Nutrient Deprivation Affects Salmonella Invasion and Its Interaction with the Gastrointestinal Microbiota

Microbial Imbalance Induces Inflammation by Promoting Salmonella Penetration through the Mucosal Barrier

The balance of microbial species in the intestine must be maintained to prevent inflammation and disease. Healthy bacteria suppress infection by pathogens and prevent disorders such as inflammatory bowel diseases (IBDs). The role of mucus in the relation between pathogens and the intestinal microbiota is poorly understood. Here, we hypothesized that healthy bacteria inhibit infection by preventing pathogens from penetrating the mucus layer and that microbial imbalance leads to inflammation by promoting the penetration of the mucosal barrier. We tested this hypothesis with an in vitro model that contains mucus, an epithelial cell layer, and resident immune cells. We found that, unlike probiotic VSL#3 bacteria, Salmonella penetrated the mucosal layers and induced the production of interleukin-8 (IL-8) and tumor necrosis factor (TNF)-α. At ratios greater than 10⁴:1, probiotic bacteria suppressed the growth and penetration of Salmonella and reduced the production of inflammatory cytokines. Counterintuitively, low densities of healthy bacteria increased both pathogen penetration and cytokine production. In all cases, mucus increased Salmonella penetration and the production of cytokines. These results suggest that mucus lessens the protective effect of probiotic bacteria by promoting barrier penetration. In this model, a more imbalanced microbial population caused infection and inflammation by selecting pathogens that are more invasive and immunogenic. Combined, the results suggest that the depletion of commensal bacteria or an insufficient dosage of probiotics could worsen an infection and cause increased inflammation. A better understanding of the interactions between pathogens, healthy microbes, and the mucosal barrier will improve the treatment of infections and inflammatory diseases.

Fasting increases microbiome-based colonization resistance and reduces host inflammatory responses during an enteric bacterial infection

Reducing food intake is a common host response to infection, yet it remains unclear whether fasting is detrimental or beneficial to an infected host. Despite the gastrointestinal tract being the primary site of nutrient uptake and a common route for infection, studies have yet to examine how fasting alters the host's response to an enteric infection. To test this, mice were fasted before and during oral infection with the invasive bacterium Salmonella enterica serovar Typhimurium. Fasting dramatically interrupted infection and subsequent gastroenteritis by suppressing Salmonella's SPI-1 virulence program, preventing invasion of the gut epithelium. Virulence suppression depended on the gut microbiota, as Salmonella's invasion of the epithelium proceeded in fasting gnotobiotic mice. Despite Salmonella's restored virulence within the intestines of gnotobiotic mice, fasting downregulated pro-inflammatory signaling, greatly reducing intestinal pathology. Our study highlights how food intake controls the complex relationship between host, pathogen and gut microbiota during an enteric infection.

Starvation and Its Effects on the Gut

There is growing awareness that intestinal dysfunction determines the clinical outcomes of situations as diverse as undernourished children in urban tropical slums and undernourished surgical patients in intensive care units. As experimental starvation in humans has only rarely been studied, and largely not using current biomedical research tools, we must draw inference from disparate clinical and experimental observations as to the derangements present in the starved gut. There is good evidence of intestinal atrophy and achlorhydria in starvation and severe undernutrition. Historical reports from concentration camps and conflict settings consistently reported a noncontagious phenomenon called "hunger diarrhea," but in settings where starved individuals are isolated from others (prisoners on hunger strike, anorexia nervosa) diarrhea is not a feature. Changes in intestinal permeability and absorption have been infrequently studied in experimental starvation; available data suggest that short-term starvation reduces sugar absorption but not permeability. Severe acute malnutrition in children is associated with severe changes in the intestinal mucosa. Experimental animal models may help explain some observations in humans. Starved rats develop a hypersecretory state and intestinal barrier defects. Starved pigs demonstrate prolongation of rotavirus diarrhea and reproduce some of the absorptive and barrier defects observed in malnourished children. However, there remains much to be learned about the effects of starvation on the gut. Given the high prevalence of undernutrition in hospitals and disadvantaged communities, the lack of attention to the interaction between undernutrition and gastrointestinal damage is surprising and needs to be corrected. Current sophisticated cellular and molecular techniques now provide the opportunity to create fresh understanding of gastrointestinal changes in pure undernutrition, using volunteer studies and samples from anorexia nervosa.

A dynamic and integrated in vitro/ex vivo gastrointestinal model for the evaluation of the probability and severity of infection in humans by Salmonella spp. vehiculated in different matrices

The lack of proper gastrointestinal models assessing the inter-strain virulence variability of foodborne pathogens and the effect of the vehicle (food matrix) affects the risk estimation. This research aimed to propose a dynamic and integrated in vitro/ex vivo gastrointestinal model to evaluate the probability and severity of infection of foodborne pathogens at different matrices. An everted gut sac was used to determine the adhesion and invasion of Salmonella enterica and tissue damage. S. Typhimurium ATCC 14028 was used as a representative bacterium, and two matrices (water and cheese) were used as vehicles. No differences (p > 0.05) in the probability of infection (P_inf) were found for intra-experimental repeatability. However, the P_inf of cheese-vehiculated S. Typhimurium was different compared to water- vehiculated S. Typhimurium, 7.2-fold higher. The histological analysis revealed Salmonella-induced tissue damage, compared with the control (p < 0.05). In silico proposed interactions between two major Salmonella outer membrane proteins (OmpA and Rck) and digested peptides from cheese casein showed high binding affinity and stability, suggesting a potential protective function from the food matrix. The results showed that the everted gut sac model is suitable to evaluate the inter-strain virulence variability, considering both physiological conditions and the effect of the food matrix.

Translating nutritional immunology into drug development for inflammatory bowel disease

PURPOSE OF REVIEW

To highlight recent advances in the understanding of nutritional immunology and in the development of novel therapeutics for inflammatory bowel disease (IBD).

RECENT FINDINGS

We highlight the variety of factors that contribute to the interaction of the immune system and nutrition including the microbiome and the nervous system stimulation of the gut. We describe the potential for therapeutic development in IBD. Further, we review the cellular metabolic effects on immune activation and promising therapeutic targets. Finally, we show how the progression of understanding the role of lanthionine synthetase C-like 2 has encompassed both nutritional and therapeutic advances and led to the development of novel oral small molecule therapeutics for IBD.

SUMMARY

Nutritional immunology and drug development research centered around immunoregulatory pathways can provide safer and more effective drugs while accelerating the path to cures.