An Enhanced Barrier Is a Better Defense: Effects of Probiotics on Intestinal Barrier Function

Probiotic bacteria enhance murine and human intestinal epithelial barrier function. Madsen K, Cornish A, Soper P, McKaigney C, Jijon H, Yachimec C, Doyle J, Jewell L, De Simone C. Gastroenterology 2001;121:580–91. Several studies have reported favorable effects of probiotic bacteria in patients with inflammatory bowel disease (IBD). However, the exact mechanisms by which probiotics exert these effects are largely unknown. In this study, the efficacy of the probiotic compound VSL#3 as a primary therapy in the treatment of colitis was studied using interleukin (IL)-10 gene-deficient mice and analyzing the effect of VSL#3 in an established in vitro model of intestinal epithelial cell monolayers. The probiotic compound VSL#3 contains viable, lyophilized bifidobacteria (B. longum, B. infantis, and B. breve), lactobacilli (L. acidophilus, L. casei, L. delbrueckii subsp. L. bulgaricus, and L. plantarium), and Streptococcus salivarius subsp. Thermophilus. In this study IL-10 gene-deficient mice, which spontaneously develop a patchy, chronic colitis that is similar to human Crohn's disease (CD), and control mice received VSL#3 for 4 weeks. Treatment of IL-10 gene-deficient mice with VSL#3 resulted in a reduction of basal and lipopolysaccharide (LPS)-induced mucosal secretion of tumor necrosis factor (TNF)-α and interferon (IFN)-γ and an improvement in histologic disease characterized by a reduction of mucosal ulceration, epithelial hyperplasia, and mononuclear and neutrophilic infiltrate into the lamina propria. VSL#3 treatment also normalized colonic physiologic function and barrier integrity as measured by short-circuit currents, transepithelial potential differences, and mannitol flux in excised tissue. In the in vitro studies, VSL#3 was applied directly to T84 cell monolayers. The authors showed that epithelial barrier function and resistance to Salmonella invasion could be enhanced by exposure to a proteinaceous soluble factor secreted by the bacteria found in the VSL#3 compound. After 6 hours of exposure of VSL#3 to T84 cells, both mannitol permeability and monolayer conductance were significantly decreased. In contrast, several other nonpathogenic bacteria increased the monolayer permeability and conductance. By applying conditioned media on T84 cells, which was obtained by removing the bacteria from these media, the authors could show that the conditioned media increase T84 cell barrier function, suggesting that a soluble factor is responsible for the enhancement of barrier integrity. This factor was inactivated by heat and proteinase K, implicating a protein as responsible component. Preexposure of T84 cell monolayers to VSL#3 resulted in a dose-dependent reduction in subsequent invasion of monolayers by S. dublin. In contrast, prior exposure of monolayers to Escherichia coli had no effect on subsequent S. dublin invasion. However, VSL#3 had only a very modest effect on S. dublin-induced IL-8 secretion, suggesting that these bacteria do not alter the ability of epithelial cells to activate a mucosal inflammatory reaction in response to a pathogenic bacterial invasion.