Viscosity gradient within the mucus layer determines the mucosal barrier function and the spatial organization of the intestinal microbiota.

BACKGROUND: Migration is an important virulence factor for intestinal bacteria. However, the role of bacterial mobility in the penetration of viscous mucus and their spatial organization within the colon is relatively unknown. METHODS: Movements of fecal bacteria were assessed in gels of varying agarose concentrations and were compared with patterns of bacterial distribution observed in colons from conventional and Enterobacter cloacae-monoassociated mice. Bacteria were visualized using fluorescence in situ hybridization. RESULTS: Long curly bacteria moved best in moderate viscosity gels, short rods and cocci preferred a low viscous environment, whereas high viscosity immobilized all bacterial groups. The spatial distribution of bacteria in the murine colon was also shape- and not taxonomy-dependent, indicating the existence of vertical (surface to lumen) and longitudinal (proximal to distal colon) viscosity gradients within the mucus layer. Our results suggest that mucus viscosity is low in goblet cells, at the crypt basis and close to the intestinal lumen, whereas sites adjacent to the columnar epithelium have a high mucus viscosity. The mucus viscosity increased progressively toward the distal colon, separating bacteria selectively in the proximal colon and completely in the distal colon. CONCLUSIONS: The site-specific regulation of mucus secretion and dehydration make the mucus layer firm and impenetrable for bacteria in regions close to the intestinal mucosa but loose and lubricating in regions adjacent to the luminal contents. Selective control of mucus secretion and dehydration may prove to be a key factor in the management of chronic diseases in which intestinal pathogens are involved.

Bacterial antigens alone can influence intestinal barrier integrity, but live bacteria are required for initiation of intestinal inflammation and injury.

Intestinal flora plays a critical role in the initiation and perpetuation of inflammatory bowel disease. This study examined whether live fecal bacteria were necessary for the initiation of this inflammatory response or whether sterile fecal material would provoke a similar response. Three preparations of fecal material were prepared: (1) a slurry of live fecal bacteria, (2) a sterile lysate of bacterial antigens, and (3) a sterile filtrate of fecal water. Each preparation was introduced via gastric gavage into the intestines of axenic interleukin-10 gene-deficient mice genetically predisposed to develop inflammatory bowel disease. Intestinal barrier integrity and degrees of mucosal and systemic inflammations were determined for each preparation group. Intestinal barrier integrity, as determined by mannitol transmural flux, was altered by both live fecal bacterial and sterile lysates of bacterial antigens, although it was not altered by sterile filtrates of fecal water. However, only live fecal bacteria initiated mucosal inflammation and injury and a systemic immune response. Fecal bacterial antigens in the presence of live bacteria and sterile fecal bacterial antigens have different effects on the initiation and perpetuation of intestinal inflammation.

Reduction of colitis by prebiotics in HLA-B27 transgenic rats is associated with microflora changes and immunomodulation.

HLA-B27 transgenic rats develop spontaneous colitis under specific pathogen-free conditions (SPF) but germ-free rats remain disease-free, emphasizing a role for intestinal bacteria in the pathogenesis of chronic intestinal inflammation. Prebiotics are dietary substances that affect the host by stimulating growth and/or activity of potentially health promoting bacteria. The aims of this study were to investigate whether prebiotics can prevent colitis in SPF HLA-B27 rats, and secondly, to explore mechanisms of protection. SPF HLA-B27 transgenic rats received orally the prebiotic combination long-chain inulin and oligofructose (Synergy 1), or not, prior to the development of clinically detectable colitis. After seven weeks, cecal and colonic tissues were collected for gross cecal scores (GCS), histologic inflammatory scores (scale 0-4), and mucosal cytokine measurement. Cecal and colonic contents were collected for analysis of the gut microbiota by PCR-denaturing gradient gel electrophoresis (PCR-DGGE) and fluorescent in-situ hybridization (FISH), and analysis of short-chain fatty acid composition. Prebiotic treatment significantly decreased GCS and inflammatory histologic scores in the cecum and colon. Prebiotic treatment also decreased cecal IL-1beta, but increased cecal TGF-beta concentrations. Inulin/oligofructose altered the cecal and colonic PCR-DGGE profiles, and FISH analysis showed significant increases in cecal Lactobacillus and Bifidobacterium populations after prebiotic treatment compared with water-treated rats. In conclusion, the prebiotic combination Synergy 1 reduced colitis in HLA-B27 transgenic rats, which effect was associated with alterations to the gut microbiota, decreased tissue proinflammatory cytokines and increased immunomodulatory molecules. These results show promise for prebiotics as primary or adjuvant maintenance therapy for chronic inflammatory bowel diseases.