Ethanolamine Signaling Promotes Salmonella Niche Recognition and Adaptation during Infection

Role of ethanolamine utilization and bacterial microcompartment formation in Listeria monocytogenes intracellular infection

Ethanolamine (EA) affects the colonization and pathogenicity of certain human bacterial pathogens in the gastrointestinal tract. However, EA can also affect the intracellular survival and replication of host cell invasive bacteria such as Listeria monocytogenes (LMO) and Salmonella enterica serovar Typhimurium (S. Typhimurium). The EA utilization (eut) genes can be categorized as regulatory, enzymatic, or structural, and previous work in LMO showed that loss of genes encoding functions for the enzymatic breakdown of EA inhibited LMO intracellular replication. In this work, we sought to further characterize the role of EA utilization during LMO infection of host cells. Unlike what was previously observed for S. Typhimurium, in LMO, an EA regulator mutant (ΔeutV) was equally deficient in intracellular replication compared to an EA metabolism mutant (ΔeutB), and this was consistent across Caco-2, RAW 264.7, and THP-1 cell lines. The structural genes encode proteins that self-assemble into bacterial microcompartments (BMCs) that encase the enzymes necessary for EA metabolism. For the first time, native EUT BMCs were fluorescently tagged, and EUT BMC formation was observed in vitro and in vivo. Interestingly, BMC formation was observed in bacteria infecting Caco-2 cells, but not the macrophage cell lines. Finally, the cellular immune response of Caco-2 cells to infection with eut mutants was examined, and it was discovered that ΔeutB and ΔeutV mutants similarly elevated the expression of inflammatory cytokines. In conclusion, EA sensing and utilization during LMO intracellular infection are important for optimal LMO replication and immune evasion but are not always concomitant with BMC formation.IMPORTANCEListeria monocytogenes (LMO) is a bacterial pathogen that can cause severe disease in immunocompromised individuals when consumed in contaminated food. It can replicate inside of mammalian cells, escaping detection by the immune system. Therefore, understanding the features of this human pathogen that contribute to its infectiousness and intracellular lifestyle is important. In this work we demonstrate that genes encoding both regulators and enzymes of EA metabolism are important for optimal growth inside mammalian cells. Moreover, the formation of specialized compartments to enable EA metabolism were visualized by tagging with a fluorescent protein and found to form when LMO infects some mammalian cell types, but not others. Interestingly, the formation of the compartments was associated with features consistent with an early stage of the intracellular infection. By characterizing bacterial metabolic pathways that contribute to survival in host environments, we hope to positively impact knowledge and facilitate new treatment strategies.

Role of Ethanolamine Utilization and Bacterial Microcompartment Formation in Listeria monocytogenes Intracellular Infection

Ethanolamine (EA) affects the colonization and pathogenicity of certain human bacterial pathogens in the gastrointestinal tract. However, EA can also affect the intracellular survival and replication of host-cell invasive bacteria such as Listeria monocytogenes (LMO) and Salmonella enterica serovar Typhimurium ( S. Typhimurium). The EA utilization ( eut) genes can be categorized as regulatory, enzymatic, or structural, and previous work in LMO showed that loss of genes encoding functions for the enzymatic breakdown of EA inhibited LMO intracellular replication. In this work, we sought to further characterize the role of EA utilization during LMO infection of host cells. Unlike what was previously observed for S. Typhimurium, in LMO, an EA regulator mutant ( ΔeutV) was equally deficient in intracellular replication compared to an EA metabolism mutant ( ΔeutB ), and this was consistent across Caco-2, RAW 264.7 and THP-1 cell lines. The structural genes encode proteins that self-assemble into bacterial microcompartments (BMCs) that encase the enzymes necessary for EA metabolism. For the first time, native EUT BMCs were fluorescently tagged, and EUT BMC formation was observed in vitro, and in vivo. Interestingly, BMC formation was observed in bacteria infecting Caco-2 cells, but not the macrophage cell lines. Finally, the cellular immune response of Caco-2 cells to infection with eut mutants was examined, and it was discovered that ΔeutB and ΔeutV mutants similarly elevated the expression of inflammatory cytokines. In conclusion, EA sensing and utilization during LMO intracellular infection are important for optimal LMO replication and immune evasion but are not always concomitant with BMC formation.

Emergence of Poultry-Associated Human Salmonella enterica Serovar Abortusovis Infections, New South Wales, Australia

Salmonella enterica serovar Abortusovis is a ovine-adapted pathogen that causes spontaneous abortion. Salmonella Abortusovis was reported in poultry in 2009 and has since been reported in human infections in New South Wales, Australia. Phylogenomic analysis revealed a clade of 51 closely related isolates from Australia originating in 2004. That clade was genetically distinct from ovine-associated isolates. The clade was widespread in New South Wales poultry production facilities but was only responsible for sporadic human infections. Some known virulence factors associated with human infections were only found in the poultry-associated clade, some of which were acquired through prophages and plasmids. Furthermore, the ovine-associated clade showed signs of genome decay, but the poultry-associated clade did not. Those genomic changes most likely led to differences in host range and disease type. Surveillance using the newly identified genetic markers will be vital for tracking Salmonella Abortusovis transmission in animals and to humans and preventing future outbreaks.

Gut-brain communication mediates the impact of dietary lipids on cognitive capacity

Cognitive impairment, as a prevalent symptom of nervous system disorders, poses one of the most challenging aspects in the management of brain diseases. Lipids present in the cell membranes of all neurons within the brain and dietary lipids can regulate the cognition and memory function. In recent years, the advancements in gut microbiome research have enabled the exploration of dietary lipids targeting the gut-brain axis as a strategy for regulating cognition. This present review provides an in-depth overview of how lipids modulate cognition via the gut-brain axis depending on metabolic, immune, neural and endocrine pathways. It also comprehensively analyzes the effects of diverse lipids on the gut microbiota and intestinal barrier function, thereby affecting the central nervous system and cognitive capacity. Moreover, comparative analysis of the positive and negative effects is presented between beneficial and detrimental lipids. The former encompass monounsaturated fatty acids, short-chain fatty acids, omega-3 polyunsaturated fatty acids, phospholipids, phytosterols, fungal sterols and bioactive lipid-soluble vitamins, as well as lipid-derived gut metabolites, whereas the latter (detrimental lipids) include medium- or long-chain fatty acids, excessive proportions of n-6 polyunsaturated fatty acids, industrial trans fatty acids, and zoosterols. To sum up, the focus of this review is on how gut-brain communication mediates the impact of dietary lipids on cognitive capacity, providing a novel theoretical foundation for promoting brain cognitive health and scientific lipid consumption patterns.

Ethanolamine enhances adhesion, promotes microcompartment formation, and modulates gene expression in Levilactobacillus brevis ATCC 14869

Ethanolamine is an abundant compound in the gastrointestinal tract and a valuable source of carbon and nitrogen for pathogenic bacteria harboring ethanolamine utilization (eut) genes. Eut-positive pathogens can consume free ethanolamine to outcompete commensal microbes, which often lack eut genes, and establish infection. Ethanolamine can also act as a host recognition signal for eut-positive pathogens to upregulate virulence genes during colonization. Therefore, reducing free ethanolamine titers may represent a novel approach to preventing infection by eut-positive pathogens. Interestingly, the commensal microorganism Levilactobacillus brevis ATCC 14869 was found to encode over 18 eut genes within its genome. This led us to hypothesize that L. brevis can compete with eut-positive pathogens by clearing free ethanolamine from the environment. Our results demonstrate that despite being unable to metabolize ethanolamine under most conditions, L. brevis ATCC 14869 responds to the compound by increasing the expression of genes encoding proteins involved in microcompartment formation and adhesion to the intestinal epithelial barrier. The improved intestinal adhesion of L. brevis in the presence of ethanolamine also enhanced the exclusion of eut-positive pathogens from adhering to intestinal epithelial cells. These findings support further studies to test whether L. brevis ATCC 14869 can counter enteric pathogens and prevent or reduce the severity of infections. Overall, the metabolic capabilities of L. brevis ATCC 14869 offer a unique opportunity to add to the armamentarium of antimicrobial therapies as well as our understanding of the mechanisms used by beneficial microbes to sense and adapt to host microenvironments.

Design and validation of a dual-fluorescence reporter system to monitor bacterial gene expression in the gut environment

Fluorescence-based reporter systems are valuable tools for studying gene expression dynamics in living cells. However, available strategies to follow gene expression in bacteria within their natural ecosystem that can be typically rich and complex are scarce. In this work, we designed a plasmid-based tool ensuring both the identification of a strain of interest in complex environments and the monitoring of gene expression through the combination of two distinct fluorescent proteins as reporter genes. The tool was validated in Escherichia coli to monitor the expression of eut genes involved in the catabolism of ethanolamine. We demonstrated that the constructed reporter strain gradually responds with a bimodal output to increasing ethanolamine concentrations during in vitro cultures. The reporter strain was next inoculated to mice, and flow cytometry was used to detect the reporter strain among the dense microbiota of intestinal samples and to analyze specifically the expression of eut genes. This novel dual-fluorescent reporter system would be helpful to evaluate transcriptional processes in bacteria within complex environments. KEY POINTS: • A reporter tool was developed to monitor bacterial gene expression in complex environments. • Ethanolamine utilization (eut) genes are expressed by commensal E. coli in the mouse gut. • Expression of eut genes follows a bimodal distribution.

Pioneer colonizers: Bacteria that alter the chicken intestinal morphology and development of the microbiota

Microbes commonly administered to chickens facilitate development of a beneficial microbiome that improves gut function, feed conversion and reduces pathogen colonization. Competitive exclusion products, derived from the cecal contents of hens and shown to reduce Salmonella colonization in chicks, possess important pioneer-colonizing bacteria needed for proper intestinal development and animal growth. We hypothesized that inoculation of these pioneer-colonizing bacteria to day of hatch chicks would enhance the development of their intestinal anatomy and microbiome. A competitive exclusion product was administered to broiler chickens, in their drinking water, at day of hatch, and its impact on intestinal morphometrics, intestinal microbiome, and production parameters, was assessed relative to a control, no treatment group. 16S rRNA gene, terminal restriction fragment length polymorphism (T-RFLP) was used to assess ileal community composition. The competitive exclusion product, administered on day of hatch, increased villus height, villus height/width ratio and goblet cell production ∼1.25-fold and expression of enterocyte sugar transporters 1.25 to 1.5-fold in chickens at 3 days of age, compared to the control group. As a next step, chicks were inoculated with a defined formulation, containing Bacteroidia and Clostridia representing pioneer-colonizing bacteria of the two major bacterial phyla present in the competitive exclusion product. The defined formulation, containing both groups of bacteria, were shown, dependent on age, to improve villus height (jejunum: 1.14 to 1.46-fold; ileum: 1.17-fold), goblet cell numbers (ileum 1.32 to 2.51-fold), and feed efficiency (1.18-fold, day 1) while decreasing Lactobacillus ileal abundance by one-third to half in birds at 16 and 42 days of age, respectively; compared to the phosphate buffered saline treatment group. Therefore, specific probiotic formulations containing pioneer colonizing species can provide benefits in intestinal development, feed efficiency and body weight gain.

<italic>Salmonella typhimurium</italic> may support cancer treatment: a review

<p indent="0mm">Antitumour treatments are evolving, including bacteria-mediated cancer therapy which is concurrently an ancient and cutting-edge approach. <italic>Salmonella typhimurium</italic> is a widely studied bacterial species that colonizes tumor tissues, showing oncolytic and immune system-regulating properties. It can be used as a delivery vector for genes and drugs, supporting conventional treatments that lack tumor-targeting abilities. This article summarizes recent evidence on the anticancer mechanisms of <italic>S</italic>. <italic>typhimurium</italic> alone and in combination with other anticancer treatments, suggesting that it may be a suitable approach to disease management. </p>.

Effect of ethanolamine utilization on the pathogenicity and metabolic profile of enterotoxigenic Escherichia coli

Bacterial pathogenicity is greatly affected by nutrient recognition and utilization in the host microenvironment. The characterization of enteral nutrients that promote intestinal pathogen virulence is helpful for developing new adjuvant therapies and inhibiting host damage. Ethanolamine (EA), as a major component of intestinal epithelial cells and bacterial membranes, is abundant in the intestine. Here, we provide the first demonstration that the critical human and porcine pathogen enterotoxigenic Escherichia coli (ETEC) can utilize EA as a nitrogen source, which affects its virulence phenotype. We found that compared with that in M9 medium (containing NH₄Cl), EA inhibited ETEC growth to a certain extent; however, the relative expression levels of virulence-related genes, such as ltA (3.0-fold), fimH (2.9-fold), CfaD (2.6-fold), gspD (3.6-fold), and qesE (1.3-fold), increased significantly with 15 mM EA as a nitrogen source (P < 0.05), and the adhesion efficiency of ETEC to Caco-2 cells increased approximately 4.2-fold. In Caco-2 cells, the relative cell viability decreased from 74.8 to 63.4%, and the transepithelial electrical resistance (TEER) cells decreased to 74.8% with intestinal EA (4 mM). In addition, the relative expression levels of proinflammatory factors, such as TNF-α (3.2-fold), INF-γ (2.9-fold), and IL-1β (1.98-fold), in ETEC-infected Caco-2 cells were significantly upregulated (P < 0.05) under EA exposure; however, the above virulence changes were not found in ΔeutR and ΔeutB ETEC. A gas chromatography-mass spectrometry (GC-MS)-based untargeted metabolomics approach was then employed to reveal EA-induced metabolic reprogramming related to ETEC virulence. The data showed that most metabolites related to carbohydrate, aspartate and glutamate metabolism, shikimic acid metabolism, and serine metabolism in ETEC exhibited a decreasing trend with increases in the EA concentration from 0 to 15 mM, but the branched-chain amino acid (BCAA) levels in ETEC increased in a dose-dependent manner under EA exposure. Our data suggest that the intestinal EA concentration can significantly affect the virulence phenotype, metabolic profile, and pathogenicity of ETEC. KEY POINTS: • ETEC growth and virulence gene expression could be regulated by ethanolamine. • The intestinal concentration of EA promoted the damaging effect of ETEC on the host epithelial barrier. • The promoting effect of EA on ETEC toxicity may be related to BCAA metabolism.

Localization and interaction studies of the Salmonella enterica ethanolamine ammonia-lyase (EutBC), its reactivase (EutA), and the EutT corrinoid adenosyltransferase

Some prokaryotes compartmentalize select metabolic capabilities. Salmonella enterica subspecies enterica serovar Typhimurium LT2 (hereafter S. Typhimurium) catabolizes ethanolamine (EA) within a proteinaceous compartment that we refer to as the ethanolamine utilization (Eut) metabolosome. EA catabolism is initiated by the adenosylcobalamin (AdoCbl)-dependent ethanolamine ammonia-lyase (EAL), which deaminates EA via an adenosyl radical mechanism to yield acetaldehyde plus ammonia. This adenosyl radical can be quenched, requiring the replacement of AdoCbl by the ATP-dependent EutA reactivase. During growth on ethanolamine, S. Typhimurium synthesizes AdoCbl from cobalamin (Cbl) using the ATP:Co(I)rrinoid adenosyltransferase (ACAT) EutT. It is known that EAL localizes to the metabolosome, however, prior to this work, it was unclear where EutA and EutT localized, and whether they interacted with EAL. Here, we provide evidence that EAL, EutA, and EutT localize to the Eut metabolosome, and that EutA interacts directly with EAL. We did not observe interactions between EutT and EAL nor between EutT and the EutA/EAL complex. However, growth phenotypes of a ΔeutT mutant strain show that EutT is critical for efficient ethanolamine catabolism. This work provides a preliminary understanding of the dynamics of AdoCbl synthesis and its uses within the Eut metabolosome.

Acinetobacter baumannii Catabolizes Ethanolamine in the Absence of a Metabolosome and Converts Cobinamide into Adenosylated Cobamides

Acinetobacter baumannii is an opportunistic pathogen typically associated with hospital-acquired infections. Our understanding of the metabolism and physiology of A. baumannii is limited. Here, we report that A. baumannii uses ethanolamine (EA) as the sole source of nitrogen and can use this aminoalcohol as a source of carbon and energy if the expression of the eutBC genes encoding ethanolamine ammonia-lyase (EAL) is increased. A strain with an ISAba1 element upstream of the eutBC genes efficiently used EA as a carbon and energy source. The A. baumannii EAL (AbEAL) enzyme supported the growth of a strain of Salmonella lacking the entire eut operon. Remarkably, the growth of the above-mentioned Salmonella strain did not require the metabolosome, the reactivase EutA enzyme, the EutE acetaldehyde dehydrogenase, or the addition of glutathione to the medium. Transmission electron micrographs showed that when Acinetobacter baumannii or Salmonella enterica subsp. enterica serovar Typhimurium strain LT2 synthesized AbEAL, the protein localized to the cell membrane. We also report that the A. baumannii genome encodes all of the enzymes needed for the assembly of the nucleotide loop of cobamides and that it uses these enzymes to synthesize different cobamides from the precursor cobinamide and several nucleobases. In the absence of exogenous nucleobases, the most abundant cobamide produced by A. baumannii was cobalamin.

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.

Streptozotocin-induced hyperglycemia alters the cecal metabolome and exacerbates antibiotic-induced dysbiosis

It is well established in the microbiome field that antibiotic (ATB) use and metabolic disease both impact the structure and function of the gut microbiome. But how host and microbial metabolism interacts with ATB susceptibility to affect the resulting dysbiosis remains poorly understood. In a streptozotocin-induced model of hyperglycemia (HG), we use a combined metagenomic, metatranscriptomic, and metabolomic approach to profile changes in microbiome taxonomic composition, transcriptional activity, and metabolite abundance both pre- and post-ATB challenge. We find that HG impacts both microbiome structure and metabolism, ultimately increasing susceptibility to amoxicillin. HG exacerbates drug-induced dysbiosis and increases both phosphotransferase system activity and energy catabolism compared to controls. Finally, HG and ATB co-treatment increases pathogen susceptibility and reduces survival in a Salmonella enterica infection model. Our data demonstrate that induced HG is sufficient to modify the cecal metabolite pool, worsen the severity of ATB dysbiosis, and decrease colonization resistance.

A Comparison of Production Performance, Egg Quality, and Cecal Microbiota in Laying Hens Receiving Graded Levels of Vitamin B12

The objective of the study was to investigate the effect of fortified diets with standard vs. high levels of vitamin B₁₂ on cecal microbiota composition, production performance, and eggshell quality of laying hens. Dietary treatments consisted of a basal diet with no supplementation of vitamin B₁₂ or supplemented with 25, 100, and 400 μg/kg vitamin B₁₂, respectively. A total of 432 laying hens were randomly assigned to four treatments with six replicates per treatment. No significant effect of dietary treatments on the production performance of hens was detected. The shell thickness of eggs from hens fed diet supplemented with 100 μg/kg of vitamin B₁₂ was higher (P < 0.01) than that of eggs from hens fed control diet or supplemented with 25 μg/kg vitamin B₁₂. The shell percentage of eggs from hens fed diet supplemented with 400 μg/kg of vitamin B₁₂ was higher (P < 0.01) than that of eggs from hens fed other treatment diets. Dietary vitamin B₁₂ did not modulate diversity of the cecal microbiota of the layers. At genus level, the cecal content from layers fed diet with supplemental level of 100 or 400 μg/kg of vitamin B₁₂ had higher (P < 0.01) abundance of Faecalibacterium and lower (P < 0.05) abundance of Acinetobacter compared with the cecal content from layers fed other two diets. The abundance of Lactobacillus in the cecal samples from layers fed 100 μg/kg of supplemental level of vitamin B₁₂ was higher (P < 0.05) than that from layers fed other three diets. The abundance of Butyricicoccus was higher (P < 0.05), while Bilophila was lower (P < 0.05) in the cecal content of layers fed 400 μg/kg of vitamin B₁₂ diet compared with those from layers fed other three diets. The results of PICRUSt analysis indicated that 10 predicted metabolic functions of the cecal microbial communities were positively correlated to dietary vitamin B₁₂ level. Overall, dietary supplementation of 100 or 400 μg/kg of vitamin B₁₂ had equivalent effects and caused the significant change in composition and metabolic functions of cecal microorganisms, which could positively impact eggshell quality, metabolism, and gut health of laying hens.

A pathogen-specific sRNA influences enterohemorrhagic Escherichia coli fitness and virulence in part by direct interaction with the transcript encoding the ethanolamine utilization regulatory factor EutR

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 relies on sRNAs to coordinate expression of metabolic and virulence factors to colonize the host. Here, we focus on the sRNA, named MavR (metabolism and virulence regulator), that is conserved among pathogenic Enterobacteriaceae. MavR is constitutively expressed under in vitro conditions that promote EHEC virulence gene expression. Using MS2-affinity purification coupled with RNA sequencing, the eutR transcript was identified as a putative target of MavR. EutR is a transcription factor that promotes expression of genes required for ethanolamine metabolism as well as virulence factors important for host colonization. MavR binds to the eutR coding sequence to protect the eutR transcript from RNase E-mediated degradation. Ultimately, MavR promotes EutR expression and in turn ethanolamine utilization and ethanolamine-dependent growth. RNAseq analyses revealed that MavR also affected expression of genes important for other metabolic pathways, motility, oxidative stress and attaching and effacing lesion formation, which contribute to EHEC colonization of the gastrointestinal tract. In support of the idea that MavR-dependent gene expression affects fitness during infection, deletion of mavR resulted in significant (∼10- to 100-fold) attenuation in colonization of the mammalian intestine. Altogether, these studies reveal an important, extensive, and robust phenotype for a bacterial sRNA in host-pathogen interactions.

Gut microbes regroup to aid defence after infection

Resident gut microbes can help to block infection, but the mechanisms involved are not fully understood. It has now been found that changes in the microbial community after infection boost the level of a molecule that combats harmful bacteria.

Genomic signatures of host adaptation in group B Salmonella enterica ST416/ST417 from harbour porpoises

A type of monophasic group B Salmonella enterica with the antigenic formula 4,12:a:- (“Fulica-like”) has been described as associated with harbour porpoises (Phocoena phocoena), most frequently recovered from lung samples. In the present study, lung tissue samples from 47 porpoises found along the Swedish coast or as bycatch in fishing nets were analysed, two of which were positive for S. enterica. Pneumonia due to the infection was considered the likely cause of death for one of the two animals. The recovered isolates were whole genome sequenced and found to belong to sequence type (ST) 416 and to be closely related to ST416/ST417 porpoise isolates from UK waters as determined by core-genome MLST. Serovars Bispebjerg, Fulica and Abortusequi were identified as distantly related to the porpoise isolates, but no close relatives from other host species were found. All ST416/417 isolates had extensive loss of function mutations in key Salmonella pathogenicity islands, but carried accessory genetic elements associated with extraintestinal infection such as iron uptake systems. Gene ontology and pathway analysis revealed reduced secondary metabolic capabilities and loss of function in terms of signalling and response to environmental cues, consistent with adaptation for the extraintestinal niche. A classification system based on machine learning identified ST416/417 as more invasive than classical gastrointestinal serovars. Genome analysis results are thus consistent with ST416/417 as a host-adapted and extraintestinal clonal population of S. enterica, which while found in porpoises without associated pathology can also cause severe opportunistic infections.

Microbes exploit death-induced nutrient release by gut epithelial cells

Regulated cell death is an integral part of life, and has broad effects on organism development and homeostasis¹. Malfunctions within the regulated cell death process, including the clearance of dying cells, can manifest in diverse pathologies throughout various tissues including the gastrointestinal tract². A long appreciated, yet elusively defined relationship exists between cell death and gastrointestinal pathologies with an underlying microbial component^3-6, but the direct effect of dying mammalian cells on bacterial growth is unclear. Here we advance a concept that several Enterobacteriaceae, including patient-derived clinical isolates, have an efficient growth strategy to exploit soluble factors that are released from dying gut epithelial cells. Mammalian nutrients released after caspase-3/7-dependent apoptosis boosts the growth of multiple Enterobacteriaceae and is observed using primary mouse colonic tissue, mouse and human cell lines, several apoptotic triggers, and in conventional as well as germ-free mice in vivo. The mammalian cell death nutrients induce a core transcriptional response in pathogenic Salmonella, and we identify the pyruvate formate-lyase-encoding pflB gene as a key driver of bacterial colonization in three contexts: a foodborne infection model, a TNF- and A20-dependent cell death model, and a chemotherapy-induced mucositis model. These findings introduce a new layer to the complex host-pathogen interaction, in which death-induced nutrient release acts as a source of fuel for intestinal bacteria, with implications for gut inflammation and cytotoxic chemotherapy treatment.

Integration of Salmonella into Combination Cancer Therapy

Simple Summary

Despite significant advances in the development of new treatments, cancer continues to be a major public health concern due to the high mortality associated with the disease. The introduction of immunotherapy as a new modality for cancer treatment has led to unprecedented clinical responses, even in terminal cancer patients. However, for reasons that remain largely unknown, the percentage of patients who respond to this treatment remains rather modest. In the present article, we highlight the potential of using attenuated Salmonella strains in cancer treatment, particularly as a means to enhance therapeutic efficacy of other cancer treatments, including immunotherapy, chemotherapy, and radiotherapy. The challenges associated with the clinical application of Salmonella in cancer therapy are discussed. An increased understanding of the potential of Salmonella bacteria in combination cancer therapy may usher in a major breakthrough in its clinical application, resulting in more favorable and durable outcomes.

Abstract

Current modalities of cancer treatment have limitations related to poor target selectivity, resistance to treatment, and low response rates in patients. Accumulating evidence over the past few decades has demonstrated the capacity of several strains of bacteria to exert anti-tumor activities. Salmonella is the most extensively studied entity in bacterial-mediated cancer therapy, and has a good potential to induce direct tumor cell killing and manipulate the immune components of the tumor microenvironment in favor of tumor inhibition. In addition, Salmonella possesses some advantages over other approaches of cancer therapy, including high tumor specificity, deep tissue penetration, and engineering plasticity. These aspects underscore the potential of utilizing Salmonella in combination with other cancer therapeutics to improve treatment effectiveness. Herein, we describe the advantages that make Salmonella a good candidate for combination cancer therapy and summarize the findings of representative studies that aimed to investigate the therapeutic outcome of combination therapies involving Salmonella. We also highlight issues associated with their application in clinical use.

Bacterial Microcompartments Coupled with Extracellular Electron Transfer Drive the Anaerobic Utilization of Ethanolamine in Listeria monocytogenes

Ethanolamine (EA) is a valuable microbial carbon and nitrogen source derived from cell membranes. EA catabolism is suggested to occur in a cellular metabolic subsystem called a bacterial microcompartment (BMC), and the activation of EA utilization (eut) genes is linked to bacterial pathogenesis. Despite reports showing that the activation of eut is regulated by a vitamin B₁₂-binding riboswitch and that upregulation of eut genes occurs in mice, it remains unknown whether EA catabolism is BMC dependent in Listeria monocytogenes Here, we provide evidence for BMC-dependent anaerobic EA utilization via metabolic analysis, proteomics, and electron microscopy. First, we show vitamin B₁₂-induced activation of the eut operon in L. monocytogenes coupled to the utilization of EA, thereby enabling growth. Next, we demonstrate BMC formation connected with EA catabolism with the production of acetate and ethanol in a molar ratio of 2:1. Flux via the ATP-generating acetate branch causes an apparent redox imbalance due to the reduced regeneration of NAD⁺ in the ethanol branch resulting in a surplus of NADH. We hypothesize that the redox imbalance is compensated by linking eut BMCs to anaerobic flavin-based extracellular electron transfer (EET). Using L. monocytogenes wild-type, BMC mutant, and EET mutant strains, we demonstrate an interaction between BMCs and EET and provide evidence for a role of Fe³⁺ as an electron acceptor. Taken together, our results suggest an important role of BMC-dependent EA catabolism in L. monocytogenes growth in anaerobic environments like the human gastrointestinal tract, with a crucial role for the flavin-based EET system in redox balancing.IMPORTANCE Listeria monocytogenes is a foodborne pathogen causing severe illness, and as such, it is crucial to understand the molecular mechanisms contributing to pathogenicity. One carbon source that allows L. monocytogenes to grow in humans is ethanolamine (EA), which is derived from phospholipids present in eukaryotic cell membranes. It is hypothesized that EA utilization occurs in bacterial microcompartments (BMCs), self-assembling subcellular proteinaceous structures and analogs of eukaryotic organelles. Here, we demonstrate that BMC-driven utilization of EA in L. monocytogenes results in increased energy production essential for anaerobic growth. However, exploiting BMCs and the encapsulated metabolic pathways also requires the balancing of oxidative and reductive pathways. We now provide evidence that L. monocytogenes copes with this by linking BMC activity to flavin-based extracellular electron transfer (EET) using iron as an electron acceptor. Our results shed new light on an important molecular mechanism that enables L. monocytogenes to grow using host-derived phospholipid degradation products.

Perspectives on Oncolytic Salmonella in Cancer Immunotherapy-A Promising Strategy

Since the first reported spontaneous regression of tumors in patients with streptococcus infection, cancer biological therapy was born and it evolved into today’s immunotherapy over the last century. Although the original strategy was unable to impart maximal therapeutic benefit at the beginning, it laid the foundations for the development of immune checkpoint blockade and CAR-T which are currently used for cancer treatment in the clinics. However, clinical applications have shown that current cancer immunotherapy can cause a series of adverse reactions and are captious for patients with preexisting autoimmune disorders. Salmonellae was first reported to exert antitumor effect in 1935. Until now, numerous studies have proved its potency as an antitumor agent in the near future. In this review, we summarize the currently available data on the antitumor effects of Salmonella, and discussed a possibility of integrating Salmonella into cancer immunotherapy to overcome current obstacles.

The Interface of Vibrio cholerae and the Gut Microbiome

The bacterium Vibrio cholerae is the etiologic agent of the severe human diarrheal disease cholera. The gut microbiome, or the native community of microorganisms found in the human gastrointestinal tract, is increasingly being recognized as a factor in driving susceptibility to infection, in vivo fitness, and host interactions of this pathogen. Here, we review a subset of the emerging studies in how gut microbiome structure and microbial function are able to drive V. cholerae virulence gene regulation, metabolism, and modulate host immune responses to cholera infection and vaccination. Improved mechanistic understanding of commensal-pathogen interactions offers new perspectives in the design of prophylactic and therapeutic approaches for cholera control.

The Cobalamin-Dependent Gene Cluster of Listeria monocytogenes: Implications for Virulence, Stress Response, and Food Safety

Several genes of the eut, pdu, and cob/cbi operons are responsible for the metabolism of ethanolamine (EA) and 1,2-propanediol (PD) and are essential during the pathogenic lifecycles of various enteric pathogens. Studies concerning EA and PD metabolism have primarily focused on bacterial genera from the family Enterobacteriaceae, especially the genus Salmonella. Listeria monocytogenes is a member of the Firmicutes phylum and is the causative agent of the rare but highly fatal foodborne disease listeriosis. The eut, pdu, and cob/cbi operons are organized as a single large locus collectively referred to as the cobalamin-dependent gene cluster (CDGC). The CDGC is well conserved in L. monocytogenes; however, functional characterization of the genes in this cluster and how they may contribute to Listeria virulence and stress tolerance in food production environments is highly limited. Previous work suggests that the degradation pathway of PD is essential for L. monocytogenes establishment in the gastrointestinal tract. In contrast, EA metabolism may be more important during intracellular replication. Other studies indicate that the CDGC is utilized when L. monocytogenes is exposed to food and food production relevant stress conditions. Perhaps most noteworthy, L. monocytogenes exhibits attenuated growth at cold temperatures when a key EA utilization pathway gene was deleted. This review aims to summarize the current knowledge of these pathways in L. monocytogenes and their significance in virulence and stress tolerance, especially considering recent developments.

The Ethanolamine-Sensing Transcription Factor EutR Promotes Virulence and Transmission during Citrobacter rodentium Intestinal Infection

Enteric pathogens exploit chemical and nutrient signaling to gauge their location within a host and control expression of traits important for infection. Ethanolamine-containing molecules are essential in host physiology and play important roles in intestinal processes. The transcription factor EutR is conserved in the Enterobacteriaceae and is required for ethanolamine sensing and metabolism. In enterohemorrhagic Escherichia coli (EHEC) O157:H7, EutR responds to ethanolamine to activate expression of traits required for host colonization and disease; however, the importance of EutR to EHEC intestinal infection has not been examined. Because EHEC does not naturally colonize or cause disease in mice, we employed the natural murine pathogen Citrobacter rodentium as a model of EHEC virulence to investigate the importance of EutR in vivo EHEC and C. rodentium possess the locus of enterocyte effacement (LEE), which is the canonical virulence trait of attaching and effacing pathogens. Our findings demonstrate that ethanolamine sensing and EutR-dependent regulation of the LEE are conserved in C. rodentium Moreover, during infection, EutR is required for maximal LEE expression, colonization, and transmission efficiency. These findings reveal that EutR not only is important for persistence during the primary host infection cycle but also is required for maintenance in a host population.

Vitamin B12 Deficiency Alters the Gut Microbiota in a Murine Model of Colitis

Purpose: Inflammatory bowel disease (IBD) refers to a spectrum of autoimmune diseases, which result in chronic intestinal inflammation. Previous findings suggest a role for diet, nutrition and dysbiosis of the gut microbiota in both the development and progression of the condition. Vitamin B12 is a key cofactor of methionine synthase and is produced solely by microbes. Previous work links increased levels of homocysteine, a substrate of methionine synthase, MetH, to IBD indicating a potential role for vitamin B12 deficiency in intestinal injury and inflammation. This study assessed the role of vitamin B12 in shaping the gut microbiota and determining responses to intestinal injury using a reproducible murine model of colitis. Methods: The effects of vitamin B12 supplementation and deficiency were assessed in vivo; 3-week-old post-weanling C57Bl/6 mice were divided into three dietary treatment groups: (1) sufficient vitamin B12 (50 mg/Kg), (2) deficient vitamin B12 (0 mg/Kg) and (3) supplemented vitamin B12 (200 mg/Kg) for a period of 4 weeks. Intestinal injury was induced with 2% dextran sodium sulphate (DSS) via drinking water for 5 days. The impact of varying levels of dietary vitamin B12 on gut microbiota composition was assessed using 16S rRNA gene sequencing from fecal samples collected at day 0 and day 28 of the dietary intervention, and 7 days following induction of colitis on day 38, when blood and colonic tissues were also collected. Results: No significant alterations were found in the gut microbiota composition of disease-free animals in response to dietary interventions. By contrast, after DSS-induced colitis, >30 genera were significantly altered in vitamin B12 deficient mice. Altered B12 levels produced no significant effect on composite disease-activity scores; however, administration of a B12 deficient diet resulted in reduced DSS-induced epithelial tissue damage. Conclusions: Vitamin B12 supplementation does not alter the gut microbiota composition under healthy conditions, but does contribute to differential microbial responses and intestinal dysbiosis following the induction of experimental colitis.

Parallel quorum-sensing system in Vibrio cholerae prevents signal interference inside the host

Many bacteria use quorum sensing (QS) to regulate virulence factor production in response to changes in population density. QS is mediated through the production, secretion, and detection of signaling molecules called autoinducers (AIs) to modulate population-wide behavioral changes. Four histidine kinases, LuxPQ, CqsS, CqsR and VpsS, have been identified in Vibrio cholerae as QS receptors to activate virulence gene expression at low cell density. Detection of AIs by these receptors leads to virulence gene repression at high cell density. The redundancy among these receptors is puzzling since any one of the four receptors is sufficient to support colonization of V. cholerae in the host small intestine. It is believed that one of the functions of such circuit architecture is to prevent interference on any single QS receptor. However, it is unclear what natural molecules can interfere V. cholerae QS and in what environment interference is detrimental. We show here mutants expressing only CqsR without the other three QS receptors are defective in colonizing the host large intestine. We identified ethanolamine, a common intestinal metabolite that can function as a chemical source of QS interference. Ethanolamine specifically interacts with the ligand-binding CACHE domain of CqsR and induces a premature QS response in V. cholerae mutants expressing only CqsR without the other three QS receptors. The effect of ethanolamine on QS gene expression and host colonization is abolished by mutations that disrupt CqsR signal sensing. V. cholerae defective in producing ethanolamine is still proficient in QS, therefore, ethanolamine functions only as an external cue for CqsR. Our findings suggest the inhibitory effect of ethanolamine on CqsR could be a possible source of QS interference but is masked by the presence of the other parallel QS pathways, allowing V. cholerae to robustly colonize the host.

Many pathogens use quorum sensing (QS) to regulate virulence gene expression for their survival and adaptation inside hosts. QS depends on the production and detection of chemical signals called autoinducers made endogenously by the bacteria. However, chemicals present in the surrounding environment could potentially lead to quorum signal interference, resulting in mis-regulation of virulence factor production and preventing effective host colonization. We show here ethanolamine, a metabolite commonly found inside the mammalian intestine, modulates the activity of one of the QS receptors in Vibrio cholerae, the etiological agent of the disease cholera. Despite the abundance of this common metabolite inside the host, by integrating multiple parallel signal inputs into its QS system, V. cholerae has evolved to maintain QS fidelity and avoids signal interference to allow robust colonization of the host.

Role of Dietary Nutrients in the Modulation of Gut Microbiota: A Narrative Review

Understanding how dietary nutrients modulate the gut microbiome is of great interest for the development of food products and eating patterns for combatting the global burden of non-communicable diseases. In this narrative review we assess scientific studies published from 2005 to 2019 that evaluated the effect of micro- and macro-nutrients on the composition of the gut microbiome using in vitro and in vivo models, and human clinical trials. The clinical evidence for micronutrients is less clear and generally lacking. However, preclinical evidence suggests that red wine- and tea-derived polyphenols and vitamin D can modulate potentially beneficial bacteria. Current research shows consistent clinical evidence that dietary fibers, including arabinoxylans, galacto-oligosaccharides, inulin, and oligofructose, promote a range of beneficial bacteria and suppress potentially detrimental species. The preclinical evidence suggests that both the quantity and type of fat modulate both beneficial and potentially detrimental microbes, as well as the Firmicutes/Bacteroides ratio in the gut. Clinical and preclinical studies suggest that the type and amount of proteins in the diet has substantial and differential effects on the gut microbiota. Further clinical investigation of the effect of micronutrients and macronutrients on the microbiome and metabolome is warranted, along with understanding how this influences host health.

Taming the Beast: Interplay between Gut Small Molecules and Enteric Pathogens

The overuse of antibiotics has led to the evolution of drug-resistant bacteria that are becoming increasingly dangerous to human health. According to the Centers for Disease Control and Prevention, antibiotic-resistant bacteria cause at least 2 million illnesses and 23,000 deaths in the United States annually. Traditionally, antibiotics are bactericidal or bacteriostatic agents that place selective pressure on bacteria, leading to the expansion of antibiotic-resistant strains. In addition, antibiotics that are effective against some pathogens can also exacerbate their pathogenesis and may lead to severe progression of the disease. Therefore, alternative strategies are needed to treat antibiotic-resistant bacterial infections. One novel approach is to target bacterial virulence to prevent or limit pathogen colonization, while also minimizing tissue damage and disease comorbidities in the host. This review focuses on the interactions between enteric pathogens and naturally occurring small molecules in the human gut as potential therapeutic targets for antivirulence strategies. Individual small molecules in the intestines modulate enteric pathogen virulence and subsequent intestinal fitness and colonization. Targeted interruption of pathogen sensing of these small molecules could therefore attenuate their virulence. This review highlights the paths of discovery for new classes of antimicrobials that could potentially mitigate the urgent problem of antibiotic resistance.

Complex Bacterial Consortia Reprogram the Colitogenic Activity of Enterococcus faecalis in a Gnotobiotic Mouse Model of Chronic, Immune-Mediated Colitis

Inflammatory bowel diseases (IBD) are associated with compositional and functional changes of the intestinal microbiota, but specific contributions of individual bacteria to chronic intestinal inflammation remain unclear. Enterococcus faecalis is a resident member of the human intestinal core microbiota that has been linked to the pathogenesis of IBD and induces chronic colitis in susceptible monoassociated IL-10-deficient (IL-10^−/−) mice. In this study, we characterized the colitogenic activity of E. faecalis as part of a simplified human microbial consortium based on seven enteric bacterial strains (SIHUMI). RNA sequencing analysis of E. faecalis isolated from monoassociated wild type and IL-10^−/− mice identified 408 genes including 14 genes of the ethanolamine utilization (eut) locus that were significantly up-regulated in response to inflammation. Despite considerable up-regulation of eut genes, deletion of ethanolamine utilization (ΔeutVW) had no impact on E. faecalis colitogenic activity in monoassociated IL-10^−/− mice. However, replacement of the E. faecalis wild type bacteria by a ΔeutVW mutant in SIHUMI-colonized IL-10^−/− mice resulted in exacerbated colitis, suggesting protective functions of E. faecalis ethanolamine utilization in complex bacterial communities. To better understand E. faecalis gene response in the presence of other microbes, we purified wild type E. faecalis cells from the colon content of SIHUMI-colonized wild type and IL-10^−/− mice using immuno-magnetic separation and performed RNA sequencing. Transcriptional profiling revealed that the bacterial environment reprograms E. faecalis gene expression in response to inflammation, with the majority of differentially expressed genes not being shared between monocolonized and SIHUMI conditions. While in E. faecalis monoassociation a general bacterial stress response could be observed, expression of E. faecalis genes in SIHUMI-colonized mice was characterized by up-regulation of genes involved in growth and replication. Interestingly, in mice colonized with SIHUMI lacking E. faecalis enhanced inflammation was observed in comparison to SIHUMI-colonized mice, supporting the hypothesis that E. faecalis ethanolamine metabolism protects against colitis in complex consortia. In conclusion, this study demonstrates that complex bacterial consortia interactions reprogram the gene expression profile and colitogenic activity of the opportunistic pathogen E. faecalis toward a protective function.

Salmonella-Based Targeted Cancer Therapy: Updates on A Promising and Innovative Tumor Immunotherapeutic Strategy

Presently, cancer is one of the leading causes of death in the world, primarily due to tumor heterogeneity associated with high-grade malignancy. Tumor heterogeneity poses a tremendous challenge, especially with the emergence of resistance not only to chemo- and radiation- therapies, but also to immunotherapy using monoclonal antibodies. The use of Salmonella, as a highly selective and penetrative antitumor agent, has shown convincing results, thus meriting further investigation. In this review, the mechanisms used by Salmonella in combating cancer are carefully explained. In essence, Salmonella overcomes the suppressive nature of the tumor microenvironment and coaxes the activation of tumor-specific immune cells to induce cell death by apoptosis and autophagy. Furthermore, Salmonella treatment suppresses tumor aggressive behavior via inhibition of angiogenesis and delay of metastatic activity. Thus, harnessing the natural potential of Salmonella in eliminating tumors will provide an avenue for the development of a promising micro-based therapeutic agent that could be further enhanced to address a wide range of tumor types.

Inflammation associated ethanolamine facilitates infection by Crohn's disease-linked adherent-invasive Escherichia coli

BACKGROUND

The predominance of specific bacteria such as adherent-invasive Escherichia coli (AIEC) within the Crohn's disease (CD) intestine remains poorly understood with little evidence uncovered to support a selective pressure underlying their presence. Intestinal ethanolamine is however readily accessible during periods of intestinal inflammation, and enables pathogens to outcompete the host microbiota under such circumstances.

METHODS

Quantitative RT-PCR (qRT-PCR) to determine expression of genes central to ethanolamine metabolism; transmission electron microscopy to detect presence of bacterial microcompartments (MCPs); in vitro infections of both murine and human macrophage cell lines examining intracellular replication of the AIEC-type strain LF82 and clinical E. coli isolates in the presence of ethanolamine; determination of E. coli ethanolamine utilization (eut) operon transcription in faecal samples from healthy patients, patients with active CD and the same patients in remission following treatment.

RESULTS

Growth on the intestinal short chain fatty acid propionic acid (PA) stimulates significantly increased transcription of the eut operon (fold change relative to glucose: >16.9; p-value <.01). Additionally ethanolamine was accessible to intra-macrophage AIEC and stimulated significant increases in growth intracellularly when it was added extracellularly at concentrations comparable to those in the human intestine. Finally, qRT-PCR indicated that expression of the E. coli eut operon was increased in children with active CD compared to healthy controls (fold change increase: >4.72; P < .02). After clinical remission post-exclusive enteral nutrition treatment, the same CD patients exhibited significantly reduced eut expression (Pre vs Post fold change decrease: >15.64; P < .01).

INTERPRETATION

Our data indicates a role for ethanolamine metabolism in selecting for AIEC that are consistently overrepresented in the CD intestine. The increased E. coli metabolism of ethanolamine seen in the intestine during active CD, and its decrease during remission, indicates ethanolamine use may be a key factor in shaping the intestinal microbiome in CD patients, particularly during times of inflammation. FUND: This work was funded by Biotechnology and Biological Sciences Research Council (BBSRC) grants BB/K008005/1 & BB/P003281/1 to DMW; by a Tenovus Scotland grant to MJO; by Glasgow Children's Hospital Charity, Nestle Health Sciences, Engineering and Physical Sciences Research Council (EPSRC) and Catherine McEwan Foundation grants awarded to KG; and by a Natural Environment Research Council (NERC) fellowship (NE/L011956/1) to UZI. The IBD team at the Royal Hospital for Children, Glasgow are supported by the Catherine McEwan Foundation and Yorkhill IBD fund. RKR and RH are supported by NHS Research Scotland Senior fellowship awards.

Nutritional Intervention for the Intestinal Development and Health of Weaned Pigs

Weaning imposes simultaneous stress, resulting in reduced feed intake, and growth rate, and increased morbidity and mortality of weaned pigs. Weaning impairs the intestinal integrity, disturbs digestive and absorptive capacity, and increases the intestinal oxidative stress, and susceptibility of diseases in piglets. The improvement of intestinal development and health is critically important for enhancing nutrient digestibility capacity and disease resistance of weaned pigs, therefore, increasing their survival rate at this most vulnerable stage, and overall productive performance during later stages. A healthy gut may include but not limited several important features: a healthy proliferation of intestinal epithelial cells, an integrated gut barrier function, a preferable or balanced gut microbiota, and a well-developed intestinal mucosa immunity. Burgeoning evidence suggested nutritional intervention are one of promising measures to enhance intestinal health of weaned pigs, although the exact protective mechanisms may vary and are still not completely understood. Previous research indicated that functional amino acids, such as arginine, cysteine, glutamine, or glutamate, may enhance intestinal mucosa immunity (i.e., increased sIgA secretion), reduce oxidative damage, stimulate proliferation of enterocytes, and enhance gut barrier function (i.e., enhanced expression of tight junction protein) of weaned pigs. A number of feed additives are marketed to assist in boosting intestinal immunity and regulating gut microbiota, therefore, reducing the negative impacts of weaning, and other environmental challenges on piglets. The promising results have been demonstrated in antimicrobial peptides, clays, direct-fed microbials, micro-minerals, milk components, oligosaccharides, organic acids, phytochemicals, and many other feed additives. This review summarizes our current understanding of nutritional intervention on intestinal health and development of weaned pigs and the importance of mechanistic studies focusing on this research area.

Host-associated niche metabolism controls enteric infection through fine-tuning the regulation of type 3 secretion

Niche-adaptation of a bacterial pathogen hinges on the ability to recognize the complexity of signals from the environment and integrate that information with the regulation of genes critical for infection. Here we report the transcriptome of the attaching and effacing pathogen Citrobacter rodentium during infection of its natural murine host. Pathogen gene expression in vivo was heavily biased towards the virulence factor repertoire and was found to be co-ordinated uniquely in response to the host. Concordantly, we identified the host-specific induction of a metabolic pathway that overlapped with the regulation of virulence. The essential type 3 secretion system and an associated suite of distinct effectors were found to be modulated co-ordinately through a unique mechanism involving metabolism of microbiota-derived 1,2-propanediol, which dictated the ability to colonize the host effectively. This study provides novel insights into how host-specific metabolic adaptation acts as a cue to fine-tune virulence.

Infection of mice with Citrobacter rodentium is a common model of infection with attaching-and-effacing pathogens. Here, Connolly et al. analyse the transcriptome of C. rodentium during mouse infection, showing host-induced coordinated upregulation of virulence factors and 1,2-propanediol metabolism.

Ethanolamine Influences Human Commensal Escherichia coli Growth, Gene Expression, and Competition with Enterohemorrhagic E. coli O157:H7

The microbiota protects the host from invading pathogens by limiting access to nutrients. In turn, bacterial pathogens selectively exploit metabolites not readily used by the microbiota to establish infection. Ethanolamine has been linked to pathogenesis of diverse pathogens by serving as a noncompetitive metabolite that enhances pathogen growth as well as a signal that modulates virulence. Although ethanolamine is abundant in the gastrointestinal tract, the prevailing idea is that commensal bacteria do not utilize EA, and thus, EA utilization has been particularly associated with pathogenesis. Here, we provide evidence that two human commensal Escherichia coli isolates readily utilize ethanolamine to enhance growth, modulate gene expression, and outgrow the pathogen enterohemorrhagic E. coli. These data indicate a more complex role for ethanolamine in host-microbiota-pathogen interactions.

A core principle of bacterial pathogenesis is that pathogens preferentially utilize metabolites that commensal bacteria do not in order to sidestep nutritional competition. The metabolite ethanolamine (EA) is well recognized to play a central role in host adaptation for diverse pathogens. EA promotes growth and influences virulence during host infection. Although genes encoding EA utilization have been identified in diverse bacteria (nonpathogenic and pathogenic), a prevailing idea is that commensal bacteria do not utilize EA to enhance growth, and thus, EA is a noncompetitive metabolite for pathogens. Here, we show that EA augments growth of two human commensal strains of Escherichia coli. Significantly, these commensal strains grow more rapidly than, and even outcompete, the pathogen enterohemorrhagic E. coli O157:H7 specifically when EA is provided as the sole nitrogen source. Moreover, EA-dependent signaling is similarly conserved in the human commensal E. coli strain HS and influences expression of adhesins. These findings suggest a more extensive role for EA utilization in bacterial physiology and host-microbiota-pathogen interactions than previously appreciated.

Enterotoxigenic E. coli virulence gene regulation in human infections

Enterotoxigenic Escherichia coli (ETEC) is a global diarrheal pathogen that utilizes adhesins and secreted enterotoxins to cause disease in mammalian hosts. Decades of research on virulence factor regulation in ETEC has revealed a variety of environmental factors that influence gene expression, including bile, pH, bicarbonate, osmolarity, and glucose. However, other hallmarks of the intestinal tract, such as low oxygen availability, have not been examined. Further, determining how ETEC integrates these signals in the complex host environment is challenging. To address this, we characterized ETEC's response to the human host using samples from a controlled human infection model. We found ETEC senses environmental oxygen to globally influence virulence factor expression via the oxygen-sensitive transcriptional regulator fumarate and nitrate reduction (FNR) regulator. In vitro anaerobic growth replicates the in vivo virulence factor expression profile, and deletion of fnr in ETEC strain H10407 results in a significant increase in expression of all classical virulence factors, including the colonization factor antigen I (CFA/I) adhesin operon and both heat-stable and heat-labile enterotoxins. These data depict a model of ETEC infection where FNR activity can globally influence virulence gene expression, and therefore proximity to the oxygenated zone bordering intestinal epithelial cells likely influences ETEC virulence gene expression in vivo. Outside of the host, ETEC biofilms are associated with seasonal ETEC epidemics, and we find FNR is a regulator of biofilm production. Together these data suggest FNR-dependent oxygen sensing in ETEC has implications for human infection inside and outside of the host.

Cancer Immunotherapy: Priming the Host Immune Response with Live Attenuated Salmonella enterica

In recent years, cancer immunotherapy has undergone great advances because of our understanding of the immune response and the mechanisms through which tumor cells evade it. A century after the first immunotherapy attempt based on bacterial products described by William Coley, the use of live attenuated bacterial vectors has become a promising alternative in the fight against cancer. This review describes the role of live attenuated Salmonella enterica as an oncolytic and immunotherapeutic agent, due to its high affinity for tumor tissue and its ability to activate innate and adaptive antitumor immune response. Furthermore, its potential use as delivery system of tumor antigens and immunomodulatory molecules that induce tumor regression is also reviewed.

Ethanolamine enhances intestinal functions by altering gut microbiome and mucosal anti-stress capacity in weaned rats

Ethanolamine (Etn) contained in milk is the base constituent of phosphatidylethanolamine and is required for the proliferation of intestinal epithelial cells and bacteria, which is important for maintenance of the gut microbiome and intestinal development. The present study investigated the effect of Etn on intestinal function and microbiome using 21-d-old Sprague-Dawley rats treated with 0, 250, 500 and 1000 μm Etn in drinking water for 2 weeks immediately after weaning. Growth performance, intestinal morphology, antioxidant capacity and mucosal immunity, as well as gut microbiota community composition, were evaluated. Metagenomic prediction and metabolic phenotype analysis based on 16S RNA sequencing were also carried out to assess changes in metabolic functions. We found that weaned rats administered 500 μm Etn enhanced mucosal antioxidant capacity, as evidenced by higher superoxide dismutase and glutathione peroxidase levels in the jejunum (P<0·05) compared with those in the control group. Predominant microbes including Bacteroidetes, Proteobacteria, Elusimicrobia and Tenericutes were altered by different levels of Etn compared with the control group. An Etn concentration of 500 µm shifted colonic microbial metabolic functions that are in favour of lipid- and sugar-related metabolism and biosynthesis. Etn also altered the metabolic phenotypes such as anaerobic microbial counts, and oxidative stress tolerance at over 250 µm. This is the first report for a role of Etn in modifying gut microbiota and intestinal functions. Our findings highlighted the important role of Etn in shaping gut microbial community and promotes intestinal functions, which may provide a better insight of breast-feeding to infant's gut health.

Loss of Ethanolamine Utilization in Enterococcus faecalis Increases Gastrointestinal Tract Colonization

Enterococcus faecalis is paradoxically a dangerous nosocomial pathogen and a normal constituent of the human gut microbiome, an environment rich in ethanolamine. E. faecalis carries the eut (ethanolamine utilization) genes, which enable the catabolism of ethanolamine (EA) as a valuable source of carbon and/or nitrogen. EA catabolism was previously shown to contribute to the colonization and growth of enteric pathogens, such as Salmonella enterica serovar Typhimurium and enterohemorrhagic Escherichia coli (EHEC), in the gut environment. We tested the ability of eut mutants of E. faecalis to colonize the gut using a murine model of gastrointestinal (GI) tract competition and report the surprising observation that these mutants outcompete the wild-type strain.

Some bacteria that are normal, harmless colonizers of the human body can cause disease in immunocompromised patients, particularly those that have been heavily treated with antibiotics. Therefore, it is important to understand the factors that promote or negate these organisms’ ability to colonize. Previously, ethanolamine, found in high concentrations in the GI tract, was shown to promote the colonization and growth of bacteria associated with food poisoning. Here, we report the surprising, opposite effect of ethanolamine utilization on the commensal colonizer E. faecalis, namely, that loss of this metabolic capacity made it a better colonizer.

The Ethanolamine Permease EutH Promotes Vacuole Adaptation of Salmonella enterica and Listeria monocytogenes during Macrophage Infection

Ethanolamine is a ubiquitous and essential molecule within a host. Significantly, bacterial pathogens exploit ethanolamine during infection to promote growth and regulate virulence. The ethanolamine permease EutH is dispensable for growth in vitro under standard conditions, whereas EutH is required for ethanolamine utilization at low pH. These findings suggested a model in which EutH facilitates diffusion of ethanolamine into the bacterial cell in acidic environments. To date, the ecological significance of this model has not been thoroughly investigated, and the importance of EutH to bacterial growth under physiologically relevant conditions is not known. During infection, immune cells internalize invading bacteria within an acidic, nutrient-depleted vacuole called the phagosome. Here, we investigated the hypothesis that EutH promotes bacterial survival following phagocytosis. Our findings indicate that EutH is important for survival and replication of the facultative intracellular pathogens Salmonella enterica serovar Typhimurium and Listeria monocytogenes during prolonged or transient exposure to the phagosome, respectively. Furthermore, in agreement with EutH being important in the acidic environment, neutralization of the vacuole abolished the requirement for EutH. Significantly, consistent with a role for EutH in promoting intramacrophage survival, EutH was not required during S Typhimurium local intestinal infection but specifically conferred an advantage upon dissemination to peripheral organs. These findings reveal a physiologically relevant and conserved role for EutH in spatiotemporal niche adaptation during infection.

Ethanolamine is a valuable nutrient source that impacts Clostridium difficile pathogenesis

Clostridium (Clostridioides) difficile is a gastrointestinal pathogen that colonizes the intestinal tract of mammals and can cause severe diarrheal disease. Although C. difficile growth is confined to the intestinal tract, our understanding of the specific metabolites and host factors that are important for the growth of the bacterium is limited. In other enteric pathogens, the membrane-derived metabolite, ethanolamine (EA), is utilized as a nutrient source and can function as a signal to initiate the production of virulence factors. In this study, we investigated the effects of ethanolamine and the role of the predicted ethanolamine gene cluster (CD1907-CD1925) on C. difficile growth. Using targeted mutagenesis, we disrupted genes within the eut cluster and assessed their roles in ethanolamine utilization, and the impact of eut disruption on the outcome of infection in a hamster model of disease. Our results indicate that the eut gene cluster is required for the growth of C. difficile on ethanolamine as a primary nutrient source. Further, the inability to utilize ethanolamine resulted in greater virulence and a shorter time to morbidity in the animal model. Overall, these data suggest that ethanolamine is an important nutrient source within the host and that, in contrast to other intestinal pathogens, the metabolism of ethanolamine by C. difficile can delay the onset of disease.

Role of Ethanolamine Utilization Genes in Host Colonization during Urinary Tract Infection

Urinary tract infection (UTI) is the second most common infection in humans, making it a global health priority. Nearly half of all women will experience a symptomatic UTI, with uropathogenic Escherichia coli (UPEC) being the major causative agent of the infection. Although there has been extensive research on UPEC virulence determinants, the importance of host-specific metabolism remains understudied. We report here that UPEC upregulates the expression of ethanolamine utilization genes during uncomplicated UTIs in humans. We further show that UPEC ethanolamine metabolism is required for effective bladder colonization in the mouse model of ascending UTI and is dispensable for bladder colonization in an immunocompromised mouse model of UTI. We demonstrate that although ethanolamine metabolism mutants do not show increased susceptibility to antimicrobial responses of neutrophils, this metabolic pathway is important for surviving the innate immune system during UTI. This study reveals a novel aspect of UPEC metabolism in the host and provides evidence for an underappreciated link between bacterial metabolism and the host immune response.

Ethanolamine Utilization in Bacteria

Ethanolamine (EA) is a valuable source of carbon and/or nitrogen for bacteria capable of its catabolism. Because it is derived from the membrane phospholipid phosphatidylethanolamine, it is particularly prevalent in the gastrointestinal tract, which is membrane rich due to turnover of the intestinal epithelium and the resident microbiota. Intriguingly, many gut pathogens carry the eut (ethanolamine utilization) genes. EA utilization has been studied for about 50 years, with most of the early work occurring in just a couple of species of Enterobacteriaceae. Once the metabolic pathways and enzymes were characterized by biochemical approaches, genetic screens were used to map the various activities to the eut genes. With the rise of genomics, the diversity of bacteria containing the eut genes and surprising differences in eut gene content were recognized. Some species contain nearly 20 genes and encode many accessory proteins, while others contain only the core catabolic enzyme. Moreover, the eut genes are regulated by very different mechanisms, depending on the organism and the eut regulator encoded. In the last several years, exciting progress has been made in elucidating the complex regulatory mechanisms that govern eut gene expression. Furthermore, a new appreciation for how EA contributes to infection and colonization in the host is emerging. In addition to providing an overview of EA-related biology, this minireview will give special attention to these recent advances.

Bacterial Chat: Intestinal Metabolites and Signals in Host-Microbiota-Pathogen Interactions

Intestinal bacteria employ microbial metabolites from the microbiota and chemical signaling during cell-to-cell communication to regulate several cellular functions. Pathogenic bacteria are extremely efficient in orchestrating their response to these signals through complex signaling transduction systems. Precise coordination and interpretation of these multiple chemical cues is important within the gastrointestinal (GI) tract. Enteric foodborne pathogens, such as enterohemorrhagic Escherichia coli (EHEC) and Salmonella enterica serovar Typhimurium, or the surrogate murine infection model for EHEC, Citrobacter rodentium, are all examples of microorganisms that modulate the expression of their virulence repertoire in response to signals from the microbiota or the host, such as autoinducer-3 (AI-3), epinephrine (Epi), and norepinephrine (NE). The QseBC and QseEF two-component systems, shared by these pathogens, are involved in sensing these signals. We review how these signaling systems sense and relay these signals to drive bacterial gene expression; specifically, to modulate virulence. We also review how bacteria chat via chemical signals integrated with metabolite recognition and utilization to promote successful associations among enteric pathogens, the microbiota, and the host.

Inflammatory bactericidal lectin RegIIIβ: Friend or foe for the host?

In the inflamed gut, the bactericidal lectin RegIIIβ is massively produced by intestinal mucosa. RegIIIβ binds peptidoglycan and lipid A respectively, and thus can kill certain Gram-positive and Gram-negative bacteria, including the gut commensal microbiota and enteropathogenic bacteria. Considering the expression pattern and bactericidal activity, RegIIIβ is believed to be a host defense factor for protecting against the infection with enteropathogenic bacteria. However, it was poorly understood how RegIIIβ recognizes the target bacteria and kill them, and how RegIIIβ plays role(s) in infectious diarrhea. Therefore, our recent study has focused on RegIIIβ-target recognition, killing of Gram-negative bacteria, and host protective functions of RegIIIβ for infectious diarrhea inflicted by Salmonella Typhimurium. Here, we discuss novel insights into the protective role of RegIIIβ in infectious diarrhea, and propose avenues towards novel therapeutic interventions for Salmonella diarrhea.

Salmonella enterica Serovar Typhimurium Strategies for Host Adaptation

Bacterial pathogens must sense and respond to newly encountered host environments to regulate the expression of critical virulence factors that allow for niche adaptation and successful colonization. Among bacterial pathogens, non-typhoidal serovars of Salmonella enterica, such as serovar Typhimurium (S. Tm), are a primary cause of foodborne illnesses that lead to hospitalizations and deaths worldwide. S. Tm causes acute inflammatory diarrhea that can progress to invasive systemic disease in susceptible patients. The gastrointestinal tract and intramacrophage environments are two critically important niches during S. Tm infection, and each presents unique challenges to limit S. Tm growth. The intestinal tract is home to billions of commensal microbes, termed the microbiota, which limits the amount of available nutrients for invading pathogens such as S. Tm. Therefore, S. Tm encodes strategies to manipulate the commensal population and side-step this nutritional competition. During subsequent stages of disease, S. Tm resists host immune cell mechanisms of killing. Host cells use antimicrobial peptides, acidification of vacuoles, and nutrient limitation to kill phagocytosed microbes, and yet S. Tm is able to subvert these defense systems. In this review, we discuss recently described molecular mechanisms that S. Tm uses to outcompete the resident microbiota within the gastrointestinal tract. S. Tm directly eliminates close competitors via bacterial cell-to-cell contact as well as by stimulating a host immune response to eliminate specific members of the microbiota. Additionally, S. Tm tightly regulates the expression of key virulence factors that enable S. Tm to withstand host immune defenses within macrophages. Additionally, we highlight the chemical and physical signals that S. Tm senses as cues to adapt to each of these environments. These strategies ultimately allow S. Tm to successfully adapt to these two disparate host environments. It is critical to better understand bacterial adaptation strategies because disruption of these pathways and mechanisms, especially those shared by multiple pathogens, may provide novel therapeutic intervention strategies.

Take Your Pick: Vitamins and Microbiota Facilitate Pathogen Clearance

The gut microbiota plays a chief role in vitamin production. Changes in its membership impact the gut metabolome and host susceptibility to pathogens. In this issue of Cell Host & Microbe, Miki et al. (2017) show that RegIIIβ impacts microbiota composition, decreasing vitamin B6 production and promoting sustained colonization by Salmonella.

Host cell-derived lactate functions as an effector molecule in Neisseria meningitidis microcolony dispersal

The development of meningococcal disease, caused by the human pathogen Neisseria meningitidis, is preceded by the colonization of the epithelial layer in the nasopharynx. After initial adhesion to host cells meningococci form aggregates, through pilus-pilus interactions, termed microcolonies from which the bacteria later detach. Dispersal from microcolonies enables access to new colonization sites and facilitates the crossing of the cell barrier; however, this process is poorly understood. In this study, we used live-cell imaging to investigate the process of N. meningitidis microcolony dispersal. We show that direct contact with host cells is not required for microcolony dispersal, instead accumulation of a host-derived effector molecule induces microcolony dispersal. By using a host-cell free approach, we demonstrated that lactate, secreted from host cells, initiate rapid dispersal of microcolonies. Interestingly, metabolic utilization of lactate by the bacteria was not required for induction of dispersal, suggesting that lactate plays a role as a signaling molecule. Furthermore, Neisseria gonorrhoeae microcolony dispersal could also be induced by lactate. These findings reveal a role of host-secreted lactate in microcolony dispersal and virulence of pathogenic Neisseria.

The human restricted pathogen Neisseria meningitidis is a major cause of bacterial meningitis and sepsis worldwide. Colonization of the mucosal layer in the upper respiratory tract is essential to establish invasive disease. The initial interaction with host cells is characterized by bacterial proliferation and adhesion as aggregates, called microcolonies. Detachment from microcolonies in the nasopharyngeal epithelium facilitates crossing of the cell barrier that can result in invasive disease, yet this process is poorly understood. Here we demonstrate that lactate, an abundant molecule in host mucosal environments, induces N. meningitidis microcolony dispersal. Interestingly, metabolic utilization of lactate by the bacteria was not required for the process, suggesting that lactate play a role as a signaling molecule in pathogenic Neisseria. We propose that the microcolony dispersal in pathogenic Neisseria is influenced by environmental concentrations of lactate. These findings will assist in better understanding the transition from asymptomatic carriage to invasive disease.