(104d) Invited Speaker: Microbial Interactions with the Intestinal Mucosal Barrier: Impact of Lumen Stimuli

The intestinal mucosal barrier is highly significant to effective oral drug delivery, nutrient absorption, and interactions between microbes and underlying tissues. The mucosa is exposed each day to dynamic and variable intestinal lumen contents, yet the impact of these contents on the mucosal barrier is not well understood. Our laboratory is studying the impact of ingested materials, such as lipids in drug delivery systems or food, on transport through the intestinal mucosa of molecules (e.g., drugs and nutrients), particulates (e.g., drug carrier systems), and microbes. Results indicate that mild stimuli, such as those presented by food, can modulate the intestinal barrier, for example to impact oral drug delivery or microbial invasion, and that permeation through mucus is highly dependent on the physical and chemical properties of the penetrating material (drug, particle, microbe).

Particle and microbe tracking studies employing quantitative videomicroscopy revealed that mucus barrier properties are altered upon exposure to common food additive emulsifiers carboxymethylcellulose (CMC) and polysorbate 80 (Tween). Intestinal mucus was collected from porcine intestine and exposed to emulsifiers together with nanoparticles (passively diffusing entities) or E. colias model mcirobes. Exposure to emulsifiers altered mucus structural and barrier properties. Overall, CMC and Tween significantly reduced effective diffusivity (Deff ) of 200 nm particles with different surface chemistries. E. colispeed was decreased in mucus in the presence of CMC, but interestingly increased in the presence of Tween. In vivoexposure (injection into intestinal lumen) to CMC compacted the mucus layer, and Tween resulted in an increase in neutral and decrease in negatively-charged mucin sugars. Overall, results indicated that the emulsifiers significantly altered the local micro-environment within mucus, but it is not clear how these observed changes in static systems would translate into effects on a mucus layer on living tissue subjected to physiological exposure conditions (luminal flow in the presence of digestive enzymes and bile), or if an inflammatory response directly results from these acute exposure effects. We are working to clarify how these effects observed upon exposure of mucus to emulsifiers under static conditions translate into impact of transient exposure with luminal flow in the presence of representative luminal contents, which could potentially result in effective “clumping” and removal of mucus, and associated altered penetration of microbes, signaling molecules, and passively diffusing particles.

Exposure to dextran sodium sulfate (DSS), a chemical used to induce acute inflammation and epithelial damage driven by innate immune response to model IBD, results in denudation of the mucus layer. It has been demonstrated that bacteria are found within the “unpenetrable” inner mucus layer and adjacent to epithelium n this model prior to inflammation and indeed in all mouse models with spontaneous colitis and patients with active colitis. It is not clear if the denudation of the mucus layer results directly from acute exposure to DSS or is a downstream effect. In the present studies, porcine intestinal mucus was exposed to1% wt/vol DSS (20 ul on 200 ul mucus). Transport of passively diffusing nanoparticles was markedly decreased through mucus, yet average E. colispeed in mucus increased from 1.8 to 2.6 µm/sec, while E. coli speed in buffer was not affected by DSS. It is possible that the DSS is effectively “clumping” the mucus, possibly through cross-linking, thus creating larger channels through which microbes can swim. The concept of DSS “clumping” mucus, and potentially removing it from the epithelial surface, is supported by confocal images of cultures exposed to DSS.

These results indicate that mucus barrier properties are modulated by lumen stimuli, including stimuli presented by certain food substances.We hypothesize, given the crucial role of intestinal mucus in modulating interactions between intestinal contents and underlying tissues, that food substances directly impact the mucus barrier, and that an altered mucus barrier modifies interactions of microbes and signaling molecules with underlying tissues (e.g., proximity of microbes, binding of antimicrobial peptides).

One limitation to studying the intestinal mucosal barrier is lack of appropriate in vitro experimental models. Our laboratory is working to develop engineered intestinal models incorporating microbiota in homeostasis with intestinal epithelium and immune cells for studying the links between ingested material and intestinal homeostasis/inflammation.