The study by Guo et al examines the in vitro protective effect of culture supernatants from 2 probiotic bacteria, Bifidobacterium infantis and Lactobacillus acidophilus, on intestinal epithelial barrier function. Using an epithelial colorectal adenocarcinoma cell line (Caco-2) assay, the authors demonstrate that preincubation with sterile, cell-free culture supernatants (CFS) from either of these 2 bacterial strains prevented interleukin-1β-induced intestinal barrier permeability as measured by transepithelial electrical resistance and fluorescein isothiocyanate-labeled dextran flux. Protection was mediated by preventing both interleukin-1β-induced increases in tight junction protein claudin-1 expression and nuclear factor-κB activation.
These observations add to a growing body of evidence that probiotic bacteria and, more specifically, their cell-associated and secreted products promote integrity of epithelial barrier function and maintenance of intestinal homeostasis (1–4). pH is a key determinant of bioactivity; L acidophilus and B infantis used in this study produce lactate that acidifies the conditioned culture medium and likely influences the bioactivity of associated microbial-derived products. In the described study, the authors adjust the cell-free supernatant to pH 7.4. Luminal pH in the healthy lower gastrointestinal tract is, however, site-specific and ranges from 5.5 to 7.5 (though patients with gastrointestinal disease frequently exhibit deviations from these mean values (5)). This raises the possibility that the bioactivity of the CFS in vitro may be distinct from in vivo conditions in the gastrointestinal tract and supports screening the bioactivity of these conditioned culture media across the range of pH conditions found in the lower gut. Questions also naturally arise as to whether this beneficial effect of probiotic strain-derived CFS is exclusive to these probiotic species, or if CFS from other bacteria, including those typically considered pathogenic, may also derive a similar effect under the epithelial culture conditions examined.
Although it is critical to understand the efficacy of single species and the mechanisms by which they promote barrier function, the complex microbial communities found in the gastrointestinal tract (human microbiome), even in the earliest phases of life, may influence bioavailability of consumed or orally administered bioactives. Microbial members within the human gastrointestinal microbiome compete with each other and the host for resources. To date, the microbial-derived products that promote barrier function include short-chain fatty acids such as acetate produced by Bifidobacteria, which prevent epithelial barrier disruption through tumor necrosis factor-α induction (1). Such products, however, represent attractive carbon sources for a broad range of gastrointestinal organisms (6–8). Hence, whether oral supplementation with probiotic-derived products would lead to effective epithelial bioavailbility in the distal gastrointestinal tract in humans remains to be determined.
Ideally, a single microbial-derived product within the conditioned media of these species promotes epithelial integrity. There are, however, multiple candidate mechanisms and likely as many microbial-derived and associated products that may promote barrier function. These may include pH modification via production of lactic acid or other short-chain fatty acids (7), lipid-derived induction of anti-inflammatory innate and adaptive immune responses (9), and/or triggering of toll-like receptor signaling pathways (4,10), which may act synergistically to promote epithelial integrity. Moreover, the gastrointestinal tract is overtly colonized from early life, so while promoting barrier function may reduce proinflammatory responses it is unlikely to correct associated gut microbial dysbioses. As such, the efficacy of such a therapeutic strategy may be enhanced by preconditioning the gastrointestinal tract with microbial-derived products to promote barrier integrity and reduce proinflammatory responses, followed by oral supplementation with a relatively simple and defined inoculum of microbial species to promote appropriate microbiome (re)development.
Probiotics have gained enormous interest in recent years as a means to promote intestinal homeostasis; however, much more work is needed to fully elucidate their mechanism(s) of action. The observations reported in this study are important because they provide initial evidence for promotion of barrier function via probiotic-derived products. Understanding the mechanism(s) by which microbes and their products beneficially alter host physiology holds much promise for a broad range of conditions, not least as a precursor treatment for highly vulnerable populations such as premature neonates at high risk of necrotizing enterocoilitis.
1. Hsieh CY, Osaka T, Moriyama E, et al. Strengthening of the intestinal epithelial tight junction by Bifidobacterium bifidum
. Physiol Rep
2. Liu HY, Roos S, Jonsson H, et al. Effects of Lactobacillus johnsonii
and Lactobacillus reuteri
on gut barrier function and heat shock proteins in intestinal porcine epithelial cells. Physiol Rep
3. Stewart L, Crumley B, Walton K. Effects of probiotic VSL#3 on cytokine and tight junction protein expression in intestinal epithelial cells. J FASEB
2015; 29: 857.1.
4. Ghadimi D, Vrese M, Heller KJ, et al. Effect of natural commensal-origin DNA on toll-like receptor 9 (TLR9) signaling cascade, chemokine IL-8 expression, and barrier integritiy of polarized intestinal epithelial cells. Inflamm Bowel Dis
5. Nugent SG, Kumar D, Rampton DS, et al. Intestinal luminal pH in inflammatory bowel disease: possible determinants and implications for therapy with aminosalicylates and other drugs. Gut
6. De Vuyst L, Leroy F. Cross-feeding between Bifidobacteria
and butyrate-producing colon bacteria explains bifdobacterial competitiveness, butyrate production, and gas production. Int J Food Microbiol
7. Keeney KM, Finlay BB. Enteric pathogen exploitation of the microbiota-generated nutrient environment of the gut. Curr Opin Microbiol
8. Sun Y, O’Riordan MXD. Regulation of bacterial pathogenesis by intestinal short-chain fatty acids. Adv Appl Microbiol
9. Mobraten K, Haug TM, Kleiveland CR, et al. Omega-3 and omega-6 PUFAs induce the same GPR120-mediated signalling events, but with different kinetics and intensity in Caco-2 cells. Lipids Health Dis
10. Vinderola G, Matar C, Perdigon G. Role of intestinal epithelial cells in immune effects mediated by Gram-positive probiotic bacteria: Involvement of Toll-like receptors. Clin Diagn Lab Immunol