What's for dinner? How Citrobacter rodentium's metabolism helps it thrive in the competitive gut

Defining enteric bacterial pathogenesis using organoids: Citrobacter rodentium uses EspC, an atypical mucinolytic protease, to penetrate mouse colonic mucus

Enteric bacterial pathogens pose significant threats to human health; however, the mechanisms by which they infect the mammalian gut in the face of daunting host defenses remain to be fully defined. For the attaching and effacing (A/E) bacterial family member and murine pathogen Citrobacter rodentium, its virulence strategy appears to involve penetration of the colonic mucus barrier to reach the underlying epithelium. To better define these interactions, we grew colonoids under air-liquid interface (ALI) conditions, producing a thick mucus layer that mimicked in vivo mucus composition and glycosylation. C. rodentium's penetration of ALI-derived mucus was dramatically enhanced upon exposure to sialic acid, in concert with the secretion of two serine protease autotransporter of Enterobacteriaceae (SPATE) proteins, Pic and EspC. Despite Pic being a class II SPATE, and already recognized as a mucinase, it was EspC, a class I SPATE family member, that degraded ALI-derived mucus, despite class I SPATEs not previously shown to possess mucinase activity. Confirming this finding, E. coli DH5α carrying a plasmid that expresses C. rodentium-derived EspC was able to degrade the mucus. Moreover, recombinant EspC alone also displayed mucinolytic activity in a dose-dependent manner. Collectively, our results reveal the utility of ALI-derived mucus in modeling microbe-host interactions at the intestinal mucosal surface, as well as identify EspC as an atypical class I SPATE that shows significant mucinolytic activity toward ALI-derived mucus.

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 nonneuronal cells. With this in mind, we assessed the immunoregulatory roles of SP receptor signaling during enteric bacterial infection with the noninvasive 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 demonstrate that SP receptor signaling can significantly reduce inflammation and prevent host-maladaptive responses without impinging upon host protection.

QcrC is a potential target for antibody therapy and vaccination to control Campylobacter jejuni infection by suppressing its energy metabolism

Introduction

Campylobacter spp. are a public health concern, yet there is still no effective vaccine or medicine available.

Methods

Here, we developed a Campylobacter jejuni-specific antibody and found that it targeted a menaquinol cytochrome c reductase complex QcrC.

Results

The antibody was specifically reactive to multiple C. jejuni strains including clinical isolates from patients with acute enteritis and was found to inhibit the energy metabolism and growth of C. jejuni. Different culture conditions produced different expression levels of QcrC in C. jejuni, and these levels were closely related not only to the energy metabolism of C. jejuni but also its pathogenicity. Furthermore, immunization of mice with recombinant QcrC induced protective immunity against C. jejuni infection.

Discussion

Taken together, our present findings highlight a possible antibody- or vaccination-based strategy to prevent or control Campylobacter infection by targeting the QcrC-mediated metabolic pathway.

Red pandas with different diets and environments exhibit different gut microbial functional composition and capacity

The red panda (Ailurus fulgens) is a distinctive mammal known for its reliance on a diet primarily consisting of bamboo. The gut microbiota and overall health of animals are strongly influenced by diets and environments. Therefore, conducting research to explore the taxonomical and functional variances within the gut microbiota of red pandas exposed to various dietary and environmental conditions could shed light on the dynamic complexities of their microbial communities. In this study, normal fecal samples were obtained from red pandas residing in captive and semi-free environments under different dietary regimes and used for metabolomic, 16S rRNA, and metagenomic sequencing analysis, with the pandas classified into four distinct cohorts according to diet and environment. In addition, metagenomic sequencing was conducted on mucus fecal samples to elucidate potential etiological agents of disease. Results revealed an increased risk of gastrointestinal diseases in red pandas consuming bamboo shoots due to the heightened presence of pathogenic bacteria, although an increased presence of microbiota-derived tryptophan metabolites appeared to facilitate intestinal balance. The red pandas fed bamboo leaves also exhibited a decrease in gut microbial diversity, which may be attributed to the antibacterial flavonoids and lower protein levels in leaves. Notably, red pandas residing in semi-free environments demonstrated an enriched gut microbial diversity. Moreover, the occurrence of mucus secretion may be due to an increased presence of species associated with diarrhea and a reduced level of microbiota-derived tryptophan metabolites. In summary, our findings substantiate the influential role of diet and environment in modulating the gut microbiota of red pandas, offering potential implications for improved captive breeding practices.

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.

In vitro digestion and fermentation behaviors of polysaccharides from Choerospondias axillaris fruit and its effect on human gut microbiota

Choerospondias axillaris fruit has attracted more and more attention due to its various pharmacological activities, which are rich in polysaccharides. This study investigated the in vitro saliva-gastrointestinal digestion and fecal fermentation behaviors of polysaccharides from Choerospondias axillaris fruit (CAP), as well as its impact on human gut microbiota. The results showed that CAP could be partially degraded during the gastrointestinal digestion. The FT-IR spectra of the digested CAP didn't change significantly, however, the morphological feature of SEM changed to disordered flocculent and rod-like structures. 16S rRNA sequencing analysis found that after in vitro fermentation, CAP could increase the relative abundances of beneficial bacteria including Megasphaera, Megamonas and Bifidobacterium to produce short-chain fatty acids (SCFAs), while it can also reduce the abundances of harmful bacteria of Collinsella, Gemmiger, Klebsiella and Citrobacter, suggesting that CAP could modulate the composition and abundance of gut microbiota. These results implied that CAP can be developed as a potential prebiotic in the future.

Sialic acid plays a pivotal role in licensing Citrobacter rodentium's transition from the intestinal lumen to a mucosal adherent niche

Enteric bacterial pathogens pose significant threats to human health; however, the mechanisms by which they infect the mammalian gut in the face of daunting host defenses and an established microbiota remain poorly defined. For the attaching and effacing (A/E) bacterial family member and murine pathogen Citrobacter rodentium, its virulence strategy likely involves metabolic adaptation to the host's intestinal luminal environment, as a necessary precursor to reach and infect the mucosal surface. Suspecting this adaptation involved the intestinal mucus layer, we found that C. rodentium was able to catabolize sialic acid, a monosaccharide derived from mucins, and utilize it as its sole carbon source for growth. Moreover, C. rodentium also sensed and displayed chemotactic activity toward sialic acid. These activities were abolished when the nanT gene, encoding a sialic acid transporter, was deleted (ΔnanT). Correspondingly, the ΔnanT C. rodentium strain was significantly impaired in its ability to colonize the murine intestine. Intriguingly, sialic acid was also found to induce the secretion of two autotransporter proteins, Pic and EspC, which possess mucinolytic and host-adherent properties. As a result, sialic acid enhanced the ability of C. rodentium to degrade intestinal mucus (through Pic), as well as to adhere to intestinal epithelial cells (through EspC). We thus demonstrate that sialic acid, a monosaccharide constituent of the intestinal mucus layer, functions as an important nutrient and a key signal for an A/E bacterial pathogen to escape the colonic lumen and directly infect its host's intestinal mucosa.

Helicobacter spp. are prevalent in wild mice and protect from lethal Citrobacter rodentium infection in the absence of adaptive immunity

Transfer of the gut microbiota from wild to laboratory mice alters the host's immune status and enhances resistance to infectious and metabolic diseases, but understanding of which microbes and how they promote host fitness is only emerging. Our analysis of metagenomic sequencing data reveals that Helicobacter spp. are enriched in wild compared with specific-pathogen-free (SPF) and conventionally housed mice, with multiple species commonly co-colonizing their hosts. We create laboratory mice harboring three non-SPF Helicobacter spp. to evaluate their effect on mucosal immunity and colonization resistance to the enteropathogen Citrobacter rodentium. Our experiments reveal that Helicobacter spp. interfere with C. rodentium colonization and attenuate C. rodentium-induced gut inflammation in wild-type (WT) mice, even preventing lethal infection in Rag2-/- SPF mice. Further analyses suggest that Helicobacter spp. interfere with tissue attachment of C. rodentium, putatively by reducing the availability of mucus-derived sugars. These results unveil pivotal protective functions of wild mouse microbiota constituents against intestinal infection.

Bacterial envelope stress responses: Essential adaptors and attractive targets

Millions of deaths a year across the globe are linked to antimicrobial resistant infections. The need to develop new treatments and repurpose of existing antibiotics grows more pressing as the growing antimicrobial resistance pandemic advances. In this review article, we propose that envelope stress responses, the signaling pathways bacteria use to recognize and adapt to damage to the most vulnerable outer compartments of the microbial cell, are attractive targets. Envelope stress responses (ESRs) support colonization and infection by responding to a plethora of toxic envelope stresses encountered throughout the body; they have been co-opted into virulence networks where they work like global positioning systems to coordinate adhesion, invasion, microbial warfare, and biofilm formation. We highlight progress in the development of therapeutic strategies that target ESR signaling proteins and adaptive networks and posit that further characterization of the molecular mechanisms governing these essential niche adaptation machineries will be important for sparking new therapeutic approaches aimed at short-circuiting bacterial adaptation.

Simulated Colonic Fluid Replicates the In Vivo Growth Capabilities of Citrobacter rodentium cpxRA Mutants and Uncovers Additive Effects of Cpx-Regulated Genes on Fitness

Citrobacter rodentium is an attaching and effacing (A/E) pathogen used to model enteropathogenic and enterohemorrhagic Escherichia coli infections in mice. During colonization, C. rodentium must adapt to stresses in the gastrointestinal tract, such as antimicrobial peptides, pH changes, and bile salts. The Cpx envelope stress response (ESR) is a two-component system used by some bacteria to remediate stress by modulating gene expression, and it is necessary for C. rodentium pathogenesis in mice. Here, we utilized simulated colonic fluid (SCF) to mimic the gastrointestinal environment, which we show strongly induces the Cpx ESR and highlights a fitness defect specific to the ΔcpxRA mutant. While investigating genes in the Cpx regulon that may contribute to C. rodentium pathogenesis, we found that the absence of the Cpx ESR resulted in higher expression of the locus of enterocyte effacement (LEE) master regulator, ler, and that the genes yebE, ygiB, bssR, and htpX relied on CpxRA for proper expression. We then determined that CpxRA and select gene mutants were essential for proper growth in SCF when in the presence of extraneous stressors and in competition. Although none of the Cpx-regulated gene mutants exhibited marked virulence phenotypes in vivo, the ΔcpxRA mutant had reduced colonization and attenuated virulence, as previously determined, which replicated the in vitro growth phenotypes specific to SCF. Overall, these results indicate that the ΔcpxRA virulence defect is not due to any single Cpx regulon gene examined. Instead, attenuation may be the result of defective growth in the colonic environment resulting from the collective impact of multiple Cpx-regulated genes.

Isolation and identification of mucin-degrading bacteria originated from human feces and their potential probiotic efficacy according to host-microbiome enterotype

AIM

Mucin-degrading bacteria are known to be beneficial for gut health. We aimed to isolate human-derived mucin-degrading bacteria and identify potential probiotic characteristics and their effects on the bacterial community and short-chain fatty acid (SCFA) production according to three different enterotypes of the host.

METHODS AND RESULTS

Bacteria with mucin decomposition ability from human feces were isolated and identified by 16S rRNA sequencing and MALDI-TOF. Heat resistance, acid resistance, antibiotic resistance, and antibacterial activity were analyzed in the selected bacteria. Their adhesion capability to the Caco-2 cell was determined by scanning electron microscopy. Their ability to alter the bacterial community and SCFA production of the isolated bacteria was investigated in three enterotypes. The three isolated strains were Bifidobacterium(B.) animalis SPM01 (CP001606.1, 99%), B. longum SPM02 (NR_043437.1, 99%), and Limosilactobacillus(L.) reuteri SPM03 (CP000705.1, 99%) deposited in Korean Collection for Type Culture (KCTC-18958P). Among them, B. animalis exhibited the highest mucin degrading ability. They exhibited strong resistance to acidic conditions, moderate resistance to heat, and the ability to adhere tightly to Caco-2 cells. Three isolated mucin-degrading bacteria incubation increased Lactobacillus in the fecal bacteria from Bacteroides and Prevotella enterotypes. However, only L. reuteri elevated Lactobacillus in the fecal bacteria from the Ruminococcus enterotype. B. longum and B. animalis increased the α-diversity in the Ruminococcus enterotype, while their incubation with other intestinal types decreased the α-diversity. B. animalis and L. reuteri increased the butyric acid level in fecal bacteria from the Prevotella enterotype, and L. reuteri elevated the acetic acid level in those from the Ruminococcus enterotype. However, the overall SCFA changes were minimal.

CONCLUSIONS

The isolated mucin-degrading bacteria act as probiotics and modulate gut microbiota and SCFA production differently according to the host's enterotypes.

SIGNIFICANCE AND IMPACT OF STUDY

Probiotics need to be personalized according to the enterotypes in clinical application.

The glycerol-3-phosphate dehydrogenases GpsA and GlpD constitute the oxidoreductive metabolic linchpin for Lyme disease spirochete host infectivity and persistence in the tick

We have identified GpsA, a predicted glycerol-3-phosphate dehydrogenase, as a virulence factor in the Lyme disease spirochete Borrelia (Borreliella) burgdorferi: GpsA is essential for murine infection and crucial for persistence of the spirochete in the tick. B. burgdorferi has a limited biosynthetic and metabolic capacity; the linchpin connecting central carbohydrate and lipid metabolism is at the interconversion of glycerol-3-phosphate and dihydroxyacetone phosphate, catalyzed by GpsA and another glycerol-3-phosphate dehydrogenase, GlpD. Using a broad metabolomics approach, we found that GpsA serves as a dominant regulator of NADH and glycerol-3-phosphate levels in vitro, metabolic intermediates that reflect the cellular redox potential and serve as a precursor for lipid and lipoprotein biosynthesis, respectively. Additionally, GpsA was required for survival under nutrient stress, regulated overall reductase activity and controlled B. burgdorferi morphology in vitro. Furthermore, during in vitro nutrient stress, both glycerol and N-acetylglucosamine were bactericidal to B. burgdorferi in a GlpD-dependent manner. This study is also the first to identify a suppressor mutation in B. burgdorferi: a glpD deletion restored the wild-type phenotype to the pleiotropic gpsA mutant, including murine infectivity by needle inoculation at high doses, survival under nutrient stress, morphological changes and the metabolic imbalance of NADH and glycerol-3-phosphate. These results illustrate how basic metabolic functions that are dispensable for in vitro growth can be essential for in vivo infectivity of B. burgdorferi and may serve as attractive therapeutic targets.

Lyme disease (borreliosis) is the most common tick-borne disease in the Northern hemisphere and its prevalence is increasing. Borrelia burgdorferi, the etiological agent of Lyme disease, is an enzootic pathogen that alternates between a tick vector and vertebrate host. Humans are considered an incidental host after transmission of B. burgdorferi following the bite of an infected tick. The mechanisms by which B. burgdorferi persists in the Ixodid tick, transmits to a vertebrate host and establishes infection are not well understood. Therefore, identifying virulence factors and uncovering the pathogenic strategies in the spirochete remain important to address the public health concerns of Lyme disease. In this study, we identify an enzyme involved in three-carbon metabolism, GpsA, as a new virulence factor with an effect on persistence in ticks. GpsA and GlpD, another enzyme, constitute a bidirectional metabolic node connecting lipid biosynthesis and glycolysis, which serves as the linchpin for regulating carbon utilization for B. burgdorferi throughout its enzootic cycle. Disruption of this node causes a lethal metabolic imbalance revealing a potential therapeutic target for the treatment of Lyme disease.