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Mucociliary clearance is an important phenomenon inside the respiratory system as a first defensive mechanism against pathogens. Therefore, any assumption considered for the mucociliary clearance and affects its functionality must be validated. The present research deals with the effects of boundary conditions on the movement of upper-convected Maxwell and high viscosity Newtonian mucus layers, numerically. Furthermore, the validity of replacing the viscoelastic mucus layer with a high viscosity Newtonian layer is evaluated. The airway surface liquid layer is considered a two-layer model including non-Newtonian mucus and Newtonian periciliary layers. Four cyclic boundary conditions are imposed at the mucus-periciliary interface as the cilia movement to obtain variations of mucociliary clearance. The upper boundary of the mucus layer is also exposed to different shear stress levels including free slip, cough, and sneeze conditions. By investigation of velocity variations inside mucus and periciliary layers, it is concluded the differences between viscoelastic and Newtonian mucus are not negligible. The maximum velocity differences between the two fluids are more than 52% and 215% during cough and sneeze, respectively. The results show there is a high order of dependency between the relaxation time and the imposed boundary conditions at the mucus-periciliary interface that leads to the invalidation of replacing two fluids with each other. Moreover, the results show substituting the viscoelastic mucus with a high viscosity Newtonian one depends on the mucus-periciliary interface boundary condition. If an independent time-varying boundary condition is used, the substitution leads to an error less than 7% under different shear stress levels. However, time-varying boundary condition shows 38% and 88% differences between high viscosity Newtonian and viscoelastic mucus layers. Furthermore, neglecting the recovery stroke leads to a velocity underestimation up to 50% by substituting viscoelastic mucus with a high viscosity Newtonian one. Therefore, replacing the viscoelastic mucus with a high viscosity Newtonian one is not acceptable for numerical simulations.

Mucus layer movement inside the airway system is an important phenomenon as the first defensive mechanism against pathogens. This research deals with the mucus velocity variations inside the nasal cavity using two different power law and thixotropic mucus layers. The cilia movement is replaced with four cyclic velocity profiles at the lower boundary of the mucus layer, while the upper boundary is exposed to the free-slip condition. The effects of boundary conditions and different fluid parameters are evaluated on the mucus flow. Furthermore, the replacement of power law and thixotropic mucus layers with a high viscous Newtonian mucus is examined under the free-slip condition at the mucus upper boundary. The adaptation rate is used as the criteria for replacing fluids instead of each other. The results show the mucus flow has enough time to adjust the changes from the lower boundary and the recovery stroke does not affect the mucus velocity in the effective stroke. Moreover, it is observed that the mucus flow variations are the same under the influence of recovery, breakdown, and breakdown exponent parameters. However, the effects of the exponent parameter on the mucus flow are more than the other two parameters in the recovery stroke. It is concluded that the assumption for replacing the power law mucus with a high viscous Newtonian one is acceptable. However, this assumption leads to the maximum error of 98.5% for thixotropic mucus in the recovery stroke.

Firefighters and workers in some industries often inhale (polluted) air at high temperatures, eg, 50–200°C, and very low relative humidity, say, 5–10%. Such severe environmental conditions may cause health problems or further aggravate existing ones. As in vivo tests are very complex, potentially harmful and costly, and in vitro experiments often lack high resolution and predictive capability, in silico studies become a priority. They may include computer simulations of the air-particle dynamics in human lung-airway models to understand the convection heat transfer as well as pollutant transport, deposition and uptake at high inlet temperatures and low relative humidity. In this study, detailed modeling, simulation and analysis focuses on two-phase flow in a human upper lung-airway model with a realistic 3D mucus lining. Especially modeling of the upper airway mucus layers was of interest in order to simulate the vapor mass transfer to the airflow because of its primary function for airway humidification. Naturally, the humidification causes water loss in the mucus layer, which leads to its reduction or even depletion when exposed to relatively high temperatures. Accounting for the changes in thickness and the rise in temperature in the mucus layer allows for the determination of locations of thermal injury in the human airways due to continuous exposure of such abnormal inhalation conditions. Different temperature profiles and local changes in mucus layer thickness were studied for ranges of severe inlet temperature conditions at a representative flow rate of 20 LPM (liters per minute). For inlet temperature reaching 100°C, mucus-layer thinning was observed in the upper airways. Interestingly, as a confirmation of the Reynolds analogy, the areas of significant wall heat flux and associated wall shear stress coincided with the regions of highest mucus evaporation, resulting in the humidification of the air with low relative humidity. Model development and mucus layer generation were done using C++ programming. All computer simulations were carried out using the open-source computational fluid dynamics toolbox OpenFOAM.

BackgroundAn interaction of the food types with the gut microbiota changes is deeply implicated in human health and disease. To verify whether animal-based diets would lead to gut dysbiosis, systemic inflammation and inflammatory pathogenesis, we fed mice with chondroitin sulfate (CS), a sulfate-containing O-glycan naturally occurring in livestock and poultry products, and monitored the dynamic changes of microbial flores, inflammatory signatures, and pathogenic hallmarks.ResultsA metagenomic gut microbiota analysis revealed the overgrowth of sulfatase-secreting bacteria and sulfate-reducing bacteria in the gastrointestinal tracts of mice upon daily CS feeding. Sulfatase-secreting bacteria compromise gut integrity through prompting mucin degradation and mucus lesions, which were evident from the upregulation of secretary leukocyte protease inhibitor (SLPI) and mucin 1/4 (MUC-1/4). A synchronous elevation of lipopolysaccharide (LPS) and tumor necrosis factor α (TNF-α) levels in the serum as well as cerebral, hepatic, cardiac and muscular tissues suggests bacterial endotoxinemia, chronic low-grade inflammation and mitochondrial dysfunction, eventually leading to the onset of global inflammatory pathogenesis towards arthritis, dementia, tumor, and fatty liver.ConclusionsCS triggers the early-phase and multi-systemic pathogenesis like arthritis, dementia, tumor, and fatty liver by enhancing gut opportunistic infection and evoking low-grade inflammation in mice. A plausible reason for the inconsistency of CS in treatment of osteoarthritis (OA) was also discussed.

The colon epithelium is protected from the luminal microbes as recently revealed by an inner firmly attached mucus layer impervious to bacteria and an outer loose mucus layer that is the habitat of bacteria. For an additional understanding of these layers, we analyzed the protein composition of these two mucus layers from the mouse colon. Proteomics using nano-LC-MS and MS/MS revealed more than 1000 protein entries. As the mucus layers contain detached cells, a majority of the proteins had an intracellular origin. However, at least 44 entries were described as secreted proteins and predicted to be mucus constituents together with extracellular/plasma and bacterial proteins, the latter largely in the loose mucus layer. A major protein was the Muc2 mucin that by its net-like disulfide-bonded polymer structure builds the mucus. When guanidinium chloride insoluble Muc2 units were analyzed, N-terminal parts of the Fc-gamma binding protein (Fcgbp) was found to be covalently attached in mouse and human colon, whereas its C-terminus was lost by reducing the disulfide bonds. In conclusion, the Fcgbp protein is probably cleaved at GD/PH and covalently attached to Muc2 via one or several of its von Willebrand D domains.

Cathepsin K has been shown to exhibit antimicrobial and anti-inflammatory activities in the mouse colon. To further elucidate its role, we used Ctsk -/- mice and demonstrated that the absence of cathepsin K was accompanied by elevated protein levels of related cysteine cathepsins (cathepsins B, L, and X) in the colon. In principle, such changes could result in altered subcellular localization; however, the trafficking of cysteine cathepsins was not affected in the colon of Ctsk -/- mice. However, cathepsin K deficiency affected the extracellular matrix constituents, as higher amounts of collagen IV and laminin were observed. Moreover, the localization pattern of the intercellular junction proteins E-cadherin and occludin was altered in the colon of Ctsk -/- mice, suggesting potential impairment of the barrier function. Thus, we used an ex vivo method for assessing the mucus layers and showed that the absence of cathepsin K had no influence on mucus organization and growth. The data of this study support the notion that cathepsin K contributes to intestinal homeostasis and tissue architecture, but the lack of cathepsin K activity is not expected to affect the mucus-depending barrier functions of the mouse colon. These results are important with regard to oral administration of cathepsin K inhibitors that are currently under investigation in clinical trials.

Elasmobranchs represent a distinct group of cartilaginous fishes that harbor a remarkable ability to heal wounds rapidly and without infection. To date very little work has addressed this phenomenon although it is suggested that antibiotic capabilities associated with epidermal surfaces may be a factor. The study of benefits derived from mutualistic interactions between unicellular and multicellular organisms is a rapidly growing area of research. Here we survey and identify bacterial associates of three ray and one skate species in order to assess the potential for antibiotic production from elasmobranch associated bacteria as a novel source for new antibiotics.

The intestinal mucus is an efficient system for protecting the epithelium from bacteria by promoting their clearance and separating them from the epithelial cells, thereby inhibiting inflammation and infection. The function of the colon inner mucus layer is especially important as this explains how we can harbor the large number of bacteria in our gut. The major component of this mucus system is the MUC2 mucin which organizes the mucus by its enormously large net-like polymers. Pathogenic microorganisms, in turn, have developed mechanisms for circumventing this well-organized mucus protective system.

We have recently shown that the colon is protected by an inner mucus layer that efficiently separates the bacteria in the outer mucus from the epithelial cells. The inner mucus is impervious for bacteria and built by a network formed by the MUC2 mucin. Lack or defects in this inner mucus layer allow bacteria to reach the epithelia, something that triggers colon inflammation.

We normally live in symbiosis with ∼10 13 bacteria present in the colon. Among the several mechanisms maintaining the bacteria/host balance, there is limited understanding of the structure, function, and properties of intestinal mucus. We now demonstrate that the mouse colonic mucus consists of two layers extending 150 μm above the epithelial cells. Proteomics revealed that both of these layers have similar protein composition, with the large gel-forming mucin Muc2 as the major structural component. The inner layer is densely packed, firmly attached to the epithelium, and devoid of bacteria. In contrast, the outer layer is movable, has an expanded volume due to proteolytic cleavages of the Muc2 mucin, and is colonized by bacteria. Muc2 −/− mice have bacteria in direct contact with the epithelial cells and far down in the crypts, explaining the inflammation and cancer development observed in these animals. These findings show that the Muc2 mucin can build a mucus barrier that separates bacteria from the colon epithelia and suggest that defects in this mucus can cause colon inflammation.

SummaryThe intestinal mucus layer provides a barrier limiting bacterial contact with the underlying epithelium. Mucus structure is shaped by intestinal location and the microbiota. To understand how commensals modulate gut mucus, we examined mucus properties under germ-free (GF) conditions and during microbial colonization. Although the colon mucus organization of GF mice was similar to that of conventionally raised (Convr) mice, the GF inner mucus layer was penetrable to bacteria-sized beads. During colonization, in which GF mice were gavaged with Convr microbiota, the small intestine mucus required 5 weeks to be normally detached and colonic inner mucus 6 weeks to become impenetrable. The composition of the small intestinal microbiota during colonization was similar to Convr donors until 3 weeks, when Bacteroides increased, Firmicutes decreased, and segmented filamentous bacteria became undetectable. These findings highlight the dynamics of mucus layer development and indicate that studies of mature microbe-mucus interactions should be conducted weeks after colonization.

The intestinal epithelium is covered with mucus with the main structural building block being the densely O-glycosylated MUC2 mucin. The intestinal epithelium is exposed to ingested material, our digestive machinery, and large amounts of microorganisms. Mucus is the first line of defense and aids to limit exposure to all these threats to the epithelium. In the small intestine, mucus acts as a matrix, which contains antimicrobial products, such as defensins and immunoglobulin A that limit epithelial exposure to the luminal bacteria. In the colon, the stratified inner mucus layer acts as a physical barrier excluding bacteria from the epithelium. Bacterial penetration of this normally restricted zone is observed in many colitis models and also in patients with ulcerative colitis. Mucus defects that allow bacteria to reach the epithelium and to stimulate an immune system response can lead to the development of intestinal inflammation. The current state of our knowledge concerning the function of the mucus layers and the main mucin component, MUC2, in inflammatory bowel disease is described in this review.

The mucus layer continuously covering the gastric mucosa consists of a loosely adherent layer that can be easily removed by suction, leaving a firmly adherent mucus layer attached to the epithelium. These two layers exhibit different gastroprotective roles; therefore, individual regulation of thickness and mucin composition were studied. Mucus thickness was measured in vivo with micropipettes in anesthetized mice [isoflurane; C57BL/6, Muc1−/−, inducible nitric oxide synthase (iNOS)−/−, and neuronal NOS (nNOS)−/−] and rats (inactin) after surgical exposure of the gastric mucosa. The two mucus layers covering the gastric mucosa were differently regulated. Luminal administration of PGE2increased the thickness of both layers, whereas luminal NO stimulated only firmly adherent mucus accumulation. A new gastroprotective role for iNOS was indicated since iNOS-deficient mice had thinner firmly adherent mucus layers and a lower mucus accumulation rate, whereas nNOS did not appear to be involved in mucus secretion. Downregulation of gastric mucus accumulation was observed in Muc1−/− mice. Both the firmly and loosely adherent mucus layers consisted of Muc5ac mucins. In conclusion, this study showed that, even though both the two mucus layers covering the gastric mucosa consist of Muc5ac, they are differently regulated by luminal PGE2and NO. A new gastroprotective role for iNOS was indicated since iNOS−/− mice had a thinner firmly adherent mucus layer. In addition, a regulatory role of Muc1 was demonstrated since downregulation of gastric mucus accumulation was observed in Muc1−/− mice.

Colon has been shown to have a two-layered mucus system where the inner layer is devoid of bacteria. However, a complete overview of the mouse gastrointestinal mucus system is lacking. We now characterize mucus release, thickness, growth over time, adhesive properties, and penetrability to fluorescent beads from stomach to distal colon. Colon displayed spontaneous mucus release and all regions released mucus in response to carbachol and PGE2, except the distal colon and domes of Peyer's patches. Stomach and colon had an inner mucus layer that was adherent to the epithelium. In contrast, the small intestine and Peyer's patches had a single mucus layer that was easily aspirated. The inner mucus layer of the distal colon was not penetrable to beads the size of bacteria and the inner layer of the proximal colon was only partly penetrable. In contrast, the inner mucus layer of stomach was fully penetrable, as was the small intestinal mucus. This suggests a functional organization of the intestinal mucus system, where the small intestine has loose and penetrable mucus that may allow easy penetration of nutrients, in contrast to the stomach, where the mucus provides physical protection, and the colon, where the mucus separates bacteria from the epithelium. This knowledge of the mucus system and its organization improves our understanding of the gastrointestinal tract physiology.

The normal intestinal microbiota inhabits the colon mucus without triggering an inflammatory response. The reason for this and how the intestinal mucus of the colon is organized have begun to be unraveled. The mucus is organized in two layers: an inner, stratified mucus layer that is firmly adherent to the epithelial cells and approximately 50 μm thick; and an outer, nonattached layer that is usually approximately 100 μm thick as measured in mouse. These mucus layers are organized around the highly glycosylated MUC2 mucin, forming a large, net-like polymer that is secreted by the goblet cells. The inner mucus layer is dense and does not allow bacteria to penetrate, thus keeping the epithelial cell surface free from bacteria. The inner mucus layer is converted into the outer layer, which is the habitat of the commensal flora. The outer mucus layer has an expanded volume due to proteolytic activities provided by the host but probably also caused by commensal bacterial proteases and glycosidases. The numerous O -glycans on the MUC2 mucin not only serve as nutrients for the bacteria but also as attachment sites and, as such, probably contribute to the selection of the species-specific colon flora. This observation that normal human individuals carry a uniform MUC2 mucin glycan array in colon may indicate such a specific selection.

Although the detection of viable probiotic bacteria following their ingestion and passage through the gastrointestinal tract (GIT) has been well documented, their mucosal attachment in vivo is more difficult to assess. In this study, we investigated the survival and mucosal attachment of multi-strain probiotics transiting the rat GIT. Rats were administered a commercial mixture of the intestinal probiotics Lactobacillus acidophilus LA742, Lactobacillus rhamnosus L2H and Bifidobacterium lactis HN019 and the oral probiotic Streptococcus salivarius K12 every 12 h for 3 days. Intestinal contents, mucus and faeces were tested 6 h, 3 days and 7 days after the last dose by strain-specific enumeration on selective media and by denaturing gradient gel electrophoresis. At 6 h, viable cells and DNA corresponding to all four probiotics were detected in the faeces and in both the lumen contents and mucus layers of the ileum and colon. Viable probiotic cells of B. lactis and L. rhamnosus were detected for 7 days and L. acidophilus for 3 days after the last dose. B. lactis and L. rhamnosus persisted in the ileal mucus and colon contents, whereas the retention of L. acidophilus appeared to be relatively higher in colonic mucus. No viable cells of S. salivarius K12 were detected in any of the samples at either day 3 or 7. The study demonstrates that probiotic strains of intestinal origin but not of oral origin exhibit temporary colonisation of the rat GIT and that these strains may have differing relative affinities for colonic and ileal mucosa.

In discussions on intestinal protection, the protective capacity of mucus has not been very much considered. The progress in the last years in understanding the molecular nature of mucins, the main building blocks of mucus, has, however, changed this. The intestinal enterocytes have their apical surfaces covered by transmembrane mucins and the whole intestinal surface is further covered by mucus, built around the gel-forming mucin MUC2. The mucus of the small intestine has only one layer, whereas the large intestine has a two-layered mucus where the inner, attached layer has a protective function for the intestine, as it is impermeable to the luminal bacteria.