentage) was for L. monocytogenes, 48±4 / 44±4 / 8±8; E. faecalis, 44±10 / 49±3 / 7±7; B. megaterium, 56±4 / 42±3 / 2±1; S. pyogenes, 48±4 / 46±3 / 6±6; E. coli, 46±4 / 47±3 / 7±7 and S. typhimurium, 38±3 / 43±7 / 19±10. A spectrum of antimicrobial peptides was identified in isolated mucus, which exhibited strong and contact-dependent antibacterial activity against both commensal as well as pathogenic bacteria.
Conclusion: These findings show that secreted antimicrobial peptides are retained by the surface-overlaying mucus and thereby provide a combined physical and antibacterial barrier to prevent bacterial attachment and invasion. This distribution facilitates high local peptide concentration on vulnerable mucosal surfaces, while still allowing the presence of an enteric microbiota.
- small intestine