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Dietary lipids disturb colonic microbiota–mucus interactions and microbial transformation of host-derived bile acids. This altered bile acid pool triggers increased lipid absorption in the small intestine, revealing an unexpected link between the colonic mucus niche and small intestinal lipid metabolism.

Background: The peptide GPR15L is produced by colonic epithelial cells and has been implicated in T cell recruitment to the large intestine. However, its role in chronic colitis has been unclear so far.

Objective: To explore the role of GPR15L in the pathogenesis of experimental colitis and IBD.

Design: We studied how genetic deletion or overexpression of Gpr15l as well as rectal application of recombinant GPR15L alters the course of acute dextran sodium sulfate colitis and T cell transfer colitis. The impact of GPR15L on microbiota was explored with co-housing, littermate and faecal microbiota transfer studies, by 16S rRNA sequencing as well as anti-microbial assays and shotgun metagenomics. The expression of GPR15L was evaluated across three independent cohorts of patients with IBD and correlated to microbial diversity and flare-free survival.

Results: GPR15L clearly mitigated experimental colitis, but this was independent of T cell recruitment and GPR15. Instead, we observed that the effects of GPR15L were mediated by altered microbiomes in the large intestine and, consistently, showed that GPR15L acts as an antimicrobial peptide under anaerobic conditions and shapes microbial communities towards a homeostatic phenotype. Rectal supplementation of GPR15L counteracted experimental colitis. In patients with IBD, GPR15L expression was decreased in active inflammation, correlated with microbial diversity and was associated with flare-free survival.

Conclusions: GPR15L is a host-defence peptide that plays a beneficial role in the pathogenesis of intestinal inflammation. It seems promising to further evaluate its potential as a future therapeutic approach in IBD.

While the excessive use of broad-spectrum antibiotics is a major driver of the global antibiotic resistance crisis, more selective therapies remain unavailable for the majority of bacterial pathogens. This includes the obligate intracellular bacterial pathogens of the genus Chlamydia, which cause millions of urogenital, ocular, and respiratory infections each year. Conducting a comprehensive search of the chemical space for novel antichlamydial activities, we identified over 60 compounds that are chemically diverse, structurally distinct from known antibiotics, non-toxic to human cells, and highly potent in preventing the growth of Chlamydia trachomatis in cell cultures. Some blocked C. trachomatis development reversibly, while others eradicated both established and persistent infections in a bactericidal manner. The top molecules displayed compelling selectivity, yet broad activity against diverse Chlamydia strains and species, including both urogenital and ocular serovars of C. trachomatis, as well as Chlamydia muridarum and Chlamydia caviae. Some compounds also displayed synergies with clinically used antibiotics. Critically, we found the most potent antichlamydial compound to inhibit fatty acid biosynthesis via covalent binding to the active site of Chlamydia FabH, identifying a new mechanism of FabH inhibition and highlighting a possible way to selectively treat Chlamydia infections.

Bacteria live in diverse communities, forming complex networks of interacting species. A central question in bacterial ecology is whether species engage in cooperative or competitive interactions. But this question often neglects the role of the environment. Here, we use genome-scale metabolic networks from two different open-access collections (AGORA and CarveMe) to assess pairwise interactions of different microbes in varying environmental conditions (provision of different environmental compounds). By computationally simulating thousands of environments for 10,000 pairs of bacteria from each collection, we found that most pairs were able to both compete and cooperate depending on the availability of environmental resources. This modeling approach allowed us to determine commonalities between environments that could facilitate the potential for cooperation or competition between a pair of species. Namely, cooperative interactions, especially obligate, were most common in less diverse environments. Further, as compounds were removed from the environment, we found interactions tended to degrade towards obligacy. However, we also found that on average at least one compound could be removed from an environment to switch the interaction from competition to facultative cooperation or vice versa. Together our approach indicates a high degree of plasticity in microbial interactions in response to the availability of environmental resources.

The small intestinal mucosa has the delicate task of allowing absorption of nutrients and limiting microbial colonization at the mucosal surface through production of antimicrobial peptides and proteins (AMPs). However, while environmental factors, including different diets, have been shown to alter AMP expression, the results from the literature are conflicting on their specific impact. Moreover, the interdependence between diet, AMPs, and metabolic health is largely unexplored. The aim of this study was thus to investigate the effect of obesogenic diets, obesity itself, and other variables, including mouse vendor, microbiota composition, and sex, on intestinal AMP expression. By using different dietary interventions in mice, we here show that prolonged intake of an obesogenic Western-style diet had a stronger impact on AMP expression than diet-independent obesity. Additionally, when comparing AMP expression under different diets in mice of both sexes from different vendors, the combined contribution of these factors had the strongest impact on absolute AMP transcript numbers, but also on the variability in small intestinal microbiota composition at the mucosa and content. Finally, we identified a novel host–microbe interaction, in which the gut commensal Faecalibaculum bloomed upon WSD-feeding and specifically induced the expression of the AMP Reg3g. Our findings thus reveal that the experimental setup, defined by mouse vendor, sex, and diet type, has a major influence on small-intestinal AMP expression. These findings could partly explain the discrepancy in the literature regarding the effect of diets on AMP expression, preventing any accurate generalization about the impact of diet on the antimicrobial response.

The colonic mucus layer is the primary interface between the host and the gut microbiota. It serves both as an ecological niche for bacteria and a barrier protecting the host from microbial exposure. Disruption of the mucus layer, particularly under Western-style diet (WSD) feeding, increases the risk of infection and inflammation. Here, we identify casein glycomacropeptide (CGMP), a milk-derived glycopeptide, as a novel dietary supplement capable of preserving mucus function under WSD consumption. Notably, we demonstrate that the sialylation level of CGMP is a key determinant of its protective effects. Supplementation of highly sialylated CGMP not only prevented WSD-induced mucus defects but also altered the gut microbiota composition, enhancing beneficial bacterial genera, particularly Bifidobacterium. Mechanistically, bacterial shifts were associated with increased production of the short-chain fatty acid propionate, which can induce mucus growth. Our findings thus reveal sialylated CGMP as a promising prebiotic supplement to counteract diet-induced mucus dysfunction, highlighting the importance of protein-bound glycan structures in modulating host-microbiota interaction.

Recent evidence indicates that repeated antibiotic usage lowers microbial diversity and ultimately changes the gut microbiota community. However, the physiological effects of repeated–but not recent–antibiotic usage on microbiota-mediated mucosal barrier function are largely unknown. By selecting human individuals from the deeply phenotyped Estonian Microbiome Cohort (EstMB), we here utilized human-to-mouse fecal microbiota transplantation to explore long-term impacts of repeated antibiotic use on intestinal mucus function. While a healthy mucus layer protects the intestinal epithelium against infection and inflammation, using ex vivo mucus function analyses of viable colonic tissue explants, we show that microbiota from humans with a history of repeated antibiotic use causes reduced mucus growth rate and increased mucus penetrability compared to healthy controls in the transplanted mice. Moreover, shotgun metagenomic sequencing identified a significantly altered microbiota composition in the antibiotic-shaped microbial community, with known mucus-utilizing bacteria, including Akkermansia muciniphila and Bacteroides fragilis, dominating in the gut. The altered microbiota composition was further characterized by a distinct metabolite profile, which may be caused by differential mucus degradation capacity. Consequently, our proof-of-concept study suggests that long-term antibiotic use in humans can result in an altered microbial community that has reduced capacity to maintain proper mucus function in the gut.

Antibiotic use is a risk factor for development of inflammatory bowel diseases (IBDs). IBDs are characterized by a damaged mucus layer, which does not separate the intestinal epithelium from the microbiota. Here, we hypothesized that antibiotics affect the integrity of the mucus barrier, which allows bacterial penetrance and predisposes to intestinal inflammation. We found that antibiotic treatment led to breakdown of the colonic mucus barrier and penetration of bacteria into the mucus layer. Using fecal microbiota transplant, RNA sequencing followed by machine learning, ex vivo mucus secretion measurements, and antibiotic treatment of germ-free mice, we determined that antibiotics induce endoplasmic reticulum stress in the colon that inhibits colonic mucus secretion in a microbiota-independent manner. This antibiotic-induced mucus secretion flaw led to penetration of bacteria into the colonic mucus layer, translocation of microbial antigens into circulation, and exacerbation of ulcerations in a mouse model of IBD. Thus, antibiotic use might predispose to intestinal inflammation by impeding mucus production.

Background: Mounting evidence suggests a significant role of the gut microbiota in the development and progression of colorectal cancer (CRC). In particular, an over-representation of oral pathogens has been linked to CRC. The aim of this study was to further investigate the faecal microbial landscape of CRC patients, with a focus on the oral pathogens Parvimonas micra and Fusobacterium nucleatum.

Methods: In this study, 16S rRNA sequencing was conducted using faecal samples from CRC patients (n = 275) and controls without pathological findings (n = 95).

Results: We discovered a significant difference in microbial composition depending on tumour location and microsatellite instability (MSI) status, with P. micra, F. nucleatum, and Peptostreptococcus stomatis found to be more abundant in patients with MSI tumours. Moreover, P. micra and F. nucleatum were associated with a cluster of CRC-related bacteria including Bacteroides fragilis as well as with other oral pathogens such as P. stomatis and various Porphyromonas species. This cluster was distinctly different in the control group, suggesting its potential linkage with CRC.

Conclusions: Our results suggest a similar distribution of several CRC-associated bacteria within CRC patients, underscoring the importance of considering the concomitant presence of bacterial species in studies investigating the mechanisms of CRC development and progression.

Beneficial gut bacteria are indispensable for developing colonic mucus and fully establishing its protective function against intestinal microorganisms. Low-fiber diet consumption alters the gut bacterial configuration and disturbs this microbe-mucus interaction, but the specific bacteria and microbial metabolites responsible for maintaining mucus function remain poorly understood. By using human-to-mouse microbiota transplantation and ex vivo analysis of colonic mucus function, we here show as a proof-of-concept that individuals who increase their daily dietary fiber intake can improve the capacity of their gut microbiota to prevent diet-mediated mucus defects. Mucus growth, a critical feature of intact colonic mucus, correlated with the abundance of the gut commensal Blautia, and supplementation of Blautia coccoides to mice confirmed its mucus-stimulating capacity. Mechanistically, B. coccoides stimulated mucus growth through the production of the short-chain fatty acids propionate and acetate via activation of the short-chain fatty acid receptor Ffar2, which could serve as a new target to restore mucus growth during mucus-associated lifestyle diseases.