Citrobacter rodentium-host-microbiota interactions: immunity, bioenergetics and metabolism

A Th17 cell-intrinsic glutathione/mitochondrial-IL-22 axis protects against intestinal inflammation

The intestinal tract generates significant reactive oxygen species (ROS), but the role of T cell antioxidant mechanisms in maintaining intestinal homeostasis is poorly understood. We used T cell-specific ablation of the catalytic subunit of glutamate cysteine ligase (Gclc), which impaired glutathione (GSH) production, crucially reducing IL-22 production by Th17 cells in the lamina propria, which is critical for gut protection. Under steady-state conditions, Gclc deficiency did not alter cytokine secretion; however, C. rodentium infection induced increased ROS and disrupted mitochondrial function and TFAM-driven mitochondrial gene expression, resulting in decreased cellular ATP. These changes impaired the PI3K/AKT/mTOR pathway, reducing phosphorylation of 4E-BP1 and consequently limiting IL-22 translation. The resultant low IL-22 levels led to poor bacterial clearance, severe intestinal damage, and high mortality. Our findings highlight a previously unrecognized, essential role of Th17 cell-intrinsic GSH in promoting mitochondrial function and cellular signaling for IL-22 protein synthesis, which is critical for intestinal integrity and defense against gastrointestinal infections.

Substance P receptor signaling contributes to host maladaptive responses during enteric bacterial infection

Immune responses in the intestine are intricately balanced to prevent pathogen entry without inducing immunopathology. The nervous system is well-established to interface with the immune system to fine-tune immunity in various organ systems including the gastrointestinal tract. Specialized sensory neurons can detect bacteria, bacterial products, and the resulting inflammation, to coordinate the immune response in the gastrointestinal tract. These sensory neurons release peptide neurotransmitters such as Substance P (SP), to induce both neuronal signaling and localized responses in non-neuronal cells. With this in mind, we assessed the immunoregulatory roles of SP receptor signaling during enteric bacterial infection with the non-invasive pathogen Citrobacter rodentium. Pharmacological antagonism of the SP receptor significantly reduced bacterial burden and prevented colonic crypt hyperplasia. Mice with SP receptor signaling blockade had significantly reduced inflammation and recruitment of T-cells in the colon. Reduced colonic T-cell recruitment is due to reduced expression of adhesion molecules on colonic endothelial cells in SP receptor antagonist-treated mice. Using SP receptor T-cell conditional knockout mice, we further confirmed SP receptor signaling enhanced select aspects of T-cell responses. Our data demonstrates that SP receptor signaling can significantly reduce inflammation and prevent host-maladaptive responses without impinging upon host protection.

Impaired neutrophil migration underpins host susceptibility to infectious colitis

Citrobacter rodentium models infection with enteropathogenic Escherichia coli and ulcerative colitis (UC). While C57BL/6 (C57) mice recover, C3H/HeN (C3H) mice succumb to infection, partially due to increased colonic neutrophil elastase activity, also seen in UC patients; however, the underlying cause was unknown. Here, we found that bone marrow, blood, and colonic C57 neutrophils expressed (CD)11bHi and reached the infected colonic lumen, where they underwent productive NETosis. In contrast, while the number of C3H neutrophils increased in the bone marrow, blood, and colon, they remained CD11bLo and got trapped in the submucosa, away from C. rodentium, where they underwent harmful NETosis. CD11bLo neutrophils in C3H mice infected with CRi9, which triggers expression of neutrophil chemoattractants, reached the colonization site, resulting in host survival. UC patient neutrophils also displayed decreased levels of the activation/differentiation markers CD16/CXCR4. These results, suggesting that neutrophil malfunction contributes to exacerbated colitis, provide insight for future therapeutic prospects.

Citrobacter rodentium infection impairs dopamine metabolism and exacerbates the pathology of Parkinson's disease in mice

Parkinson's disease (PD) is a prevalent neurodegenerative disorder with indistinct etiology and ill-defined pathophysiology. Intestinal inflammation involved in the pathogenesis of PD, but the underlying mechanism is not fully understood. Citrobacter rodentium (C.R) is a gram-negative bacterium that can be used to induce human inflammatory bowel disease in mice. Here, we investigated whether the proinflammatory effects caused by C.R infection initiate PD-like injury and/or exacerbate PD pathology and extensively studied the underlying mechanism. Mice were gavaged once with C.R and monitored for several pathological features at 9 days post infection. The results showed that C.R delivery in mice induced IBD-like symptoms, including significant weight loss, increased fecal water content, an impaired intestinal barrier, intestinal hyperpermeability and inflammation, and intestinal microbiota disturbances. Notably, C.R infection modified dopamine (DA) metabolism in the brains of both male and female mice. Subsequently, a single high dose of MPTP or normal saline was administered at 6 days post infection. At 3 days after MPTP administration, the feces were collected for 16 S rRNA analysis, and PD-like phenotypes and mechanisms were systemically analyzed. Compared with C.R or MPTP injection alone, the injection of C.R and MPTP combined worsened behavioral performance. Moreover, such combination triggered more severe dopaminergic degeneration and glial cell overactivation in the nigrostriatal pathway of mice. Mechanistically, the combination of C.R and MPTP increased the expression of TLR4 and NF-κB p65 in the colon and striatum and upregulated proinflammatory cytokine expression. Therefore, C.R infection-induced intestinal inflammation can impair dopamine metabolism and exacerbate PD pathological processes.

Histone demethylase JMJD2D protects against enteric bacterial infection via up-regulating colonic IL-17F to induce β-defensin expression

Histone demethylase JMJD2D (also known as KDM4D) can specifically demethylate H3K9me2/3 to activate its target gene expression. Our previous study has demonstrated that JMJD2D can protect intestine from dextran sulfate sodium (DSS)-induced colitis by activating Hedgehog signaling; however, its involvement in host defense against enteric attaching and effacing bacterial infection remains unclear. The present study was aimed to investigate the role of JMJD2D in host defense against enteric bacteria and its underlying mechanisms. The enteric pathogen Citrobacter rodentium (C. rodentium) model was used to mimic clinical colonic infection. The responses of wild-type and JMJD2D-/- mice to oral infection of C. rodentium were investigated. Bone marrow chimeric mice were infected with C. rodentium. JMJD2D expression was knocked down in CMT93 cells by using small hairpin RNAs, and Western blot and real-time PCR assays were performed in these cells. The relationship between JMJD2D and STAT3 was studied by co-immunoprecipitation and chromatin immunoprecipitation. JMJD2D was significantly up-regulated in colonic epithelial cells of mice in response to Citrobacter rodentium infection. JMJD2D-/- mice displayed an impaired clearance of C. rodentium, more body weight loss, and more severe colonic tissue pathology compared with wild-type mice. JMJD2D-/- mice exhibited an impaired expression of IL-17F in the colonic epithelial cells, which restricts C. rodentium infection by inducing the expression of antimicrobial peptides. Accordingly, JMJD2D-/- mice showed a decreased expression of β-defensin-1, β-defensin-3, and β-defensin-4 in the colonic epithelial cells. Mechanistically, JMJD2D activated STAT3 signaling by inducing STAT3 phosphorylation and cooperated with STAT3 to induce IL-17F expression by interacting with STAT3 and been recruited to the IL-17F promoter to demethylate H3K9me3. Our study demonstrates that JMJD2D contributes to host defense against enteric bacteria through up-regulating IL-17F to induce β-defensin expression.

Possible link between colonization of the gastrointestinal tract by Citrobacter rodentium in C57BL/6 mice and microbiota composition

Colonization resistance, conferred by the host's microbiota through both direct and indirect protective actions, serves to protect the host from enteric infections. Here, we identified the specific members of the gut microbiota that impact gastrointestinal colonization by Citrobacter rodentium, a murine pathogen causing colonic crypt hyperplasia. The gut colonization levels of C. rodentium in C57BL/6 mice varied among breeding facilities, probably due to differences in microbiota composition. A comprehensive analysis of the microbiota revealed that specific members of the microbiota may influence gut colonization by C. rodentium, thus providing a potential link between the two.

Diet-driven differential response of Akkermansia muciniphila modulates pathogen susceptibility

The erosion of the colonic mucus layer by a dietary fiber-deprived gut microbiota results in heightened susceptibility to an attaching and effacing pathogen, Citrobacter rodentium. Nevertheless, the questions of whether and how specific mucolytic bacteria aid in the increased pathogen susceptibility remain unexplored. Here, we leverage a functionally characterized, 14-member synthetic human microbiota in gnotobiotic mice to deduce which bacteria and functions are responsible for the pathogen susceptibility. Using strain dropouts of mucolytic bacteria from the community, we show that Akkermansia muciniphila renders the host more vulnerable to the mucosal pathogen during fiber deprivation. However, the presence of A. muciniphila reduces pathogen load on a fiber-sufficient diet, highlighting the context-dependent beneficial effects of this mucin specialist. The enhanced pathogen susceptibility is not owing to altered host immune or pathogen responses, but is driven by a combination of increased mucus penetrability and altered activities of A. muciniphila and other community members. Our study provides novel insights into the mechanisms of how discrete functional responses of the same mucolytic bacterium either resist or enhance enteric pathogen susceptibility.

Metabolism of L-arabinose converges with virulence regulation to promote enteric pathogen fitness

Virulence and metabolism are often interlinked to control the expression of essential colonisation factors in response to host-associated signals. Here, we identified an uncharacterised transporter of the dietary monosaccharide ʟ-arabinose that is widely encoded by the zoonotic pathogen enterohaemorrhagic Escherichia coli (EHEC), required for full competitive fitness in the mouse gut and highly expressed during human infection. Discovery of this transporter suggested that EHEC strains have an enhanced ability to scavenge ʟ-arabinose and therefore prompted us to investigate the impact of this nutrient on pathogenesis. Accordingly, we discovered that ʟ-arabinose enhances expression of the EHEC type 3 secretion system, increasing its ability to colonise host cells, and that the underlying mechanism is dependent on products of its catabolism rather than the sensing of ʟ-arabinose as a signal. Furthermore, using the murine pathogen Citrobacter rodentium, we show that ʟ-arabinose metabolism provides a fitness benefit during infection via virulence factor regulation, as opposed to supporting pathogen growth. Finally, we show that this mechanism is not restricted to ʟ-arabinose and extends to other pentose sugars with a similar metabolic fate. This work highlights the importance integrating central metabolism with virulence regulation in order to maximise competitive fitness of enteric pathogens within the host-niche.

IL-22-dependent responses and their role during Citrobacter rodentium infection

The mouse pathogen Citrobacter rodentium is utilized as a model organism for studying infections caused by the human pathogens enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) and to elucidate mechanisms of mucosal immunity. In response to C. rodentium infection, innate lymphoid cells and T cells secrete interleukin (IL)-22, a cytokine that promotes mucosal barrier function. IL-22 plays a pivotal role in enabling mice to survive and recover from C. rodentium infection, although the exact mechanisms involved remain incompletely understood. Here, we investigated whether particular components of the host response downstream of IL-22 contribute to the cytokine's protective effects during C. rodentium infection. In line with previous research, mice lacking the IL-22 gene (Il22-/- mice) were highly susceptible to C. rodentium infection. To elucidate the role of specific antimicrobial proteins modulated by IL-22, we infected the following knockout mice: S100A9-/- (calprotectin), Lcn2-/- (lipocalin-2), Reg3b-/- (Reg3β), Reg3g-/- (Reg3γ), and C3-/- (C3). All knockout mice tested displayed a considerable level of resistance to C. rodentium infection, and none phenocopied the lethality observed in Il22-/- mice. By investigating another arm of the IL-22 response, we observed that C. rodentium-infected Il22-/- mice exhibited an overall decrease in gene expression related to intestinal barrier integrity as well as significantly elevated colonic inflammation, gut permeability, and pathogen levels in the spleen. Taken together, these results indicate that host resistance to lethal C. rodentium infection may depend on multiple antimicrobial responses acting in concert, or that other IL-22-regulated processes, such as tissue repair and maintenance of epithelial integrity, play crucial roles in host defense to attaching and effacing pathogens.

Group 3 innate lymphoid cells: A trained Gutkeeper

Group 3 innate lymphoid cells (ILC3s) are tissue-resident immune lymphocytes that critically regulate intestinal homeostasis, organogenesis, and immunity. ILC3s possess the capacity to "sense" the inflammatory environment within tissues, especially in the context of pathogen challenges that imprints durable non-antigen-specific changes in ILC3 function. As such, ILC3s become a new actor in the emerging field of trained innate immunity. Here, we summarize recent discoveries regarding ILC3 responses to bacterial challenges and the role these encounters play in triggering trained innate immunity. We further discuss how signaling events throughout ILC3 ontogeny potentially control the development and function of trained ILC3s. Finally, we highlight the open questions surrounding ILC3 "training" the answers to which may reveal new insights into innate immunity. Understanding the fundamental concepts behind trained innate immunity could potentially lead to the development of new strategies for improving immunity-based modulation therapies for inflammation, infectious diseases, and cancer.

Why put yourself on a pedestal? The pathogenic role of the A/E pedestal

Certain Escherichia coli (E. coli) strains are attaching and effacing (A/E) lesion pathogens that primarily infect intestinal epithelial cells. They cause actin restructuring and polymerization within the host cell to create an actin-rich protrusion below the site of adherence, termed the pedestal. Although there is clarity on the pathways initiating pedestal formation, the underlying purpose(s) of the pedestal remains ambiguous. The conservation of pedestal-forming activity across multiple pathogens and redundancy in formation pathways indicate a pathogenic advantage. However, few decisive conclusions have been drawn, given that the results vary between model systems. Some research argues that the pedestal increases the colonization capability of the bacterium. These studies utilize A/E pathogens specifically deficient in pedestal formation to evaluate adhesion and intestinal colonization following infection. There have been many proposed mechanisms for the colonization benefit conferred by the pedestal. One suggested benefit is that the pedestal allows for direct cytosolic anchoring through incorporation of the established host cortical actin, causing a stable link between the pathogen and cell structure. The pedestal may confer enhanced motility, as enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) are better able to migrate on the surface of host cells and infect neighboring cells in the presence of the pedestal. Additionally, some research suggests that the pedestal improves effector delivery. This review will investigate the purpose of pedestal formation using evidence from recent literature and will critically evaluate the methodology and model systems. Most importantly, we will contextualize the proposed functions to reconcile potential synergistic effects.

Dopamine receptor D2 confers colonization resistance via microbial metabolites

The gut microbiome has major roles in modulating host physiology. One such function is colonization resistance, or the ability of the microbial collective to protect the host against enteric pathogens1-3, including enterohaemorrhagic Escherichia coli (EHEC) serotype O157:H7, an attaching and effacing (AE) food-borne pathogen that causes severe gastroenteritis, enterocolitis, bloody diarrhea and acute renal failure4,5 (haemolytic uremic syndrome). Although gut microorganisms can provide colonization resistance by outcompeting some pathogens or modulating host defence provided by the gut barrier and intestinal immune cells6,7, this phenomenon remains poorly understood. Here, we show that activation of the neurotransmitter receptor dopamine receptor D2 (DRD2) in the intestinal epithelium by gut microbial metabolites produced upon dietary supplementation with the essential amino acid L-tryptophan protects the host against Citrobacter rodentium, a mouse AE pathogen that is widely used as a model for EHEC infection8,9. We further find that DRD2 activation by these tryptophan-derived metabolites decreases expression of a host actin regulatory protein involved in C. rodentium and EHEC attachment to the gut epithelium via formation of actin pedestals. Our results reveal a noncanonical colonization resistance pathway against AE pathogens that features an unconventional role for DRD2 outside the nervous system in controlling actin cytoskeletal organization in the gut epithelium. Our findings may inspire prophylactic and therapeutic approaches targeting DRD2 with dietary or pharmacological interventions to improve gut health and treat gastrointestinal infections, which afflict millions globally.

Pericytes modulate endothelial inflammatory response during bacterial infection

Pericytes are located around blood vessels, in close contact with endothelial cells. We discovered that pericytes dampen pro-inflammatory endothelial cell responses. Endothelial cells co-cultured with pericytes had markedly reduced expression of adhesion molecules (PECAM-1 and ICAM-1) and proinflammatory cytokines (CCL-2 and IL-6) in response to bacterial stimuli (Brucella ovis, Listeria monocytogenes, or Escherichia coli lipopolysaccharide). Pericyte-depleted mice intraperitoneally inoculated with either B. ovis, a stealthy pathogen that does not trigger detectable inflammation, or Listeria monocytogenes, developed peritonitis. Further, during Citrobacter rodentium infection, pericyte-depleted mice developed severe intestinal inflammation, which was not evident in control mice. The anti-inflammatory effect of pericytes required connexin 43, as either chemical inhibition or silencing of connexin 43 abrogated pericyte-mediated suppression of endothelial inflammatory responses. Our results define a mechanism by which pericytes modulate inflammation during infection, which shifts our understanding of pericyte biology: from a structural cell to a pro-active player in modulating inflammation.

IMPORTANCE

A previously unknown mechanism by which pericytes modulate inflammation was discovered. The absence of pericytes or blocking interaction between pericytes and endothelium through connexin 43 results in stronger inflammation, which shifts our understanding of pericyte biology, from a structural cell to a player in controlling inflammation.

Fecal microbiota impacts development of Cryptosporidium parvum in the mouse

The dependence of Cryptosporidium parasites on host cell metabolites suggests that the development of nutritional interventions to limit parasite proliferation should be feasible. Based on this concept, we are testing dietary interventions to affect the enterocytes' metabolism in a manner that limits intracellular multiplication of the parasite. We hypothesize that changes in the metabolic pathways encoded by the gastro-intestinal tract microbiota may restrict parasite proliferation. To identify taxonomic and metabolic features of the microbiota associated with severity of cryptosporidiosis, as determined by estimating oocyst output, we characterized the fecal microbiota from mice experimentally infected with Cryptosporidium parvum. To eliminate the confounding effect of the interaction between co-housed mice, as well as facilitate the identification of microbiota markers associated with severity of cryptosporidiosis, fecal microbiota from individually caged mice were analyzed. Variation partitioning analysis applied to 16S sequence data from 25 mice belonging to four experiments shows that experiment was by far the biggest source of microbiota variation. Severity of cryptosporidiosis explained a smaller, though significant, fraction of microbiota variation. Notably, this effect was significant in the pre-patent phase of the infection, before mice excreted oocysts. These results are consistent with the pre-patent intestinal microbiota having a modest, but measurable, effect on cryptosporidiosis.

The Effect of Opuntia ficus Mucilage Pectin and Citrus aurantium Extract Added to a Food Matrix on the Gut Microbiota of Lean Humans and Humans with Obesity

Experimental studies have provided evidence that physicochemical interactions in the food matrix can modify the biologically beneficial effects of bioactive compounds, including their effect on gut microbiota. This work aimed to evaluate the effect of a food gel matrix with Opuntia ficus cladodes mucilage pectin and Citrus Aurantium extract on the growth of four beneficial gut bacteria obtained from the fecal microbiota of people who are lean or who have obesity after digestion in the upper digestive system. To accomplish this, a base formulation of Opuntia ficus cladodes mucilage with or without C. aurantium extract was submitted to an ex vivo fecal fermentation in an automatic and robotic intestinal system. The changes in the intestinal microbiota were determined by means of plate culture and 16S sequencing, while short-chain fatty acids (SCFA) produced in the colon were determined via gas chromatography. In the presence of the extract in formulation, greater growth of Bifidobacterium spp. (+1.6 Log10 Colonic Forming Unit, UFC) and Lactobacillus spp. (+2 Log10 UFC) in the microbiota of lean people was observed. Only the growth in Salmonella spp. (-1 Log10 UFC) from both microbiota was affected in the presence of the extract, which decreased in the ascending colon. SCFA was mainly produced by the microbiota of people who were lean rather than those who had obesity in the presence of the extract, particularly in the ascending colon. The effect of sour orange extract seems to depend on the origin of the microbiota, whether in people who have obesity (25 mM/L) or are lean (39 mM/L).

Gut complement induced by the microbiota combats pathogens and spares commensals

Canonically, the complement system is known for its rapid response to remove microbes in the bloodstream. However, relatively little is known about a functioning complement system on intestinal mucosal surfaces. Herein, we report the local synthesis of complement component 3 (C3) in the gut, primarily by stromal cells. C3 is expressed upon commensal colonization and is regulated by the composition of the microbiota in healthy humans and mice, leading to an individual host's specific luminal C3 levels. The absence of membrane attack complex (MAC) components in the gut ensures that C3 deposition does not result in the lysis of commensals. Pathogen infection triggers the immune system to recruit neutrophils to the infection site for pathogen clearance. Basal C3 levels directly correlate with protection against enteric infection. Our study reveals the gut complement system as an innate immune mechanism acting as a vigilant sentinel that combats pathogens and spares commensals.

Complement in breast milk modifies offspring gut microbiota to promote infant health

Breastfeeding offers demonstrable benefits to newborns and infants by providing nourishment and immune protection and by shaping the gut commensal microbiota. Although it has been appreciated for decades that breast milk contains complement components, the physiological relevance of complement in breast milk remains undefined. Here, we demonstrate that weanling mice fostered by complement-deficient dams rapidly succumb when exposed to murine pathogen Citrobacter rodentium (CR), whereas pups fostered on complement-containing milk from wild-type dams can tolerate CR challenge. The complement components in breast milk were shown to directly lyse specific members of gram-positive gut commensal microbiota via a C1-dependent, antibody-independent mechanism, resulting in the deposition of the membrane attack complex and subsequent bacterial lysis. By selectively eliminating members of the commensal gut community, complement components from breast milk shape neonate and infant gut microbial composition to be protective against environmental pathogens such as CR.

A novel antibacterial strategy for targeting the bacterial methionine biosynthesis pathway

Bacterial pathogens reprogramme their metabolic networks to support growth and establish infection at specific sites. Bacterial central metabolism has been considered attractive for developing antimicrobial drugs; however, most metabolic enzymes are conserved between humans and bacteria. This study found that blockade of methionine biosynthesis in Citrobacter rodentium and Salmonella enteritidis inhibited bacterial growth and activity of the type III secretion system, resulting in severe defects in colonization and pathogenicity. In addition, α-methyl-methionine was found to inhibit the activity of methionine biosynthetic enzyme MetA, and consequently reduce the virulence and pathogenicity of enteric pathogens. These findings highlight the crucial role of methionine in bacterial virulence, and describe a potential new drug target.

Comparison of the gut microbiota and metabolism in different regions of Red Swamp Crayfish (Procambarus clarkii)

Background

The gut microbiota is very important for maintaining the homeostasis and health of crustaceans. Many factors affect the gut microbiota of crustaceans, one of which is temperature. However, it is currently unclear how temperature affects the gut microbiota and metabolites of Procambarus clarkii.

Methods

Using metagenomic sequencing and gas chromatography-mass spectrometry (GC-MS) techniques, the gut microbiota and metabolites of P. clarkii from Hubei (HB), Jiangsu (JS), Shandong (SD), and Zhejiang (ZJ) in China were investigated.

Results

Under the impact of temperature, the gut microbiota and metabolites of P. clarkii exhibit a specific trend of change. The primary pathogenic bacteria affecting P. clarkii are Citrobacter, Enterobacterium, and Aeromonas, which are affected by temperature. Two metabolites, namely, sugars and amino acids, are regulated by temperature.

Implication

This study demonstrated that the gut microbiota and gut metabolites of P. clarkii were considerably affected by temperature. It provides a theoretical basis for the systematic study of P. clarkii and provides a basis for a healthy culture of P. clarkii.

Pomegranate (Punica granatum L.) and Its Rich Ellagitannins as Potential Inhibitors in Ulcerative Colitis

Ulcerative colitis, an immune-mediated inflammatory disease of the gastrointestinal tract, places a significant financial burden on patients and the healthcare system. Recently, reviews of the pomegranate and the abundant medicinal applications of its ellagitannins, as well as its pharmacological action, phytochemicals, metabolism, and pharmacokinetics, have been completed. However, summaries on their anti-ulcerative colitis effects are lacking. Numerous preclinical animal investigations and clinical human trial reports demonstrated the specific therapeutic effects of pomegranate and the effect of its ellagitannins against ulcerative colitis. According to the literature collected by Sci-finder and PubMed databases over the past 20 years, this is the first review that has compiled references regarding how the rich ellagitannins found in pomegranate have altered the ulcerative colitis. It was suggested that the various parts of pomegranates and their rich ellagitannins (especially their primary components, punicalagin, and ellagic acid) can inhibit oxidant and inflammatory processes, regulate the intestinal barrier and flora, and provide an anti-ulcerative colitis resource through dietary management.

Diet-driven differential response of Akkermansia muciniphila modulates pathogen susceptibility

The erosion of the colonic mucus layer by a dietary fiber-deprived gut microbiota results in heightened susceptibility to an attaching and effacing pathogen, Citrobacter rodentium. Nevertheless, the questions of whether and how specific mucolytic bacteria aid in the increased pathogen susceptibility remain unexplored. Here, we leverage a functionally characterized, 14-member synthetic human microbiota in gnotobiotic mice to deduce which bacteria and functions are responsible for the pathogen susceptibility. Using strain dropouts of mucolytic bacteria from the community, we show that Akkermansia muciniphila renders the host more vulnerable to the mucosal pathogen during fiber deprivation. However, the presence of A. muciniphila reduces pathogen load on a fiber-sufficient diet, highlighting the context-dependent beneficial effects of this mucin specialist. The enhanced pathogen susceptibility is not owing to altered host immune or pathogen responses, but is driven by a combination of increased mucus penetrability and altered activities of A. muciniphila and other community members. Our study provides novel insights into the mechanisms of how discrete functional responses of the same mucolytic bacterium either resist or enhance enteric pathogen susceptibility.

The autoinflammation-associated NLRC4V341A mutation increases microbiota-independent IL-18 production but does not recapitulate human autoinflammatory symptoms in mice

Background

Autoinflammation with infantile enterocolitis (AIFEC) is an often fatal disease caused by gain-of-function mutations in the NLRC4 inflammasome. This inflammasomopathy is characterized by macrophage activation syndrome (MAS)-like episodes as well as neonatal-onset enterocolitis. Although elevated IL-18 levels were suggested to take part in driving AIFEC pathology, the triggers for IL-18 production and its ensuing pathogenic effects in these patients are incompletely understood.

Methods

Here, we developed and characterized a novel genetic mouse model expressing a murine version of the AIFEC-associated NLRC4V341A mutation from its endogenous Nlrc4 genomic locus.

Results

NLRC4V341A expression in mice recapitulated increased circulating IL-18 levels as observed in AIFEC patients. Housing NLRC4V341A-expressing mice in germfree (GF) conditions showed that these systemic IL-18 levels were independent of the microbiota, and unmasked an additional IL-18-inducing effect of NLRC4V341A expression in the intestines. Remarkably, elevated IL-18 levels did not provoke detectable intestinal pathologies in NLRC4V341A-expressing mice, even not upon genetically ablating IL-18 binding protein (IL-18BP), which is an endogenous IL-18 inhibitor that has been used therapeutically in AIFEC. In addition, NLRC4V341A expression did not alter susceptibility to the NLRC4-activating gastrointestinal pathogens Salmonella Typhimurium and Citrobacter rodentium.

Conclusion

As observed in AIFEC patients, mice expressing a murine NLRC4V341A mutant show elevated systemic IL-18 levels, suggesting that the molecular mechanisms by which this NLRC4V341A mutant induces excessive IL-18 production are conserved between humans and mice. However, while our GF and infection experiments argue against a role for commensal or pathogenic bacteria, identifying the triggers and mechanisms that synergize with IL-18 to drive NLRC4V341A-associated pathologies will require further research in this NLRC4V341A mouse model.

Turicibacterales protect mice from severe Citrobacter rodentium infection

One of the major contributors to child mortality in the world is diarrheal diseases, with an estimated 800,000 deaths per year. Many pathogens are causative agents of these illnesses, including the enteropathogenic or enterohemorrhagic forms of Escherichia coli. These bacteria are characterized by their ability to cause attaching and effacing lesions in the gut mucosa. Although much has been learned about the pathogenicity of these organisms and the immune response against them, the role of the intestinal microbiota during these infections is not well characterized. Infection of mice with E. coli requires pre-treatment with antibiotics in most mouse models, which hinders the study of the microbiota in an undisturbed environment. Using Citrobacter rodentium as a murine model for attaching and effacing bacteria, we show that C57BL/6 mice deficient in granzyme B expression are highly susceptible to severe disease caused by C. rodentium infection. Although a previous publication from our group shows that granzyme B-deficient CD4+ T cells are partially responsible for this phenotype, in this report, we present data demonstrating that the microbiota, in particular members of the order Turicibacterales, have an important role in conferring resistance. Mice deficient in Turicibacter sanguinis have increased susceptibility to severe disease. However, when these mice are co-housed with resistant mice or colonized with T. sanguinis, susceptibility to severe infection is reduced. These results clearly suggest a critical role for this commensal in the protection against enteropathogens.

Review article: The complex interplay between diet and Escherichia coli in inflammatory bowel disease

BACKGROUND

Although no causative microbe has been yet identified or successfully targeted in the treatment of inflammatory bowel disease (IBD), the role of Escherichia coli in the pathogenesis of Crohn's disease has attracted considerable interest.

AIM

In this review, we present a literature overview of the interactions between diet and E. coli and other Proteobacteria in the aetiology, outcomes and management of IBD and suggest future research directions.

METHODS

An extensive literature search was performed to identify in vitro studies and research in animal models that explored mechanisms by which dietary components can interact with E. coli or Proteobacteria to initiate or propagate gut inflammation. We also explored the effect diet and dietary therapies have on the levels of E. coli or Proteobacteria in patients with IBD.

RESULTS

Preclinical data suggest that the Western diet and its components influence the abundance, colonisation and phenotypic behaviour of E. coli in the gut, which may in turn initiate or contribute to gut inflammation. In contrast, the Mediterranean diet and specific dietary fibres may abrogate these effects and protect from inflammation. There are limited data from clinical trials, mostly from patients with Crohn's disease during treatment with exclusive enteral nutrition, with findings often challenging observations from preclinical research. Data from patients with ulcerative colitis are sparse.

CONCLUSIONS

Preclinical and some clinical trial data suggest that E. coli and other Proteobacteria interact with certain dietary components to promote gut inflammation. Well-designed clinical trials are required before dietary recommendations for disease management can be made.

The impact of helminth-induced immunity on infection with bacteria or viruses

Different human and animal pathogens trigger distinct immune responses in their hosts. The infection of bacteria or viruses can trigger type I pro-inflammatory immune responses (e.g., IFN-γ, TNF-α, TH1 cells), whereas infection by helminths typically elicits a type II host resistance and tolerizing immune response (e.g., IL-4, IL-5, IL-13, TH2 cells). In some respects, the type I and II immune responses induced by these different classes of pathogens are antagonistic. Indeed, recent studies indicate that infection by helminths differentially shapes the response and outcome of subsequent infection by viruses and bacteria. In this review, we summarize the current knowledge on how helminth infections influence concurrent or subsequent microbial infections and also discuss the implications for helminth-mediated immunity on the outcome of SARS-CoV-2 disease.

Xianglian Zhixie Tablet Antagonizes Dextran Sulfate Sodium-Induced Ulcerative Colitis by Attenuating Systemic Inflammation and Modulating Gut Microbiota

Purpose

Xianglian Zhixie Tablet (XLZXT), a classical traditional Chinese medicine formulation, is commonly used to treat Ulcerative Colitis (UC) in China. However, the therapeutic mechanisms of XLZXT for UC have yet to be fully understood. This study aimed to investigate the curative benefits of XLZXT and its associated mechanisms for healing UC in mice.

Methods

In the present study, the 1% dextran sulfate sodium (DSS) solution was used to establish the UC model in C57BL/6N mice. To investigate the therapeutic effects of XLZXT on DSS-induced UC mice, several parameters were measured, including DAI score, colon length, spleen index, pathological changes in colon tissue, and levels of inflammatory factors in plasma and colon tissue. By investigating the gut microbiota, assessing the levels of intestinal mucosal protein expression, and looking at the proteins involved in the TLR4/MyD88/NF-B p65 signaling pathway, the mechanisms of XLZXT impact on UC were investigated. Mouse feces were examined for patterns of gut microbiota expression using high-throughput sequencing of 16S rRNA.

Results

XLZXT effectively alleviated UC symptoms and colon pathological damage in DSS-induced UC mice. It improved body weight loss, stool consistency, and hematochezia, while also repairing colon damage. Moreover, it down-regulated pro-inflammatory cytokines (such as TNF-α, IL-1β, and IL-6), and up-regulated anti-inflammatory cytokines (such as IL-10). XLZXT also increased the expression of MUC-2, Occludin and ZO-1, while decreasing the expression of NF-κB, MyD88 and TLR4. Additionally, it regulated gut microbiota disorder by increasing the abundance of beneficial bacteria and reducing the adhesion of intestinal harmful bacteria.

Conclusion

XLZXT demonstrated therapeutic effects on DSS-induced UC mice. The mechanisms may be associated with repairing the intestinal mucosal barrier, regulating the TLR4/MyD88/NF-κB p65 signaling pathway, and restoring the balance of gut microbiota.

Epithelial NAD+ depletion drives mitochondrial dysfunction and contributes to intestinal inflammation

Introduction

We have previously demonstrated that a pathologic downregulation of peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC1α) within the intestinal epithelium contributes to the pathogenesis of inflammatory bowel disease (IBD). However, the mechanism underlying downregulation of PGC1α expression and activity during IBD is not yet clear.

Methods

Mice (male; C57Bl/6, Villincre/+;Pgc1afl/fl mice, and Pgc1afl/fl) were subjected to experimental colitis and treated with nicotinamide riboside. Western blot, high-resolution respirometry, nicotinamide adenine dinucleotide (NAD+) quantification, and immunoprecipitation were used to in this study.

Results

We demonstrate a significant depletion in the NAD+ levels within the intestinal epithelium of mice undergoing experimental colitis, as well as humans with ulcerative colitis. While we found no decrease in the levels of NAD+-synthesizing enzymes within the intestinal epithelium of mice undergoing experimental colitis, we did find an increase in the mRNA level, as well as the enzymatic activity, of the NAD+-consuming enzyme poly(ADP-ribose) polymerase-1 (PARP1). Treatment of mice undergoing experimental colitis with an NAD+ precursor reduced the severity of colitis, restored mitochondrial function, and increased active PGC1α levels; however, NAD+ repletion did not benefit transgenic mice that lack PGC1α within the intestinal epithelium, suggesting that the therapeutic effects require an intact PGC1α axis.

Discussion

Our results emphasize the importance of PGC1α expression to both mitochondrial health and homeostasis within the intestinal epithelium and suggest a novel therapeutic approach for disease management. These findings also provide a mechanistic basis for clinical trials of nicotinamide riboside in IBD patients.

A Th17 cell-intrinsic glutathione/mitochondrial-IL-22 axis protects against intestinal inflammation

Although the intestinal tract is a major site of reactive oxygen species (ROS) generation, the mechanisms by which antioxidant defense in gut T cells contribute to intestinal homeostasis are currently unknown. Here we show, using T cell-specific ablation of the catalytic subunit of glutamate cysteine ligase (Gclc), that the ensuing loss of glutathione (GSH) impairs the production of gut-protective IL-22 by Th17 cells within the lamina propria. Although Gclc ablation does not affect T cell cytokine secretion in the gut of mice at steady-state, infection with C. rodentium increases ROS, inhibits mitochondrial gene expression and mitochondrial function in Gclc-deficient Th17 cells. These mitochondrial deficits affect the PI3K/AKT/mTOR pathway, leading to reduced phosphorylation of the translation repressor 4E-BP1. As a consequence, the initiation of translation is restricted, resulting in decreased protein synthesis of IL-22. Loss of IL-22 results in poor bacterial clearance, enhanced intestinal damage, and high mortality. ROS-scavenging, reconstitution of IL-22 expression or IL-22 supplementation in vivo prevent the appearance of these pathologies. Our results demonstrate the existence of a previously unappreciated role for Th17 cell-intrinsic GSH coupling to promote mitochondrial function, IL-22 translation and signaling. These data reveal an axis that is essential for maintaining the integrity of the intestinal barrier and protecting it from damage caused by gastrointestinal infection.

Modulation of innate lymphoid cells by enteric bacterial pathogens

Innate lymphoid cells (ILCs) are key regulators of tissue homeostasis, inflammation, and immunity to infections. ILCs rapidly respond to environmental cues such as cytokines, microbiota and invading pathogens which regulate their function and phenotype. Even though ILCs are rare cells, they are enriched at barrier surfaces such as the gastrointestinal (GI) tract, and they are often critical to the host's immune response to eliminate pathogens. On the other side of host-pathogen interactions, pathogenic bacteria also have the means to modulate these immune responses. Manipulation or evasion of the immune cells is often to the pathogen's benefit and/or to the detriment of competing microbiota. In some instances, specific bacterial virulence factors or toxins have been implicated in how the pathogen modulates immunity. In this review, we discuss the recent progress made towards understanding the role of non-cytotoxic ILCs during enteric bacterial infections, how these pathogens can modulate the immune response, and the implications these have on developing new therapies to combat infection.

Cooperation between physiological defenses and immune resistance produces asymptomatic carriage of a lethal bacterial pathogen

Animals evolved two defense strategies to survive infections. Antagonistic strategies include immune resistance mechanisms that operate to kill invading pathogens. Cooperative or physiological defenses mediate host adaptation to the infected state, limiting physiological damage and disease, without killing the pathogen, and have been shown to cause asymptomatic carriage and transmission of lethal pathogens. Here, we demonstrate that physiological defenses cooperate with the adaptive immune response to generate long-term asymptomatic carriage of the lethal enteric murine pathogen, Citrobacter rodentium. Asymptomatic carriage of genetically virulent C. rodentium provided immune resistance against subsequent infections. Immune protection was dependent on systemic antibody responses and pathogen virulence behavior rather than the recognition of specific virulent antigens. Last, we demonstrate that an avirulent strain of C. rodentium in the field has background mutations in genes that are important for LPS structure. Our work reveals insight into how asymptomatic infections can arise mechanistically with immune resistance, mediating exclusion of phenotypically virulent enteric pathogen to promote asymptomatic carriage.

Temporary consumption of western diet trains the immune system to reduce future gut inflammation

Urbanization drives the popularity of western diet (WD), which increased burden in metabolic diseases but also in inflammatory diseases. Here, we show continuous WD disrupted the gut barrier, initiating low-grade inflammation and enhancing the colitis response. Nevertheless, transient WD consumption followed by ad libitum normal diet enhanced mucin production and tight junction protein expression in recovered mice. Furthermore, transient WD consumption surprisingly reduced the subsequent inflammatory response in DSS colitis and Citrobacter rodentium-infection induced colitis. The protective effect of WD training was not sex-dependent, and co-housing experiments suggested microbiota changes were not responsible. We identified important roles for cholesterol biosynthesis pathway and macrophages, pointing to innate myeloid training. Together, these data suggest detrimental effects of WD consumption can be reversed on return to a healthier diet. Furthermore, transient WD consumption leads to beneficial immune training, suggesting an evolutionary mechanism to benefit from feasting when abundant food is available.

IEC-intrinsic IL-1R signaling holds dual roles in regulating intestinal homeostasis and inflammation

Intestinal epithelial cells (IECs) constitute a critical first line of defense against microbes. While IECs are known to respond to various microbial signals, the precise upstream cues regulating diverse IEC responses are not clear. Here, we discover a dual role for IEC-intrinsic interleukin-1 receptor (IL-1R) signaling in regulating intestinal homeostasis and inflammation. Absence of IL-1R in epithelial cells abrogates a homeostatic antimicrobial program including production of antimicrobial peptides (AMPs). Mice deficient for IEC-intrinsic IL-1R are unable to clear Citrobacter rodentium (C. rodentium) but are protected from DSS-induced colitis. Mechanistically, IL-1R signaling enhances IL-22R-induced signal transducer and activator of transcription 3 (STAT3) phosphorylation in IECs leading to elevated production of AMPs. IL-1R signaling in IECs also directly induces expression of chemokines as well as genes involved in the production of reactive oxygen species. Our findings establish a protective role for IEC-intrinsic IL-1R signaling in combating infections but a detrimental role during colitis induced by epithelial damage.

Turicibacterales protect mice from severe Citrobacter rodentium infection

One of the major contributors to child mortality in the world is diarrheal diseases, with an estimated 800,000 deaths per year. Many pathogens are causative agents of these illnesses, including the enteropathogenic (EPEC) or enterohemorrhagic (EHEC) forms of Escherichia coli. These bacteria are characterized by their ability to cause attaching and effacing lesions in the gut mucosa. Although much has been learned about the pathogenicity of these organisms and the immune response against them, the role of the intestinal microbiota during these infections is not well characterized. Infection of mice with E. coli requires pre-treatment with antibiotics in most mouse models, which hinders the study of the microbiota in an undisturbed environment. Using Citrobacter rodentium as a murine model for attaching and effacing bacteria, we show that C57BL/6 mice deficient in granzyme B expression are highly susceptible to severe disease caused by C. rodentium infection. Although a previous publication from our group shows that granzyme B-deficient CD4+ T cells are partially responsible for this phenotype, in this report we present data demonstrating that the microbiota, in particular members of the order Turicibacterales, have an important role in conferring resistance. Mice deficient in Turicibacter sanguinis have increased susceptibility to severe disease. However, when these mice are co-housed with resistant mice, or colonized with T. sanguinis, susceptibility to severe infection is reduced. These results clearly suggest a critical role for this commensal in the protection against entero-pathogens.

Ginkgo biloba Extract Preventively Intervenes in Citrobacter Rodentium-Induced Colitis in Mice

Inflammatory bowel disease (IBD) represents a highly recurrent gastrointestinal disorder and global public health issue. However, it lacks effective and safe strategies for its control. Although Ginkgo biloba extract (GBE) has been suggested to exhibit preventive and therapeutic activity for the control of IBD, whether its activity is associated with its ability to modulate intestinal microbiota remains to be addressed. To investigate the effect of GBE on controlling IBD, a Citrobacter Rodentium (CR)-induced mouse colitis model was used, and then histopathological examinations, biochemical assays, immunohistochemistry, and immunoblotting were performed to detect histological changes, cytokines, and tight junction (TJ) proteins in the intestine samples. We also studied 16s rRNA to detect changes in intestinal microbiota and used GC-MS to determine the microbiota-related metabolites short chain fatty acids (SCFAs). The results of our studies revealed that pre-treatment with GBE was sufficient for protecting the animals from CR-induced colitis. As a mechanism for GBE activity, GBE treatment was able to modulate the intestinal microbiota and increase the SCFAs capable of decreasing the pro-inflammatory factors and up-regulating the anti-inflammatory factors while elevating the intestinal-barrier-associated proteins to maintain the integrity of the intestines. Accordingly, our results led to a strong suggestion that GBE should be seriously considered in the preventive control of CR-induced colitis and in the development of effective and safe therapeutic strategies for controlling IBD.

Dopamine receptor D2 confers colonization resistance via gut microbial metabolites

The gut microbiome plays major roles in modulating host physiology. One such function is colonization resistance, or the ability of the microbial collective to protect the host against enteric pathogens1-3, including enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7, an attaching and effacing (AE) food-borne pathogen that causes severe gastroenteritis, enterocolitis, bloody diarrhea, and acute renal failure (hemolytic uremic syndrome)4,5. Although gut microbes can provide colonization resistance by outcompeting some pathogens or modulating host defense provided by the gut barrier and intestinal immune cells, this phenomenon remains poorly understood. Emerging evidence suggests that small-molecule metabolites produced by the gut microbiota may mediate this process6. Here, we show that tryptophan (Trp)-derived metabolites produced by the gut bacteria protect the host against Citrobacter rodentium, a murine AE pathogen widely used as a model for EHEC infection7,8, by activation of the host neurotransmitter dopamine receptor D2 (DRD2) within the intestinal epithelium. We further find that these Trp metabolites act through DRD2 to decrease expression of a host actin regulatory protein involved in C. rodentium and EHEC attachment to the gut epithelium via formation of actin pedestals. Previously identified mechanisms of colonization resistance either directly affect the pathogen by competitive exclusion or indirectly by modulation of host defense mechanisms9,10, so our results delineate a noncanonical colonization resistance pathway against AE pathogens featuring an unconventional role for DRD2 outside the nervous system in controlling actin cytoskeletal organization within the gut epithelium. Our findings may inspire prophylactic and therapeutic approaches for improving gut health and treating gastrointestinal infections, which afflict millions globally.

Modelling the Gastrointestinal Carriage of Klebsiella pneumoniae Infections

Klebsiella pneumoniae is a leading cause of nosocomial and community acquired infections, making K. pneumoniae the pathogen that is associated with the second largest number of deaths attributed to any antibiotic resistant infection. K. pneumoniae colonizes the nasopharynx and the gastrointestinal tract in an asymptomatic manner without dissemination to other tissues. Importantly, gastrointestinal colonization is a requisite for infection. Our understanding of K. pneumoniae colonization is still based on interrogating mouse models in which animals are pretreated with antibiotics to disturb the colonization resistance imposed by the gut microbiome. In these models, infections disseminate to other tissues. Here, we report a murine model to allow for the study of the gastrointestinal colonization of K. pneumoniae without tissue dissemination. Hypervirulent and antibiotic resistant strains stably colonize the gastrointestinal tract of in an inbred mouse population without antibiotic treatment. The small intestine is the primary site of colonization and is followed by a transition to the colon over time, without dissemination to other tissues. Our model recapitulates the disease dynamics of the metastatic K. pneumoniae strains that are able to disseminate from the gastrointestinal tract to other sterile sites. Colonization is associated with mild to moderate histopathology, no significant inflammation, and no effect on the richness of the microbiome. Our model sums up the clinical scenario in which antibiotic treatment disturbs the colonization of K. pneumoniae and results in dissemination to other tissues. Finally, we establish that the capsule polysaccharide is necessary for the colonization of the large intestine, whereas the type VI secretion system contributes to colonization across the gastrointestinal tract. IMPORTANCE Klebsiella pneumoniae is one of the pathogens that is sweeping the world in the antibiotic resistance pandemic. Klebsiella colonizes the nasopharynx and the gut of healthy subjects in an asymptomatic manner, making gut colonization a requisite for infection. This makes it essential to understand the gastrointestinal carriage in preventing Klebsiella infections. Current research models rely on the perturbation of the gut microbiome by antibiotics, resulting in an invasive infection. Here, we report a new model of K. pneumoniae gut colonization that recapitulates key features of the asymptomatic human gastrointestinal tract colonization. In our model, there is no need to disturb the microbiota to achieve stable colonization, and there is no dissemination to other tissues. Our model sums up the clinical scenario in which antibiotic treatment triggers invasive infection. We envision that our model will be an excellent platform upon which to investigate factors enhancing colonization and invasive infections and to test therapeutics to eliminate Klebsiella asymptomatic colonization.

Cooperation between physiological defenses and immune resistance produces asymptomatic carriage of a lethal bacterial pathogen

Animals have evolved two defense strategies to survive infections. Antagonistic strategies include mechanisms of immune resistance that operate to sense and kill invading pathogens. Cooperative or physiological defenses mediate host adaptation to the infected state, limiting physiological damage and disease, without killing the pathogen, and have been shown to cause asymptomatic carriage and transmission of lethal pathogens. Here we demonstrate that physiological defenses cooperate with the adaptive immune response to generate long-term asymptomatic carriage of the lethal enteric murine pathogen, Citrobacter rodentium. Asymptomatic carriage of genetically virulent C. rodentium provided immune resistance against subsequent infections. Host immune protection was dependent on systemic antibody responses and pathogen virulence behavior, rather than the recognition of specific virulent factor antigens. Finally, we demonstrate that an avirulent strain of C. rodentium in the field has background mutations in two genes that are important for LPS structure. Our work reveals novel insight into how asymptomatic infections can arise mechanistically with immune resistance, mediating exclusion of phenotypically virulent enteric pathogen to promote asymptomatic carriage.

Microbiome induced complement synthesized in the gut protects against enteric infections

Canonically, complement is a serum-based host defense system that protects against systemic microbial invasion. Little is known about the production and function of complement components on mucosal surfaces. Here we show gut complement component 3 (C3), central to complement function, is regulated by the composition of the microbiota in healthy humans and mice, leading to host-specific gut C3 levels. Stromal cells in intestinal lymphoid follicles (LFs) are the predominant source of intestinal C3. During enteric infection with Citrobacter rodentium or enterohemorrhagic Escherichia coli, luminal C3 levels increase significantly and are required for protection. C. rodentium is remarkably more invasive to the gut epithelium of C3-deficient mice than of wild-type mice. In the gut, C3-mediated phagocytosis of C. rodentium functions to clear pathogens. Our study reveals that variations in gut microbiota determine individuals’ intestinal mucosal C3 levels, dominantly produced by LF stromal cells, which directly correlate with protection against enteric infection.

Highlights

Gut complement component 3 (C3) is induced by the microbiome in healthy humans and mice at a microbiota-specific level.Gut stromal cells located in intestinal lymphoid follicles are a major source of luminal C3 During enteric infections with Citrobacter rodentium or enterohemorrhagic Escherichia coli, gut luminal C3 levels increase and are required for protection. C. rodentium is significantly more invasive of the gut epithelium in C3-deficient mice when compared to WT mice. In the gut, C3-mediated opsonophagocytosis of C. rodentium functions to clear pathogens.

Biological pathways via which the anthocyanin malvidin alleviated the murine colitis induced by Citrobacter rodentium

Enteropathogenic E. coli (EPEC) is a causal agent for diarrheal diseases and contributes to morbidity and mortality in children under the age of five years. The emergence and rapid spread of antibiotic resistant EPEC strains necessitate the search for novel alternatives to antibiotics. In this study, we used Citrobacter rodentium, a natural mouse pathogen that mimics many aspects of human EPEC infections, to investigate the antimicrobial properties of the blueberry anthocyanin malvidin 3-glucoside (MG) using a multi-omics approach. MG supplementation reversed the bodyweight loss induced by C. rodentium infection and improved colonic hyperplasia and histopathological scores. In the colon tissue, MG supplementation significantly increased the expression of Hace1, a key regulator of TNFα-driven signaling, and impacted multiple pathways, such as TGFβ signaling. MG partially restored C. rodentium-induced microbial dysbiosis and significantly enhanced the abundance of the probiotic Bifidobacterium animalis. Moreover, MG disrupted the interactions of E. coli with other gut microbes. MG significantly mediated several host- and microbiota-derived metabolites, such as cytosine, ureidopropionic acid, and glutaric acid. MG normalized the bioactive lipid oleoylethanolamine, a member of the endocannabinoid system, from the dysregulated level in infected mice, directly contributing to its overall beneficial effects. Our findings provided novel insights into molecular processes via which the flavonoid malvidin exerts its biological effects in the gastrointestinal tract.

Raw potato starch alters the microbiome, colon and cecal gene expression, and resistance to Citrobacter rodentium infection in mice fed a Western diet

Resistant starches (RS) are fermented in the cecum and colon to produce short-chain fatty acids and other microbial metabolites that can alter host physiology and the composition of the microbiome. We previously showed that mice fed a Total Western Diet (TWD) based on NHANES data that mimics the composition of a typical American diet, containing resistant potato starch (RPS), produced concentration dependent changes to the cecal short-chain fatty acids, the microbiome composition as well as gene expression changes in the cecum and colon that were most prevalent in mice fed the 10% RPS diet. We were then interested in whether feeding TWD/RPS would alter the resistance to bacterial-induced colitis caused by Citrobacter rodentium (Cr), a mouse pathogen that shares 66.7% of encoded genes with Enteropathogenic Escherichia coli. Mice were fed the TWD for 6 weeks followed by a 3-weeks on the RPS diets before infecting with Cr. Fecal Cr excretion was monitored over time and fecal samples were collected for 16S sequencing. Mice were euthanized on day 12 post-infection and cecal contents collected for 16S sequencing. Cecum and colon tissues were obtained for gene expression analysis, histology and to determine the level of mucosa-associated Cr. Feeding RPS increased the percentage of mice productively infected by Cr and fecal Cr excretion on day 4 post-infection. Mice fed the TWD/10% RPS diet also had greater colonization of colonic tissue at day 12 post-infection and colonic pathology. Both diet and infection altered the fecal and cecal microbiome composition with increased levels of RPS resulting in decreased α-diversity that was partially reversed by Cr infection. RNASeq analysis identified several mechanistic pathways that could be associated with the increased colonization of Cr-infected mice fed 10% RPS. In the distal colon we found a decrease in enrichment for genes associated with T cells, B cells, genes associated with the synthesis of DHA-derived SPMs and VA metabolism/retinoic acid signaling. We also found an increase in the expression of the potentially immunosuppressive gene, Ido1. These results suggest that high-level consumption of RPS in the context of a typical American diet, may alter susceptibility to gastrointestinal bacterial infections.

Mouse models for bacterial enteropathogen infections: insights into the role of colonization resistance

Globally, enteropathogenic bacteria are a major cause of morbidity and mortality.1-3 Campylobacter, Salmonella, Shiga-toxin-producing Escherichia coli, and Listeria are among the top five most commonly reported zoonotic pathogens in the European Union.4 However, not all individuals naturally exposed to enteropathogens go on to develop disease. This protection is attributable to colonization resistance (CR) conferred by the gut microbiota, as well as an array of physical, chemical, and immunological barriers that limit infection. Despite their importance for human health, a detailed understanding of gastrointestinal barriers to infection is lacking, and further research is required to investigate the mechanisms that underpin inter-individual differences in resistance to gastrointestinal infection. Here, we discuss the current mouse models available to study infections by non-typhoidal Salmonella strains, Citrobacter rodentium (as a model for enteropathogenic and enterohemorrhagic E. coli), Listeria monocytogenes, and Campylobacter jejuni. Clostridioides difficile is included as another important cause of enteric disease in which resistance is dependent upon CR. We outline which parameters of human infection are recapitulated in these mouse models, including the impact of CR, disease pathology, disease progression, and mucosal immune response. This will showcase common virulence strategies, highlight mechanistic differences, and help researchers from microbiology, infectiology, microbiome research, and mucosal immunology to select the optimal mouse model.

Immobilization of Lactobacillus plantarum NCIMB 8826 ameliorates Citrobacter rodentium induced lesions and enhances the gut inflammatory response in C57BL/6 weanling mice

Infectious diarrhea is a major cause of infant mortality in most developing countries. In this research, we evaluated the potential of immobilized Lactobacillus plantarum NCIMB 8826 on weanimix infant cereal and its effectiveness in reducing the severity of Citrobacter rodentium-induced diarrhea in weanling mice. Thirty-six C57BL/6 weanling mice were placed into four groups (n = 9 each; negative, positive, prevention and cure). Mice received either L. plantarum (109 CFU/g) immobilized on weanimix infant cereal 3 days before C. rodentium (109 CFU/ ml) infection (Prevention) or 3 days after C. rodentium infection (Cure). A positive control group was infected with C. rodentium only, while a negative control group received neither L. plantarum nor C. rodentium. Positive control mice showed colonic mucosal and submucosal inflammation, erosion, and mucosal epithelia hyperplasia with the C. rodentium infection. Mice in the prevention and cure groups had less severe histologic alterations in the colon. Some beneficial effect of L. plantarum was observed in cecal short-chain fatty acid concentrations, which stimulates water and electrolytes absorption to reduce diarrhea. Our findings demonstrated that L. plantarum NCIMB 8826 could be immobilized on weanimix infant cereal to help reduce diarrhea during weaning.

Graphical Abstract

RIPK1 and RIPK3 in antibacterial defence

Upon sensing pathogenic bacterial infection, host cells activate a multitude of inflammatory and immunogenic responses to promote bacterial clearance and restore tissue homeostasis. RIPK1 and RIPK3 are two key players in antimicrobial defence, by either driving inflammatory signalling or inducing programmed cell death activation, ranging from apoptosis, pyroptosis to necroptosis. In this review, we first discuss the mechanisms by which RIPK1 and RIPK3 promote the assembly of death-inducing complexes and how these cell death pathways are activated as host responses to counteract pathogenic bacteria. We further outline the immunological importance of cell death in antibacterial defence and highlight outstanding questions in the field.

ILCs-Crucial Players in Enteric Infectious Diseases

Research of the last decade has remarkably increased our understanding of innate lymphoid cells (ILCs). ILCs, in analogy to T helper (Th) cells and their cytokine and transcription factor profile, are categorized into three distinct populations: ILC1s express the transcription factor T-bet and secrete IFNγ, ILC2s depend on the expression of GATA-3 and release IL-5 and IL-13, and ILC3s express RORγt and secrete IL-17 and IL-22. Noteworthy, ILCs maintain a level of plasticity, depending on exposed cytokines and environmental stimuli. Furthermore, ILCs are tissue resident cells primarily localized at common entry points for pathogens such as the gut-associated lymphoid tissue (GALT). They have the unique capacity to initiate rapid responses against pathogens, provoked by changes of the cytokine profile of the respective tissue. Moreover, they regulate tissue inflammation and homeostasis. In case of intracellular pathogens entering the mucosal tissue, ILC1s respond by secreting cytokines (e.g., IFNγ) to limit the pathogen spread. Upon infection with helminths, intestinal epithelial cells produce alarmins (e.g., IL-25) and activate ILC2s to secrete IL-13, which induces differentiation of intestinal stem cells into tuft and goblet cells, important for parasite expulsion. Additionally, during bacterial infection ILC3-derived IL-22 is required for bacterial clearance by regulating antimicrobial gene expression in epithelial cells. Thus, ILCs can limit infectious diseases via secretion of inflammatory mediators and interaction with other cell types. In this review, we will address the role of ILCs during enteric infectious diseases.

Defects in the expression of colonic host defense factors associate with barrier dysfunction induced by a high-fat/high-cholesterol diet

The effect of Western diets in the gastrointestinal system is largely mediated by their ability to promote alterations in the immunity and physiology of the intestinal epithelium, and to affect the composition of the commensal microbiota. To investigate the response of the colonic epithelium to high-fat/high-cholesterol diets (HFHCDs), we evaluated the synthesis of host defense factors involved in the maintenance of the colonic homeostasis. C57BL/6 mice were fed an HFHCD for 3 weeks and their colons were evaluated for histopathology, gene expression, and microbiota composition. In addition, intestinal permeability and susceptibility to Citrobacter rodentium were also studied. HFHCD caused colonic hyperplasia, loss of goblet cells, thinning of the mucus layer, moderate changes in the composition of the intestinal microbiota, and an increase in intestinal permeability. Gene expression analyses revealed significant drops in the transcript levels of Muc1, Muc2, Agr2, Atoh1, Spdef, Ang4, Camp, Tff3, Dmbt1, Fcgbp, Saa3, and Retnlb. The goblet cell granules of HFHCD-fed mice were devoid of Relmβ and Tff3, indicating defective production of those two factors critical for intestinal epithelial defense and homeostasis. In correspondence with these defects, colonic bacteria were in close contact with, and invading the epithelium. Fecal shedding of C. rodentium showed an increased bacterial burden in HFHCD-fed animals accompanied by increased epithelial damage. Collectively, our results show that HFHCD perturbs the synthesis of colonic host defense factors, which associate with alterations in the commensal microbiota, the integrity of the intestinal barrier, and the host's susceptibility to enteric infections.

Insights from DOCK2 in cell function and pathophysiology

Dedicator of cytokinesis 2 (DOCK2) can activate the downstream small G protein Rac and regulate cytoskeletal reorganization. DOCK2 is essential for critical physiological processes such as migration, activation, proliferation, and effects of immune cells, including lymphocytes, neutrophils, macrophages, and dendritic cells. For example, DOCK2 is involved in the development and activation of T and B lymphocytes by affecting synapse formation and inhibiting the development of the Th2 lineage by downregulating IL-4Rα surface expression. Not only that, DOCK2 may be a molecular target for controlling cardiac transplant rejection and Alzheimer’s disease (AD). Patients with defects in the DOCK2 gene also exhibit a variety of impaired cellular functions, such as chemotactic responses of lymphocytes and reactive oxygen species (ROS) production by neutrophils. To date, DOCK2 has been shown to be involved in the development of various diseases, including AD, pneumonia, myocarditis, colitis, tumors, etc. DOCK2 plays different roles in these diseases and the degree of inflammatory response has a different impact on the progression of disease. In this paper, we present a review of recent advances in the function of DOCK2 in various immune cells and its role in various diseases.

Interferon regulatory factor 1 (IRF-1) promotes intestinal group 3 innate lymphoid responses during Citrobacter rodentium infection

Group 3 innate lymphoid cells (ILC3s) are crucial mediators of immunity and epithelial barrier function during immune responses against extracellular bacteria. Here, we identify Interferon regulatory factor 1 (IRF-1), a transcription factor previously associated with type 1 immunity, as an essential regulator of intestinal ILC3 accumulation and effector cytokine production. We demonstrate that IRF-1 is upregulated in the context of infection with the enteropathogen Citrobacter rodentium and that its presence is central for anatomical containment and prevention of pathogen dissemination. We furthermore show that IRF-1 is required in order for intestinal ILC3s to produce large amounts of the protective effector cytokine IL-22 early in the course of infection. On a molecular level, our data indicate that IRF-1 controls ILC3 numbers and their activation by direct transcriptional regulation of the IL-12Rβ1 chain, thereby allowing ILCs to physiologically respond to IL-23 stimulation.

Innate lymphoid cells (ILC) are involved with different immune responses. Here the authors show that Interferon regulatory factor 1 (IRF1) is important for intestinal ILC3 accumulation during Citrobacter rodentium infection and promotes release of the protective cytokine IL-22 and response to IL-23.

Heterologous Assembly of the Type VI Secretion System Empowers Laboratory Escherichia coli with Antimicrobial and Cell Penetration Capabilities

The synthetic biology toolbox has amassed a vast number of diverse functional modules, but protein translocation modules for cell penetration and cytosol-to-cytosol delivery remain relatively scarce. The type VI secretion system (T6SS), commonly found in many Gram-negative pathogens, functions as a contractile device to translocate protein toxins to prokaryotic and eukaryotic cells. Here, we have assembled the T6SS of Aeromonas dhakensis, an opportunistic waterborne pathogen, in the common laboratory strain Escherichia coli BL21(DE3). We constructed a series of plasmids (pT6S) carrying the T6SS structural and effector genes under native or tetracycline-inducible promoters, the latter for controlled expression. Using fluorescence microscopy and biochemical analyses, we demonstrate a functional T6SS in E. coli capable of secreting proteins directly into the cytosol of neighboring bacteria and outcompeting a number of drug-resistant pathogens. The heterologous assembly of T6SS not only confers the lab workhorse E. coli with the cytosol-to-cytosol protein delivery capability but also demonstrates the potential for harnessing the T6SS of various pathogens for general protein delivery and antibacterial applications. IMPORTANCE The T6SS is a powerful and versatile protein delivery system. However, the complexity of its macromolecular structure and gene regulation makes it not a trivial task to reconstitute the T6SSs of pathogens in a nonpathogenic host. In this study, we have assembled an inducible T6SS in E. coli BL21(DE3) and demonstrated its functions in protein delivery and antimicrobial activities. The engineered T6SS empowers E. coli to deliver protein cargos into a wide range of prokaryotic and eukaryotic cells.