Protective role of commensals against Clostridium difficile infection via an IL-1β-mediated positive-feedback loop

Reviewing the Clostridioides difficile Mouse Model: Insights into Infection Mechanisms

Clostridioides difficile is an anaerobic, spore-forming bacterium associated with intestinal infection, manifesting a broad spectrum of gastrointestinal symptoms, ranging from mild diarrhea to severe colitis. A primary risk factor for the development of C. difficile infection (CDI) is antibiotic exposure. Elderly and immunocompromised individuals are particularly vulnerable to CDI. A pivotal aspect for comprehending the complexities of this infection relies on the utilization of experimental models that mimic human CDI transmission, pathogenesis, and progression. These models offer invaluable insights into host-pathogen interactions and disease dynamics, and serve as essential tools for testing potential therapeutic approaches. In this review, we examine the animal model for CDI and delineate the stages of infection, with a specific focus on mice. Our objective is to offer an updated description of experimental models employed in the study of CDI, emphasizing both their strengths and limitations.

Identifying microbiota community patterns important for plant protection using synthetic communities and machine learning

Plant-associated microbiomes contribute to important ecosystem functions such as host resistance to biotic and abiotic stresses. The factors that determine such community outcomes are inherently difficult to identify under complex environmental conditions. In this study, we present an experimental and analytical approach to explore microbiota properties relevant for a microbiota-conferred host phenotype, here plant protection, in a reductionist system. We screened 136 randomly assembled synthetic communities (SynComs) of five bacterial strains each, followed by classification and regression analyses as well as empirical validation to test potential explanatory factors of community structure and composition, including evenness, total commensal colonization, phylogenetic diversity, and strain identity. We find strain identity to be the most important predictor of pathogen reduction, with machine learning algorithms improving performances compared to random classifications (94-100% versus 32% recall) and non-modelled predictions (0.79-1.06 versus 1.5 RMSE). Further experimental validation confirms three strains as the main drivers of pathogen reduction and two additional strains that confer protection in combination. Beyond the specific application presented in our study, we provide a framework that can be adapted to help determine features relevant for microbiota function in other biological systems.

Host Immune Responses to Clostridioides difficile Infection and Potential Novel Therapeutic Approaches

Clostridioides difficile infection (CDI) is a leading nosocomial infection, posing a substantial public health challenge within the United States and globally. CDI typically occurs in hospitalized elderly patients who have been administered antibiotics; however, there has been a rise in the occurrence of CDI in the community among young adults who have not been exposed to antibiotics. C. difficile releases toxins, which damage large intestinal epithelium, leading to toxic megacolon, sepsis, and even death. Unfortunately, existing antibiotic therapies do not always prevent these consequences, with up to one-third of treated patients experiencing a recurrence of the infection. Host factors play a crucial role in the pathogenesis of CDI, and accumulating evidence shows that modulation of host immune responses may potentially alter the disease outcome. In this review, we provide an overview of our current knowledge regarding the role of innate and adaptive immune responses on CDI outcomes. Moreover, we present a summary of non-antibiotic microbiome-based therapies that can effectively influence host immune responses, along with immunization strategies that are intended to tackle both the treatment and prevention of CDI.

Dual RNA-seq identifies genes and pathways modulated during Clostridioides difficile colonization

IMPORTANCE

The initial interactions between the colonic epithelium and the bacterium are likely critical in the establishment of Clostridioides difficile infection, one of the major causes of hospital-acquired diarrhea worldwide. Molecular interactions between C. difficile and human gut cells have not been well defined mainly due to the technical challenges of studying cellular host-pathogen interactions with this anaerobe. Here we have examined transcriptional changes occurring in the pathogen and host cells during the initial 24 hours of infection. Our data indicate several changes in metabolic pathways and virulence-associated factors during the initial bacterium-host cell contact and early stages of infection. We describe canonical pathways enriched based on the expression profiles of a dual RNA sequencing in the host and bacterium, and functions of bacterial factors that are modulated during infection. This study thus provides fresh insight into the early C. difficile infection process.

Butyrate Protects against Clostridium difficile Infection by Regulating Bile Acid Metabolism

Clostridium difficile infection (CDI) is caused by a prevalent nosocomial enteric pathogen, leading to high morbidity and mortality. CDI recurrence after antibiotic treatment is high; therefore, it is necessary to develop novel therapeutics against this enteric pathogen. Butyrate is used to treat many diseases because it provides energy, has anti-inflammatory properties, and maintains intestinal barrier function. An anti-CDI effect for butyrate has been reported; however, the specific mechanism remains elusive. This study aimed to explore the potential role and mechanism of butyrate in the treatment of CDI. Using a CDI mouse model, we found that butyrate significantly inhibited CDI development by regulating bile acid metabolism. Dysregulation of fecal bile acid was significantly higher, and levels of short-chain fatty acids were significantly lower in patients with CDI than those in controls. In CDI mice, butyrate exhibited a protective role by enhancing barrier protection, exerting anti-inflammatory effects, and regulating bile acid metabolism. Butyrate treatment also regulated the production of bile salt hydrolase (BSH) flora and activated farnesoid X receptor (FXR), and its therapeutic effects were reduced in CDI mice treated with BSH or FXR inhibitors. Thus, butyrate treatment may serve as a novel therapeutic approach for patients with CDI. IMPORTANCE Here, we show that levels of fecal short-chain fatty acids (SCFAs), particularly butyrate, are reduced, and normal colon structure is damaged in patients with CDI compared with those in healthy individuals. Bile acid (BA) metabolic disorder in patients with CDI is characterized by increased primary BA levels and decreased secondary BAs. In mice, butyrate alters BA metabolism in CDI and may play a vital role in CDI treatment by promoting secondary BA metabolism. Lastly, butyrate-mediated therapeutic effects in CDI require FXR. Our findings demonstrate that butyrate treatment significantly decreases the severity of CDI-induced colitis in mice and affects BA metabolism and FXR activation, which provides a potential alternative treatment for CDI.

Returning to Nature for the Next Generation of Antimicrobial Therapeutics

Antibiotics found in and inspired by nature are life-saving cures for bacterial infections and have enabled modern medicine. However, the rise in resistance necessitates the discovery and development of novel antibiotics and alternative treatment strategies to prevent the return to a pre-antibiotic era. Once again, nature can serve as a source for new therapies in the form of natural product antibiotics and microbiota-based therapies. Screening of soil bacteria, particularly actinomycetes, identified most of the antibiotics used in the clinic today, but the rediscovery of existing molecules prompted a shift away from natural product discovery. Next-generation sequencing technologies and bioinformatics advances have revealed the untapped metabolic potential harbored within the genomes of environmental microbes. In this review, we first highlight current strategies for mining this untapped chemical space, including approaches to activate silent biosynthetic gene clusters and in situ culturing methods. Next, we describe how using live microbes in microbiota-based therapies can simultaneously leverage many of the diverse antimicrobial mechanisms found in nature to treat disease and the impressive efficacy of fecal microbiome transplantation and bacterial consortia on infection. Nature-provided antibiotics are some of the most important drugs in human history, and new technologies and approaches show that nature will continue to offer valuable inspiration for the next generation of antibacterial therapeutics.

Inhibition of CD82 improves colitis by increasing NLRP3 deubiquitination by BRCC3

CD82 is a transmembrane protein that is involved in cancer suppression and activates immune cells; however, information on the NLRP3 inflammasome is limited. Herein, we show that although CD82 suppressed the activation of the NLRP3 inflammasome in vivo and in vitro, CD82 deficiency decreased the severity of colitis in mice. Furthermore, two binding partners of CD82, NLRP3 and BRCC3, were identified. CD82 binding to these partners increased the degradation of NLRP3 by blocking BRCC3-dependent K63-specific deubiquitination. Previous studies have shown that CD82-specific bacteria in the colon microbiota called Bacteroides vulgatus (B. vulgatus) regulated the expression of CD82 and promoted the activation of the NLRP3 inflammasome. Accordingly, we observed that B. vulgatus administration increased mouse survival by mediating CD82 expression and activating NLRP3 in mice with colitis. Overall, this study showed that CD82 suppression reduced the pathogenesis of colitis by elevating the activation of the NLRP3 inflammasome through BRCC3-dependent K63 deubiquitination. Based on our findings, we propose that B. vulgatus is a novel therapeutic candidate for colitis.

Association of Single Nucleotide Polymorphisms in Nucleotide-Binding Domain Leucine-Rich Repeat Protein 1 with Clostridioides difficile Colonization or Infection

Introduction

Nucleotide-binding domain leucine-rich repeat protein (NLRP) is critical in the inflammasome-activation pathway, which is important for host survival and the clearance of Clostridioides difficile. Therefore, the influence of NLRP1 polymorphisms on C. difficile colonization (CdC) or infection (CDI) was analyzed.

Materials and Methods

A prospective cohort study consisted of hospitalized adults was conducted from January 2011 to January 2013. Single nucleotide polymorphisms (SNPs) of NLRP1, including rs12150220, rs2670660, rs6502867, rs878329, rs8182352, rs3744717, and rs11078571, were incorporating in analyses. The episodes of CdC and CDI were the primary and secondary outcome, respectively.

Results

Of the total of 509 eligible patients, 376 (73.9%) had neither CdC nor CDI, 104 (21.8%) had CdC without developing CDI, and 29 (4.3%) developed CDI during the study period. Through multivariate analyses, comorbid diabetes mellitus (adjusted odds ratio [AOR] 1.59, P=0.04) and CC genotype in NLRP1 rs3744717 (AOR 1.70, P=0.02) were recognized as the risk factor of CdC. After adjusting the independent predictors of CDI, in terms of comorbid diabetes mellitus (AOR 3.18, P=0.005) and prior exposure to ceftazidime/ceftriaxone (AOR 2.87, P=0.04) or proton pump inhibitors (AOR 3.86, P=0.001), patients with CC+GC genotype in NLRP1, rs878329 (AOR 2.39, P=0.03) remained a higher risk of CDI.

Conclusion

For hospitalized adults, the association of CC genotype in NLRP1 rs3744717 and CdC as well as the CC+GC genotype in NLRP1 rs878329 and CDI was respectively evidenced. We believed the prompt identification of patients having specific genotype in NLRP1 would prevent and improve the quality of care in CDI.

Intervention and potential mechanism of non-starch polysaccharides from natural resources on ulcerative colitis: A review

Ulcerative colitis (UC) is a chronic inflammatory bowel disease of unknown etiology that affects the colon and rectum. It has evolved into a global burden due to the high incidence in developed countries and the highly-increased incidence in developing countries. Non-starch polysaccharides (NSPs) from natural resources, as a type of functional carbohydrates, have a significant therapeutic effect on UC because of their good anti-inflammatory and immunomodulatory activities. Based on the etiology and pathogenesis of UC, this review summarizes the intervention effects and mechanisms of NSPs in the prevention and treatment of UC. The results showed that NSPs can improve UC by protecting the intestinal mucosal barrier, regulating the immune response of the intestinal mucosa, and remodeling the intestinal flora and metabolites. These contents provide theoretical basis for the application of polysaccharides in the prevention and treatment of UC.

Mechanism of the Gut Microbiota Colonization Resistance and Enteric Pathogen Infection

The mammalian gut microbial community, known as the gut microbiota, comprises trillions of bacteria, which co-evolved with the host and has an important role in a variety of host functions that include nutrient acquisition, metabolism, and immunity development, and more importantly, it plays a critical role in the protection of the host from enteric infections associated with exogenous pathogens or indigenous pathobiont outgrowth that may result from healthy gut microbial community disruption. Microbiota evolves complex mechanisms to restrain pathogen growth, which included nutrient competition, competitive metabolic interactions, niche exclusion, and induction of host immune response, which are collectively termed colonization resistance. On the other hand, pathogens have also developed counterstrategies to expand their population and enhance their virulence to cope with the gut microbiota colonization resistance and cause infection. This review summarizes the available literature on the complex relationship occurring between the intestinal microbiota and enteric pathogens, describing how the gut microbiota can mediate colonization resistance against bacterial enteric infections and how bacterial enteropathogens can overcome this resistance as well as how the understanding of this complex interaction can inform future therapies against infectious diseases.

Host Immune Responses to Clostridioides difficile: Toxins and Beyond

Clostridioides difficile is often resistant to the actions of antibiotics to treat other bacterial infections and the resulting C. difficile infection (CDI) is among the leading causes of nosocomial infectious diarrhea worldwide. The primary virulence mechanism contributing to CDI is the production of toxins. Treatment failures and recurrence of CDI have urged the medical community to search for novel treatment options. Strains that do not produce toxins, so called non-toxigenic C. difficile, have been known to colonize the colon and protect the host against CDI. In this review, a comprehensive description and comparison of the immune responses to toxigenic C. difficile and non-toxigenic adherence, and colonization factors, here called non-toxin proteins, is provided. This revealed a number of similarities between the host immune responses to toxigenic C. difficile and non-toxin proteins, such as the influx of granulocytes and the type of T-cell response. Differences may reflect genuine variation between the responses to toxigenic or non-toxigenic C. difficile or gaps in the current knowledge with respect to the immune response toward non-toxigenic C. difficile. Toxin-based and non-toxin-based immunization studies have been evaluated to further explore the role of B cells and reveal that plasma cells are important in protection against CDI. Since the success of toxin-based interventions in humans to date is limited, it is vital that future research will focus on the immune responses to non-toxin proteins and in particular non-toxigenic strains.

Dendrobium fimbriatum Hook polysaccharide ameliorates dextran-sodium-sulfate-induced colitis in mice via improving intestinal barrier function, modulating intestinal microbiota, and reducing oxidative stress and inflammatory responses

The ameliorative effect of Dendrobium fimbriatum polysaccharide (cDFPW1) on ulcerative colitis (UC) was investigated using a dextran-sodium-sulfate-induced (DSS-induced) mouse model in the present study. The results showed that cDFPW1 effectively improved colitis in mice by ameliorating weight loss, disease activity index (DAI) and colonic pathological damage, and by protecting the intestinal barrier function integrity. Moreover, cDFPW1 modulated the composition and metabolism of intestinal microbiota through enhancing Romboutsia, Lactobacillus and Odoribacter, and reducing Parasutterella, Burkholderia-Caballeronia-Paraburkholderia and Acinetobacter in colitis mice. Notably, cDFPW1 significantly restored the homeostasis of Th17/regulatory T (Treg) cells and the expression of specific cytokines. Western blotting of colon tissues showed that cDFPW1 markedly up-regulated the expression of Nrf2 and inhibited the phosphorylation of NF-κB signaling. These results indicated that cDFPW1 possesses the potential of improving UC and its effect on palliating colitis may be connected with the regulation of Nrf2/NF-κB signaling.

Listeriosis, a model infection to study host-pathogen interactions in vivo

Listeria monocytogenes (Lm) is a foodborne pathogen and the etiological agent of listeriosis. This facultative intracellular Gram-positive bacterium has the ability to colonize the intestinal lumen, cross the intestinal, blood-brain and placental barriers, leading to bacteremia, neurolisteriosis and maternal-fetal listeriosis. Lm is a model microorganism for the study of the interplay between a pathogenic microbe, host tissues and microbiota in vivo. Here we review how animal models permissive to Lm-host interactions allow deciphering some of the key steps of the infectious process, from the intestinal lumen to the crossing of host barriers and dissemination within the host. We also highlight recent investigations using tagged Lm and clinically relevant strains that have shed light on within-host dynamics and the purifying selection of Lm virulence factors. Studying Lm infection in vivo is a way forward to explore host biology and unveil the mechanisms that have selected its capacity to closely associate with its vertebrate hosts.

Endotoxin Acts Synergistically With Clostridioides difficile Toxin B to Increase Interleukin 1β Production: A Potential Role for the Intestinal Biome in Modifying the Severity of C. difficile Colitis

BACKGROUND

Inflammation is a crucial driver of host damage in patients with Clostridioides difficile colitis. We examined the potential for the intestinal microbiome to modify inflammation in patients with C. difficile colitis via the effects of gut-derived endotoxin on cytokine production.

METHODS

Endotoxin from Escherichia coli and Pseudomonas aeruginosa as well as stool-derived endotoxin were tested for their ability to enhance interleukin 1β (IL-1β) and tumor necrosis factor alpha (TNF-α) production by toxin B-stimulated peripheral blood mononuclear cells. Inflammasome and Toll-like receptor 4 (TLR4) blocking studies were done to discern the importance of these pathways, while metagenomic studies were done to characterize predominant organisms from stool samples.

RESULTS

Endotoxin significantly enhanced the ability of C. difficile toxin B to promote IL-1β production but not TNF-α. The magnitude of this effect varied by endotoxin type and was dependent on combined inflammasome and TLR4 activation. Stool-derived endotoxin exhibited a similar synergistic effect on IL-1β production with less synergy observed for stools that contained a high proportion of γ-proteobacteria.

CONCLUSIONS

The ability of endotoxin to enhance IL-1β production highlights a manner by which the microbiome can modify inflammation and severity of C. difficile disease. This information may be useful in devising new therapies for severe C. difficile colitis.

NLRP3 Inflammasome: Checkpoint Connecting Innate and Adaptive Immunity in Autoimmune Diseases

Autoimmune diseases are a broad spectrum of human diseases that are characterized by the breakdown of immune tolerance and the production of autoantibodies. Recently, dysfunction of innate and adaptive immunity is considered to be a key step in the initiation and maintenance of autoimmune diseases. NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is a multimeric protein complex, which can detect exogenous pathogen irritants and endogenous danger signals. The main function of NLRP3 inflammasome is to promote secretion of interleukin (IL)-1β and IL-18, and pyroptosis mediated by caspase-1. Served as a checkpoint in innate and adaptive immunity, aberrant activation and regulation of NLRP3 inflammasome plays an important role in the pathogenesis of autoimmune diseases. This paper reviewed the roles of NLRP3 inflammasome in autoimmune diseases, which shows NLRP3 inflammasome may be a potential target for autoimmune diseases deserved further study.

S100B Inhibition Attenuates Intestinal Damage and Diarrhea Severity During Clostridioides difficile Infection by Modulating Inflammatory Response

The involvement of the enteric nervous system, which is a source of S100B, in Clostridioides difficile (C. difficile) infection (CDI) is poorly understood although intestinal motility dysfunctions are known to occur following infection. Here, we investigated the role of S100B in CDI and examined the S100B signaling pathways activated in C. difficile toxin A (TcdA)- and B (TcdB)-induced enteric glial cell (EGC) inflammatory response. The expression of S100B was measured in colon tissues and fecal samples of patients with and without CDI, as well as in colon tissues from C. difficile-infected mice. To investigate the role of S100B signaling in IL-6 expression induced by TcdA and TcdB, rat EGCs were used. Increased S100B was found in colonic biopsies from patients with CDI and colon tissues from C. difficile-infected mice. Patients with CDI-promoted diarrhea exhibited higher levels of fecal S100B compared to non-CDI cases. Inhibition of S100B by pentamidine reduced the synthesis of IL-1β, IL-18, IL-6, GMCSF, TNF-α, IL-17, IL-23, and IL-2 and downregulated a variety of NFκB-related genes, increased the transcription (SOCS2 and Bcl-2) of protective mediators, reduced neutrophil recruitment, and ameliorated intestinal damage and diarrhea severity in mice. In EGCs, TcdA and TcdB upregulated S100B-mediated IL-6 expression via activation of RAGE/PI3K/NFκB. Thus, CDI appears to upregulate colonic S100B signaling in EGCs, which in turn augment inflammatory response. Inhibition of S100B activity attenuates the intestinal injury and diarrhea caused by C. difficile toxins. Our findings provide new insight into the role of S100B in CDI pathogenesis and opens novel avenues for therapeutic interventions.

Clostridioides difficile infection induces a rapid influx of bile acids into the gut during colonization of the host

Clostridioides difficile is the leading cause of nosocomial intestinal infections in the United States. Ingested C. difficile spores encounter host bile acids and other cues that are necessary for germinating into toxin-producing vegetative cells. While gut microbiota disruption (often by antibiotics) is a prerequisite for C. difficile infection (CDI), the mechanisms C. difficile employs for colonization remain unclear. Here, we pioneered the application of imaging mass spectrometry to study how enteric infection changes gut metabolites. We find that CDI induces an influx of bile acids into the gut within 24 h of the host ingesting spores. In response, the host reduces bile acid biosynthesis gene expression. These bile acids drive C. difficile outgrowth, as mice receiving the bile acid sequestrant cholestyramine display delayed colonization and reduced germination. Our findings indicate that C. difficile may facilitate germination upon infection and suggest that altering flux through bile acid pathways can modulate C. difficile outgrowth in CDI-prone patients.

Therapeutic implications of inflammasome in inflammatory bowel disease

Inflammatory bowel disease (IBD) remains a persistent health problem with a global burden surging over 6.8 million cases currently. Clinical pathology of IBD is complicated; however, hyperactive inflammatory and immune responses in the gut is shown to be one of the persistent causes of the disease. Human gut inflammasome, the activator of innate immune system is believed to be a primary underlying cause for the pathology and is largely associated with the progression of IBD. To manage IBD, there is a need to fully understand the role of inflammasome activation in IBD. Since inflammasome potentially play a significant role in IBD, systemic modulation of inflammasome may provide an effective therapeutic and clinical approach to control IBD symptoms. In this review, we have focused on this association between IBD and gut inflammasome, and recent advances in the research and therapeutic strategies for IBD. We have discussed inflammasomes and their components, outcomes from the experimental animals and human studies, inflammasome inhibitors, and developments in the inflammasome-targeted therapies for IBD.

Loss of Interleukin-10 (IL-10) Signaling Promotes IL-22-Dependent Host Defenses against Acute Clostridioides difficile Infection

Infection with the bacterial pathogen Clostridioides difficile causes severe damage to the intestinal epithelium that elicits a robust inflammatory response. Markers of intestinal inflammation accurately predict clinical disease, however, the extent to which host-derived proinflammatory mediators drive pathogenesis versus promote host protective mechanisms remains elusive. In this report, we employed Il10-/- mice as a model of spontaneous colitis to examine the impact of constitutive intestinal immune activation, independent of infection, on C. difficile disease pathogenesis. Upon C. difficile challenge, Il10-/- mice exhibited significantly decreased morbidity and mortality compared to littermate Il10 heterozygote (Il10HET) control mice, despite a comparable C. difficile burden, innate immune response, and microbiota composition following infection. Similarly, antibody-mediated blockade of interleukin-10 (IL-10) signaling in wild-type C57BL/6 mice conveyed a survival advantage if initiated 3 weeks prior to infection. In contrast, no advantage was observed if blockade was initiated on the day of infection, suggesting that the constitutive activation of inflammatory defense pathways prior to infection mediated host protection. IL-22, a cytokine critical in mounting a protective response against C. difficile infection, was elevated in the intestine of uninfected, antibiotic-treated Il10-/- mice, and genetic ablation of the IL-22 signaling pathway in Il10-/- mice negated the survival advantage following C. difficile challenge. Collectively, these data demonstrate that constitutive loss of IL-10 signaling, via genetic ablation or antibody blockade, enhances IL-22-dependent host defense mechanisms to limit C. difficile pathogenesis.

Leptin Receptor q223r Polymorphism Influences Clostridioides difficile Infection-Induced Neutrophil CXCR2 Expression in an Interleukin-1β Dependent Manner

Neutrophils are key first-responders in the innate immune response to C. difficile infection (CDI) and play a central role in disease pathogenesis. Studies have clearly shown that tissue neutrophil numbers need to be tightly regulated for optimal CDI outcomes: while excessive colonic neutrophilia is associated with severe CDI, neutrophil depletion also results in worse outcomes. However, the biological mechanisms that control CDI-induced neutrophilia remain poorly defined. C-X-C chemokine receptor 2 (CXCR2) is a chemotactic receptor that is critical in neutrophil mobilization from bone marrow to blood and tissue sites. We have previously reported that a single nucleotide polymorphism (SNP) in leptin receptor (LEPR), present in up to 50% of people, influenced CDI-induced neutrophil CXCR2 expression and tissue neutrophilia. Homozygosity for mutant LEPR (i.e. RR genotype) was associated with higher CXCR2 expression and more tissue neutrophils. Here, we investigated the biological mechanisms that regulate neutrophil CXCR2 expression after CDI, and the influence of host genetics on this process. Our data reveal that: a) CXCR2 plays a key role in CDI-induced neutrophil extravasation from blood to colonic tissue; b) plasma from C. difficile-infected mice upregulated CXCR2 on bone marrow neutrophils; c) plasma from C. difficile-infected RR mice induced a higher magnitude of CXCR2 upregulation and had more IL-1β; and d) IL-1β neutralization reduced CXCR2 expression on bone marrow and blood neutrophils and their subsequent accrual to colonic tissue. In sum, our data indicate that IL-1β is a key molecular mediator that communicates between gastro-intestinal tract (i.e. site of CDI) and bone marrow (i.e. primary neutrophil reservoir) and regulates the intensity of CDI-induced tissue neutrophilia by modulating CXCR2 expression. Further, our studies highlight the importance of host genetics in affecting these innate immune responses and provide novel insights into the mechanisms by which a common SNP influences CDI-induced neutrophilia.

Mechanisms of Colonization Resistance Against Clostridioides difficile

Clostridioides difficile is an urgent antimicrobial-resistant bacterium, causing mild to moderate and sometimes life-threatening disease. Commensal gut microbes are critical for providing colonization resistance against C difficile and can be leveraged as non-antibiotic alternative therapeutics for the prevention and treatment of C difficile infection.

Inflammatory Bowel Disease: The Emergence of New Trends in Lifestyle and Nanomedicine as the Modern Tool for Pharmacotherapy

The human intestine, which harbors trillions of symbiotic microorganisms, may enter into dysbiosis when exposed to a genetic defect or environmental stress. The naissance of chronic inflammation due to the battle of the immune system with the trespassing gut bacteria leads to the rise of inflammatory bowel disease (IBD). Though the genes behind the scenes and their link to the disease are still unclear, the onset of IBD occurs in young adults and has expanded from the Western world into the newly industrialized countries. Conventional drug deliveries depend on a daily heavy dosage of immune suppressants or anti-inflammatory drugs targeted for the treatment of two types of IBD, ulcerative colitis (UC) and Crohn's disease (CD), which are often associated with systemic side effects and adverse toxicities. Advances in oral delivery through nanotechnology seek remedies to overcome the drawbacks of these conventional drug delivery systems through improved drug encapsulation and targeted delivery. In this review, we discuss the association of genetic factors, the immune system, the gut microbiome, and environmental factors like diet in the pathogenesis of IBD. We also review the various physiological concerns required for oral delivery to the gastrointestinal tract (GIT) and new strategies in nanotechnology-derived, colon-targeting drug delivery systems.

The absence of murine cathelicidin-related antimicrobial peptide impacts host responses enhancing Salmonella enterica serovar Typhimurium infection

BACKGROUND

Cathelicidins are a class of antimicrobial peptide, and the murine cathelicidin-related antimicrobial peptide (mCRAMP) has been demonstrated in vitro to impair Salmonella enterica serovar Typhimurium proliferation. However, the impact of mCRAMP on host responses and the microbiota following S. Typhimurium infection has not been determined. In this study mCRAMP^-/- and mCRAMP^+/+ mice (± streptomycin) were orally inoculated with S. enterica serovar Typhimurium DT104 (SA +), and impacts on the host and enteric bacterial communities were temporally evaluated.

RESULTS

Higher densities of the pathogen were observed in cecal digesta and associated with mucosa in SA+/mCRAMP^-/- mice that were pretreated (ST+) and not pretreated (ST-) with streptomycin at 24 h post-inoculation (hpi). Both SA+/ST+/mCRAMP^-/- and SA+/ST-/mCRAMP^-/- mice were more susceptible to infection exhibiting greater histopathologic changes (e.g. epithelial injury, leukocyte infiltration, goblet cell loss) at 48 hpi. Correspondingly, immune responses in SA+/ST+/mCRAMP^-/- and SA+/ST-/mCRAMP^-/- mice were affected (e.g. Ifnγ, Kc, Inos, Il1β, RegIIIγ). Systemic dissemination of the pathogen was characterized by metabolomics, and the liver metabolome was affected to a greater degree in SA+/ST+/mCRAMP^-/- and SA+/ST-/mCRAMP^-/- mice (e.g. taurine, cadaverine). Treatment-specific changes to the structure of the enteric microbiota were associated with infection and mCRAMP deficiency, with a higher abundance of Enterobacteriaceae and Veillonellaceae observed in infected null mice. The microbiota of mice that were administered the antibiotic and infected with Salmonella was dominated by Proteobacteria.

CONCLUSION

The study findings showed that the absence of mCRAMP modulated both host responses and the enteric microbiota enhancing local and systemic infection by Salmonella Typhimurium.

Heat shock transcription factor 2 reduces the secretion of IL-1β by inhibiting NLRP3 inflammasome activation in ulcerative colitis

Ulcerative colitis (UC) is a chronic inflammatory disease with unknown aetiology. As a pro-inflammatory cytokine, interleukin-1β (IL-1β) plays a critical, damaging role in UC. Heat shock proteins (HSPs) are important anti-inflammatory factors that maintain intestinal epithelial cells (IECs) homeostasis. Heat shock transcription factor 2 (HSF2) is an important regulator of HSPs. In our previous research, we found that HSF2 is highly expressed in UC, is negatively related to colon inflammation of mice, and inhibits the expression of IL-1β, but the specific mechanism is still unclear. As a product of the NLRP3 inflammasome, the expression of IL-1β is closely related to NLRP3 inflammasome activation. Therefore, we hypothesised that HSF2 affects the secretion of IL-1β by regulating activation of the NLRP3 inflammasome. In this study, hsf^-/- DSS model mice showed highest levels of expression of the NLRP3 inflammasome and the secretion of IL-1β. In Caco-2 cells, the levels of expression of the NLRP3 inflammasome and the secretion of IL-1β were inhibited by overexpression of HSF2, and inhibited HSF2 increased activation of the NLRP3 inflammasome and the secretion of IL-1β. These findings indicated that HSF2 might be an important target for inflammatory modulation in UC.

Detection and elimination of a novel non-toxigenic Clostridioides difficile strain from the microbiota of a mouse colony

Clostridioides difficile is an enteric bacterial pathogen that can a cause nosocomial infection leading to debilitating colitis. The development of a murine model of C. difficile infection has led to fundamental discoveries in disease pathogenesis and the host immune response to infection. Recently, C. difficile endogenously present in the microbiota of mice has been reported and was found to complicate interpretation of mouse studies. Here, we report a novel C. difficile strain, named NTCD-035, isolated from the microbiota of our mouse colony. The presence of NTCD-035 in mice prior to challenge with a highly pathogenic C. difficile strain (VPI10463) led to significantly reduced disease severity. Phylogenetic characterization derived from whole genome sequencing and PCR ribotyping identified the isolate as a novel clade 1, ribotype 035 strain that lacks the pathogenicity locus required to produce toxins. Deficiency in toxin production along with sporulation capacity and secondary bile acid sensitivity was confirmed using in vitro assays. Inoculation of germ-free mice with NTCD-035 did not cause morbidity despite the strain readily colonizing the large intestine. Implementation of a culture-based screening procedure enabled the identification of mice harboring C. difficile in their microbiota, the establishment of a C. difficile-free mouse colony, and a monitoring system to prevent future contamination. Taken together, these data provide a framework for screening mice for endogenously harbored C. difficile and support clinical findings that demonstrate the therapeutic potential of non-toxigenic strains in preventing C. difficile associated disease. Abbreviations: PaLoc - Pathogenicity locus, CFUs - Colony forming units, TcdA - toxin-A, TcdB - toxin-B, CdtA - binary toxin A, CdtB - binary toxin B, CdtR - binary toxin R, NTCD - non-toxigenic C. difficile.

Contradictory Effects of NLRP3 Inflammasome Regulatory Mechanisms in Colitis

The inflammasome is an intracellular molecular complex, which is mainly involved in innate immunity. Inflammasomes are formed in response to danger signals, associated with infection and injury, and mainly regulate the secretion of interleukin-1β and interleukin-18. Inflammasome dysregulation is known to be associated with various diseases and conditions, and its regulatory mechanisms have become of great interest in recent years. In the colon, inflammasomes have been reported to be associated with autophagy and the microbiota, and their dysregulation contributes to colitis and. However, the detailed role of inflammasomes in inflammatory bowel disease is still under debate because the mechanisms that regulate the inflammasome are complex and the inflammasome components and cytokines show seemingly contradictory multiple effects. Herein, we comprehensively review the literature on inflammasome functioning in the colon and describe the complex interactions of the NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome components with inflammatory cytokines, autophagy, and the microbiota in experimental colitis models and patients with inflammatory bowel disease.

Acetate coordinates neutrophil and ILC3 responses against C. difficile through FFAR2

Microbiota-derived acetate coordinates innate immune responses during intestinal Clostridium difficile infection through its cognate receptor FFAR2. Acetate accelerates early neutrophil recruitment and increases ILC3 expression of the IL-1 receptor, boosting ILC3 production of IL-22 in response to neutrophil-derived IL-1β.

Antibiotic-induced dysbiosis is a key predisposing factor for Clostridium difficile infections (CDIs), which cause intestinal disease ranging from mild diarrhea to pseudomembranous colitis. Here, we examined the impact of a microbiota-derived metabolite, short-chain fatty acid acetate, on an acute mouse model of CDI. We found that administration of acetate is remarkably beneficial in ameliorating disease. Mechanistically, we show that acetate enhances innate immune responses by acting on both neutrophils and ILC3s through its cognate receptor free fatty acid receptor 2 (FFAR2). In neutrophils, acetate-FFAR2 signaling accelerates their recruitment to the inflammatory sites, facilitates inflammasome activation, and promotes the release of IL-1β; in ILC3s, acetate-FFAR2 augments expression of the IL-1 receptor, which boosts IL-22 secretion in response to IL-1β. We conclude that microbiota-derived acetate promotes host innate responses to C. difficile through coordinate action on neutrophils and ILC3s.

Lipopolysaccharide O structure of adherent and invasive Escherichia coli regulates intestinal inflammation via complement C3

Gut dysbiosis associated with intestinal inflammation is characterized by the blooming of particular bacteria such as adherent-invasive E. coli (AIEC). However, the precise mechanisms by which AIEC impact on colitis remain largely unknown. Here we show that antibiotic-induced dysbiosis worsened chemically-induced colitis in IL-22-deficient mice, but not in wild-type mice. The increase in intestinal inflammation was associated with the expansion of E. coli strains with genetic and functional features of AIEC. These E. coli isolates exhibited high ability to out compete related bacteria via colicins and resistance to the host complement system in vitro. Mutation of wzy, the lipopolysaccharide O polymerase gene, rendered AIEC more sensitive to the complement system and more susceptible to engulfment and killing by phagocytes while retaining its ability to outcompete related bacteria in vitro. The wzy AIEC mutant showed impaired fitness to colonize the intestine under colitic conditions, but protected mice from chemically-induced colitis. Importantly, the ability of the wzy mutant to protect from colitis was blocked by depletion of complement C3 which was associated with impaired intestinal eradication of AIEC in colitic mice. These studies link surface lipopolysaccharide O-antigen structure to the regulation of colitic activity in commensal AIEC via interactions with the complement system.

Microbially competent 3D skin: a test system that reveals insight into host-microbe interactions and their potential toxicological impact

The skin`s microbiome is predominantly commensalic, harbouring a metabolic potential far exceeding that of its host. While there is clear evidence that bacteria-dependent metabolism of pollutants modulates the toxicity for the host there is still a lack of models for investigating causality of microbiome-associated pathophysiology or toxicity. We now report on a biologically characterised microbial-skin tissue co-culture that allows studying microbe-host interactions for extended periods of time in situ. The system is based on a commercially available 3D skin model. In a proof-of-concept, this model was colonised with single and mixed cultures of two selected skin commensals. Two different methods were used to quantify the bacteria on the surface of the skin models. While Micrococcus luteus established a stable microbial-skin tissue co-culture, Pseudomonas oleovorans maintained slow continuous growth over the 8-day cultivation period. A detailed skin transcriptome analysis showed bacterial colonisation leading to up to 3318 significant changes. Additionally, FACS, ELISA and Western blot analyses were carried out to analyse secretion of cytokines and growth factors. Changes found in colonised skin varied depending on the bacterial species used and comprised immunomodulatory functions, such as secretion of IL-1α/β, Il-6, antimicrobial peptides and increased gene transcription of IL-10 and TLR2. The colonisation also influenced the secretion of growth factors such as VFGFA and FGF2. Notably, many of these changes have already previously been associated with the presence of skin commensals. Concomitantly, the model gained first insights on the microbiome's influence on skin xenobiotic metabolism (i.e., CYP1A1, CYP1B1 and CYP2D6) and olfactory receptor expression. The system provides urgently needed experimental access for assessing the toxicological impact of microbiome-associated xenobiotic metabolism in situ.

The adaptive immune response to Clostridioides difficile: A tricky balance between immunoprotection and immunopathogenesis

Clostridioides difficile (C. difficile) is the major cause of hospital-acquired gastrointestinal infections in individuals following antibiotics treatment. The pathogenesis of C. difficile infection (CDI) is mediated mainly by the production of toxins that induce tissue damage and host inflammatory responses. While innate immunity is well characterized in human and animal models of CDI, adaptive immune responses remain poorly understood. In this review, the current understanding of adaptive immunity is summarized and its influence on pathogenesis and disease outcome is discussed. The perspectives on what we believe to be the main pending questions and the focus of future research are also provided. There is no doubt that the innate immune response provides a first line of defense to CDI. But, is the adaptive immune response a friend or a foe? Probably it depends on the course of the disease. Adaptive immunity is essential for pathogen eradication, but may also trigger uncontrolled or pathological inflammation. Most of the understanding of the role of T cells is based on findings from experimental models. While they are a very valuable tool for research studies, more studies in human are needed to translate these findings into human disease. Another main challenge is to unravel the role of the different T cell populations on protection or induction of immunopathogenesis.

IL-17-producing γδ T cells protect against Clostridium difficile infection

Colitis caused by Clostridium difficile infection is a growing cause of human morbidity and mortality, especially after antibiotic use in health care settings. The natural immunity of newborn infants and protective host immune mediators against C. difficile infection are not fully understood, with data suggesting that inflammation can be either protective or pathogenic. Here, we show an essential role for IL-17A produced by γδ T cells in host defense against C. difficile infection. Fecal extracts from children with C. difficile infection showed increased IL-17A and T cell receptor γ chain expression, and IL-17 production by intestinal γδ T cells was efficiently induced after infection in mice. C. difficile-induced tissue inflammation and mortality were markedly increased in mice deficient in IL-17A or γδ T cells. Neonatal mice, with naturally expanded RORγt+ γδ T cells poised for IL-17 production were resistant to C. difficile infection, whereas elimination of γδ T cells or IL-17A each efficiently overturned neonatal resistance against infection. These results reveal an expanded role for IL-17-producing γδ T cells in neonatal host defense against infection and provide a mechanistic explanation for the clinically observed resistance of infants to C. difficile colitis.

The pyrin inflammasome in host-microbe interactions

Pyrin is an inflammasome sensor in phagocytes that is activated in response to bacterial toxins and effectors that modify RhoA. Pathogen effector-triggered pyrin activation is analogous to an indirect guard mechanism in plants. Pyrin activation appears to be triggered when RhoA GTPases in a host cell are prevented from binding downstream signaling proteins (transducers). RhoA transducers that control this response include PRK kinases, which negatively regulate pyrin by phosphorylation and binding of 14-3-3 proteins. Microtubules regulate pyrin at different levels and may serve as a platform for inflammasome nucleation. Pyrin increases inflammation in the lung, gut or systemically during infection or intoxication in mouse models and protects against systemic infection by decreasing bacterial loads. Pathogenic Yersinia spp. overcome this protective response using effectors that inhibit the pyrin inflammasome. Gain of function mutations in MEFV, the gene encoding pyrin, cause the autoinflammatory disease Familial Mediterranean Fever. Yersinia pestis may have selected for gain of function MEFV mutations in the human population.

Impact of Antibiotic-Resistant Bacteria on Immune Activation and Clostridioides difficile Infection in the Mouse Intestine

Antibiotic treatment of patients undergoing complex medical treatments can deplete commensal bacterial strains from the intestinal microbiota, thereby reducing colonization resistance against a wide range of antibiotic-resistant pathogens. Loss of colonization resistance can lead to marked expansion of vancomycin-resistant Enterococcus faecium (VRE), Klebsiella pneumoniae, and Escherichia coli in the intestinal lumen, predisposing patients to bloodstream invasion and sepsis. The impact of intestinal domination by these antibiotic-resistant pathogens on mucosal immune defenses and epithelial and mucin-mediated barrier integrity is unclear. We used a mouse model to study the impact of intestinal domination by antibiotic-resistant bacterial species and strains on the colonic mucosa. Intestinal colonization with K. pneumoniae, Proteus mirabilis, or Enterobacter cloacae promoted greater recruitment of neutrophils to the colonic mucosa. To test the hypothesis that the residual microbiota influences the severity of colitis caused by infection with Clostridioides difficile, we coinfected mice that were colonized with ampicillin-resistant bacteria with a virulent strain of C. difficile and monitored colonization and pathogenesis. Despite the compositional differences in the gut microbiota, the severity of C. difficile infection (CDI) and mortality did not differ significantly between mice colonized with different ampicillin-resistant bacterial species. Our results suggest that the virulence mechanisms enabling CDI and epithelial destruction outweigh the relatively minor impact of less-virulent antibiotic-resistant intestinal bacteria on the outcome of CDI.

Persistent Systemic Microbial Translocation, Inflammation, and Intestinal Damage During Clostridioides difficile Infection

BACKGROUND

Clostridioides difficile infection (CDI) might be complicated by the development of nosocomial bloodstream infection (n-BSI). Based on the hypothesis that alteration of the normal gut integrity is present during CDI, we evaluated markers of microbial translocation, inflammation, and intestinal damage in patients with CDI.

METHODS

Patients with documented CDI were enrolled in the study. For each subject, plasma samples were collected at T0 and T1 (before and after CDI therapy, respectively), and the following markers were evaluated: lipopolysaccharide-binding protein (LPB), EndoCab IgM, interleukin-6, intestinal fatty acid binding protein (I-FABP). Samples from nonhospitalized healthy controls were also included. The study population was divided into BSI+/BSI- and fecal microbiota transplantation (FMT) +/FMT- groups, according to the development of n-BSI and the receipt of FMT, respectively.

RESULTS

Overall, 45 subjects were included; 8 (17.7%) developed primary n-BSI. Markers of microbial translocation and intestinal damage significantly decreased between T0 and T1, however, without reaching values similar to controls (P < .0001). Compared with BSI-, a persistent high level of microbial translocation in the BSI+ group was observed. In the FMT+ group, markers of microbial translocation and inflammation at T1 tended to reach control values.

CONCLUSIONS

CDI is associated with high levels of microbial translocation, inflammation, and intestinal damage, which are still present at clinical resolution of CDI. The role of residual mucosal perturbation and persistence of intestinal cell damage in the development of n-BSI following CDI, as well as the possible effect of FMT in the restoration of mucosal integrity, should be further investigated.

The xenobiotic sensing pregnane X receptor regulates tissue damage and inflammation triggered by C difficile toxins

Clostridioides difficile (formerly Clostridium difficile; C difficile), the leading cause of nosocomial antibiotic‐associated colitis and diarrhea in the industrialized world, triggers colonic disease through the release two toxins, toxin A (TcdA) and toxin B (TcdB), glucosyltransferases that modulate monomeric G‐protein function and alter cytoskeletal function. The initial degree of the host immune response to C difficile and its pathogenic toxins is a common indicator of disease severity and infection recurrence. Thus, targeting the intestinal inflammatory response during infection could significantly decrease disease morbidity and mortality. In the current study, we sought to interrogate the influence of the pregnane X receptor (PXR), a modulator of xenobiotic and detoxification responses, which can sense and respond to microbial metabolites and modulates inflammatory activity, during exposure to TcdA and TcdB. Following intrarectal exposure to TcdA/B, PXR‐deficient mice (Nr1i2^−/−) exhibited reduced survival, an effect that was associated with increased levels of innate immune cell influx. This exacerbated response was associated with a twofold increase in the expression of Tlr4. Furthermore, while broad‐spectrum antibiotic treatment (to deplete the intestinal microbiota) did not alter the responses in Nr1i2^−/− mice, blocking TLR4 signaling significantly reduced TcdA/B‐induced disease severity and immune responses in these mice. Lastly, to assess the therapeutic potential of targeting the PXR, we activated the PXR with pregnenolone 16α‐carbonitrile (PCN) in wild‐type mice, which greatly reduced the severity of TcdA/B‐induced damage and intestinal inflammation. Taken together, these data suggest that the PXR plays a role in the host's response to TcdA/B and may provide a novel target to dampen the inflammatory tissue damage in C difficile infections.

The liver as a nursery for leukocytes

Leukocytes are a large population of cells spread within most tissues in the body. These cells may be either sessile (called as resident cells) or circulating leukocytes, which travel long journeys inside the vessels during their lifespan. Although production and maturation of these leukocytes in adults primarily occur in the bone marrow, it is well known that this process-called hematopoiesis-started in the embryonic life in different sites, including the yolk sac, placenta, and the liver. In this review, we will discuss how the liver acts as a pivotal site for leukocyte maturation during the embryo phase, and also how the most frequent liver-resident immune cell populations-namely Kupffer cells, dendritic cells, and lymphocytes-play a vital role in both tolerance and inflammatory responses to antigens from food, microbiota, and pathogens.

Enterotoxic Clostridia: Clostridioides difficile Infections

Clostridioides difficile is a Gram-positive, anaerobic, spore forming pathogen of both humans and animals and is the most common identifiable infectious agent of nosocomial antibiotic-associated diarrhea. Infection can occur following the ingestion and germination of spores, often concurrently with a disruption to the gastrointestinal microbiota, with the resulting disease presenting as a spectrum, ranging from mild and self-limiting diarrhea to severe diarrhea that may progress to life-threating syndromes that include toxic megacolon and pseudomembranous colitis. Disease is induced through the activity of the C. difficile toxins TcdA and TcdB, both of which disrupt the Rho family of GTPases in host cells, causing cell rounding and death and leading to fluid loss and diarrhea. These toxins, despite their functional and structural similarity, do not contribute to disease equally. C. difficile infection (CDI) is made more complex by a high level of strain diversity and the emergence of epidemic strains, including ribotype 027-strains which induce more severe disease in patients. With the changing epidemiology of CDI, our understanding of C. difficile disease, diagnosis, and pathogenesis continues to evolve. This article provides an overview of the current diagnostic tests available for CDI, strain typing, the major toxins C. difficile produces and their mode of action, the host immune response to each toxin and during infection, animal models of disease, and the current treatment and prevention strategies for CDI.

Whole-genome analysis of the colonization-resistant bacterium Phytobacter sp. SCO41T isolated from Bacillus nematocida B16-fed adult Caenorhabditis elegans

Colonization resistance is an important attribute for bacterial interactions with hosts, but the mechanism is still not completely clear. In this study, we found that Phytobacter sp. SCO41^T can effectively inhibit the in vivo colonization of Bacillus nematocida B16 in Caenorhabditis elegans, and we revealed the colonization resistance mechanism. Three strains of colonization-resistant bacteria, SCO41^T, BX15, and BC7, were isolated from the intestines of the free-living nematode C. elegans derived from rotten fruit and soil. The primary characteristics and genome map of one of the three isolates was investigated to explore the underlying mechanism of colonization resistance in C. elegans. In addition, we performed exogenous iron supplementation and gene cluster knockout experiments to validate the sequencing results. The results showed that relationship was close among the three strains, which was identified as belonging to the genus Phytobacter. The type strain is SCO41^T (= CICC 24103^T = KCTC 52362^T). Whole genome analysis showed that csgA, csgB, csgC, csgE, csgF, and csgG were involved in the curli adhesive process and that fepA, fepB, fepC, fepD, and fepG played important roles in SCO41^T against the colonization of B. nematocida B16 in C. elegans by competing for iron. Exogenous iron supplementation showed that exogenous iron can increase the colonization of B. nematocida B16, which was additionally confirmed by a deletion mutant strain. The csg gene family contributes to the colonization of SCO41^T in C. elegans. Curli potentially contribute to the colonization of SCO41^T in C. elegans, and enterobactin has a key role in SCO41^T to resist the colonization of B. nematocida B16 by competing for iron.

Epithelium-specific MyD88 signaling, but not DCs or macrophages, control acute intestinal infection with Clostridium difficile

Infection with Clostridium difficile is one of the major causes of health care acquired diarrhea and colitis. Signaling though MyD88 downstream of TLRs is critical for initiating the early protective host response in mouse models of C. difficile infection (CDI). In the intestine, MyD88 is expressed in various tissues and cell types, such as the intestinal epithelium and mononuclear phagocytes (MNP), including DC or macrophages. Using a genetic gain-of-function system, we demonstrate here that restricting functional MyD88 signaling to the intestinal epithelium, but also to MNPs is sufficient to protect mice during acute CDI by upregulation of the intestinal barrier function and recruitment of neutrophils. Nevertheless, we also show that mice depleted for CD11c-expressing MNPs in the intestine display no major defects in mounting an effective inflammatory response, indicating that the absence of these cells is irrelevant for inducing host protection during acute infection. Together, our results highlight the importance of epithelial-specific MyD88 signaling and demonstrate that although functional MyD88 signaling in DC and macrophages alone is sufficient to correct the phenotype of MyD88-deficiency, these cells do not seem to be essential for host protection in MyD88-sufficient animals during acute infection with C. difficile.

NLRP3 Inflammasome and Inflammatory Bowel Disease

NLRP3 inflammasome can be widely found in epithelial cells and immune cells. The NOD-like receptors (NLRs) family member NLRP3 contains a central nucleotide-binding and oligomerization (NACHT) domain which facilitates self-oligomerization and has ATPase activity. The C-terminal conserves a leucine-rich repeats (LRRs) domain which can modulate NLRP3 activity and sense endogenous alarmins and microbial ligands. In contrast, the N-terminal pyrin domain (PYD) can account for homotypic interactions with the adaptor protein-ASC of NLRP3 inflammasome. These characters enable it function in innate immunity. Its downstream effector proteins include caspase-1 and IL-1β etc. which exhibit protective or detrimental roles in mucosal immunity in different studies. Here, we comprehensively review the current literature regarding the physiology of NLRP3 inflammasome and its potential roles in the pathogenesis of IBD. We also discuss about the complex interactions among the NLRP3 inflammasome, mucosal immune response, and gut homeostasis as found in experimental models and IBD patients.

Nonsteroidal Anti-inflammatory Drugs Alter the Microbiota and Exacerbate Clostridium difficile Colitis while Dysregulating the Inflammatory Response

Clostridium difficile infection (CDI) is a major public health threat worldwide. The use of nonsteroidal anti-inflammatory drugs (NSAIDs) is associated with enhanced susceptibility to and severity of CDI; however, the mechanisms driving this phenomenon have not been elucidated. NSAIDs alter prostaglandin (PG) metabolism by inhibiting cyclooxygenase (COX) enzymes. Here, we found that treatment with the NSAID indomethacin prior to infection altered the microbiota and dramatically increased mortality and the intestinal pathology associated with CDI in mice. We demonstrated that in C. difficile-infected animals, indomethacin treatment led to PG deregulation, an altered proinflammatory transcriptional and protein profile, and perturbed epithelial cell junctions. These effects were paralleled by increased recruitment of intestinal neutrophils and CD4⁺ cells and also by a perturbation of the gut microbiota. Together, these data implicate NSAIDs in the disruption of protective COX-mediated PG production during CDI, resulting in altered epithelial integrity and associated immune responses.IMPORTANCEClostridium difficile infection (CDI) is a spore-forming anaerobic bacterium and leading cause of antibiotic-associated colitis. Epidemiological data suggest that use of nonsteroidal anti-inflammatory drugs (NSAIDs) increases the risk for CDI in humans, a potentially important observation given the widespread use of NSAIDs. Prior studies in rodent models of CDI found that NSAID exposure following infection increases the severity of CDI, but mechanisms to explain this are lacking. Here we present new data from a mouse model of antibiotic-associated CDI suggesting that brief NSAID exposure prior to CDI increases the severity of the infectious colitis. These data shed new light on potential mechanisms linking NSAID use to worsened CDI, including drug-induced disturbances to the gut microbiome and colonic epithelial integrity. Studies were limited to a single NSAID (indomethacin), so future studies are needed to assess the generalizability of our findings and to establish a direct link to the human condition.

Aging impairs protective host defenses against Clostridioides (Clostridium) difficile infection in mice by suppressing neutrophil and IL-22 mediated immunity

BACKGROUND

Morbidity and mortality associated with Clostridioides (formerly Clostridium) difficile infection (CDI) rises progressively with advanced age (≥65 years) due in part to perturbations of the gut microbiota and immune dysfunction. Epidemiological data of community-acquired CDI suggests increased susceptibility may begin earlier during middle-age (45-64 years) but the causation remains unknown.

METHODS

Middle-aged (12-14 months) and young (2-4 months) adult mice were infected with C. difficile, and disease severity, gut microbiome and innate immune response were compared. Cytokine reconstitution studies were performed in infected middle-aged mice.

RESULTS

Infection of middle-aged mice with C. difficile led to greater disease compared to young controls, which was associated with increases in C. difficile burden and toxin titers, and elevated bacterial translocation. With the exception of an expansion of C. difficile in middle-aged mice, microbiome analysis revealed no age-related differences. In contrast, middle-aged mice displayed a significant defect in neutrophil recruitment to the colon, with diminished levels of innate immune cytokines IL-6, IL-23 and IL-22. Importantly, recombinant IL-22 administration during CDI reduced morbidity and prevented death in middle-aged mice.

CONCLUSION

Increased susceptibility to C. difficile occurs in middle-aged mice modeling the community-acquired CDI demographics and is driven by an impaired innate immune response.

Apoptosis of intestinal epithelial cells restricts Clostridium difficile infection in a model of pseudomembranous colitis

Clostridium difficile is the leading cause of pseudomembranous colitis in hospitalized patients. C. difficile enterotoxins TcdA and TcdB promote this inflammatory condition via a cytotoxic response on intestinal epithelial cells (IECs), but the underlying mechanisms are incompletely understood. Additionally, TcdA and TcdB engage the Pyrin inflammasome in macrophages, but whether Pyrin modulates CDI pathophysiology is unknown. Here we show that the Pyrin inflammasome is not functional in IECs and that Pyrin signaling is dispensable for CDI-associated IEC death and for in vivo pathogenesis. Instead, our studies establish that C. difficile enterotoxins induce activation of executioner caspases 3/7 via the intrinsic apoptosis pathway, and demonstrate that caspase-3/7-mediated IEC apoptosis is critical for in vivo host defense during early stages of CDI. In conclusion, our findings dismiss a critical role for inflammasomes in CDI pathogenesis, and identify IEC apoptosis as a host defense mechanism that restricts C. difficile infection in vivo.

The role of zinc and nutritional immunity in Clostridium difficile infection

Clostridium difficile in one of the most commonly reported nosocomial pathogens worldwide. Beyond antibiotic use, little is known about the host, microbiota, and environmental factors that contribute to susceptibility to and severity of C. difficile infection (CDI). We recently observed that in a mouse model of CDI, excess dietary zinc (Zn) alters the gut microbiota and decreases resistance to CDI. Moreover, we determined that high levels of Zn exacerbate C. difficile-associated disease and calprotectin-mediated Zn limitation is an essential host response to infection. In this addendum, we discuss how these findings add to our understanding of CDI and consider the potential implications of excess metal intake on the microbiota and infection.

Control of Clostridium difficile Infection by Defined Microbial Communities

Each year in the United States, billions of dollars are spent combating almost half a million Clostridium difficile infections (CDIs) and trying to reduce the ∼29,000 patient deaths in which C. difficile has an attributed role. In Europe, disease prevalence varies by country and level of surveillance, though yearly costs are estimated at €3 billion. One factor contributing to the significant health care burden of C. difficile is the relatively high frequency of recurrent CDIs. Recurrent CDI, i.e., a second episode of symptomatic CDI occurring within 8 weeks of successful initial CDI treatment, occurs in ∼25% of patients, with 35 to 65% of these patients experiencing multiple episodes of recurrent disease. Using microbial communities to treat recurrent CDI, either as whole fecal transplants or as defined consortia of bacterial isolates, has shown great success (in the case of fecal transplants) or potential promise (in the case of defined consortia of isolates). This review will briefly summarize the epidemiology and physiology of C. difficile infection, describe our current understanding of how fecal microbiota transplants treat recurrent CDI, and outline potential ways that knowledge can be used to rationally design and test alternative microbe-based therapeutics.

The ATP-P2X7 Signaling Axis Is an Essential Sentinel for Intracellular Clostridium difficile Pathogen-Induced Inflammasome Activation

Clostridium difficile infection (CDI) is the leading cause of nosocomial infection in hospitalized patients receiving long-term antibiotic treatment. An excessive host inflammatory response is believed to be the major mechanism underlying the pathogenesis of C. difficile infection, and various proinflammatory cytokines such as IL-1β are detected in patients with C. difficile infection. IL-1β is known to be processed by caspase-1, a cysteine protease that is regulated by a protein complex called the inflammasome, which leads to a specialized form of cell death called pyroptosis. The function of inflammasome activation-induced pyroptosis is to clear or limit the spread of invading pathogens via infiltrated neutrophils. Here, we focused on inflammasome activation induced by intact C. difficile to re-evaluate the nature of inflammasome activation in CDI pathogenesis, which could provide information that leads to an alternative therapeutic strategy for the treatment of this condition in humans. First, we found that caspase-1-dependent IL-1β production was induced by C. difficile pathogens in macrophages and increased in a time-dependent manner. Moreover, intracellular toxigenic C. difficile was essential for ATP-P2X₇ pathway of inflammasome activation and subsequent caspase-1-dependent pyroptotic cell death, leading to the loss of membrane integrity and release of intracellular contents such as LDH. Notably, we also observed that bacterial components such as surface layer proteins (SLPs) were released from pyroptotic cells. In addition, pro-IL-1β production was completely MyD88 and partially TLR2 dependent. Finally, to investigate the role of the caspase-1-dependent inflammasome in host defense, we found that colonic inflammasome activation was also induced by CDI and that caspase-1 inhibition by Ac-YVAD-CMK led to increased disease progression and C. difficile load. Taken together, the present results suggest that MyD88 and TLR2 are critical component in pro-IL-1β production and intracellular C. difficile following the ATP-P2X₇ pathway of inflammasome activation and pyroptosis, which play important roles in host defense through the utilization of inflammation-mediated bacterial clearance mechanisms during C. difficile infection.

Is Clostridium difficile infection a risk factor for subsequent bloodstream infection?

BACKGROUND

Clostridium difficile infection (CDI) is a common nosocomial diarrheal illness increasingly associated with mortality in United States. The underlying factors and mechanisms behind the recent increases in morbidity from CDI have not been fully elucidated. Murine models suggest a mucosal barrier breakdown leads to bacterial translocation and subsequent bloodstream infection (BSI). This study tests the hypothesis that CDI is associated with subsequent BSI in humans.

METHODS

We conducted a retrospective cohort study on 1132 inpatients hospitalized >72 h with available stool test results for toxigenic C. difficile. The primary outcome was BSI following CDI. Secondary outcomes included 30-day mortality, colectomy, readmission, and ICU admission. Unadjusted and adjusted logistic regression models were developed.

RESULTS

CDI occurred in 570 of 1132 patients (50.4%). BSI occurred in 86 (7.6%) patients. Enterococcus (14%) and Klebsiella (14%) species were the most common organisms. Patients with BSI had higher comorbidity scores and were more likely to be male, on immunosuppression, critically ill, and have a central venous catheter in place. Of the patients with BSI, 36 (42%) had CDI. CDI was not associated with subsequent BSI (OR 0.69; 95% CI 0.44-1.08; P = 0.103) in unadjusted analysis. In multivariable modeling, CDI appeared protective against subsequent BSI (OR 0.57; 95% CI 0.34-0.96; P = 0.036). Interaction modeling suggests a complicated relationship among CDI, BSI, antibiotic exposure, and central venous catheter use.

CONCLUSIONS

In this cohort of inpatients that underwent testing for CDI, CDI was not a risk factor for developing subsequent BSI.