Siderophore-based immunization strategy to inhibit growth of enteric pathogens

Regulatory mechanisms of the probiotic-targeted gut-liver axis for the alleviation of alcohol-related liver disease: a review

Alcohol abuse-triggered alcohol-related liver disease (ALD) has become as a global public health concern that substantially affects the well-being and clinical status of patients. Although modern medicine provides various treatments for ALD, their effectiveness is limited and can lead to adverse side effects. Probiotics have been employed to prevent, alleviate, and even treat ALD, with promising results. However, few comprehensive reviews are available on how they mitigate ALD by targeting the gut-liver axis. This review systematically clarifies the specific mediators of the gut-liver axis in healthy states. It also describes the alterations observed in ALD. Furthermore, this review thoroughly summarizes the underlying mechanisms through which probiotics act on the gut-liver axis to relieve ALD. It also discusses the current status and challenges faced in clinical research applications. Finally, we discuss the challenges and future prospects of using probiotics to treat ALD. This review improves our understanding of ALD and supports the development and application of probiotics that target the gut-liver axis for therapeutic use.

Metals in the gut: microbial strategies to overcome nutritional immunity in the intestinal tract

Trace metals are indispensable nutritional factors for all living organisms. During host-pathogen interactions, they serve as crucial resources that dictate infection outcomes. Accordingly, the host uses a defense strategy known as nutritional immunity, which relies on coordinated metal chelation to mitigate bacterial advances. In response, pathogens employ complex strategies to secure these resources at sites of infection. In the gastrointestinal (GI) tract, the microbiota must also acquire metals for survival, making metals a central line of competition in this complex ecosystem. In this minireview, we outline how bacteria secure iron, zinc, and manganese from the host with a focus on the GI tract. We also reflect on how host dietary changes impact disease outcomes and discuss therapeutic opportunities to target bacterial metal uptake systems. Ultimately, we find that recent discoveries on the dynamics of transition metals at the host-pathogen-microbiota interface have reshaped our understanding of enteric infections and provided insights into virulence strategies, microbial cooperation, and antibacterial strategies.

Evaluation of the Stereochemistry of Staphyloferrin A for Developing Staphylococcus-Specific Targeting Conjugates

Bacteria in the genus Staphylococcus are pathogenic and harmful to humans. Alarmingly, some Staphylococcus, such as methicillin-resistant S. aureus (MRSA) and vancomycin-resistant S. aureus (VRSA) have spread worldwide and become notoriously resistant to antibiotics, threatening and concerning public health. Hence, the development of new Staphylococcus-targeting diagnostic and therapeutic agents is urgent. Here, we chose the S. aureus-secreted siderophore staphyloferrin A (SA) as a guiding unit. We developed a series of Staphyloferrin A conjugates (SA conjugates) and showed the specific targeting ability to Staphylococcus bacteria. Furthermore, among the structural factors we evaluated, the stereo-chemistry of the amino acid backbone of SA conjugates is essential to efficiently target Staphylococci. Finally, we demonstrated that fluorescent Staphyloferrin A probes (SA-FL probes) could specifically target Staphylococci in complex bacterial mixtures.

Native metabolomics for mass spectrometry-based siderophore discovery

Microorganisms, plants, and animals alike have specialized acquisition pathways for obtaining metals, with microorganisms and plants biosynthesizing and secreting small molecule natural products called siderophores and metallophores with high affinities and specificities for iron or other non-iron metals, respectively. This chapter details a novel approach to discovering metal-binding molecules, including siderophores and metallophores, from complex samples ranging from microbial supernatants to biological tissue to environmental samples. This approach, called Native Metabolomics, is a mass spectrometry method in which pH adjustment and metal infusion post-liquid chromatography are interfaced with ion identity molecular networking (IIMN). This rule-based data analysis workflow that enables the identification of metal-binding species based on defined mass (m/z) offsets with the same chromatographic profiles and retention times. Ion identity molecular networking connects compounds that are structurally similar by their fragmentation pattern and species that are ion adducts of the same compound by chromatographic shape correlations. This approach has previously revealed new insights into metal binding metabolites, including that yersiniabactin can act as a biological zincophore (in addition to its known role as a siderophore), that the recently elucidated lepotchelin natural products are cyanobacterial metallophores, and that antioxidants in traditional medicine bind iron. Native metabolomics can be conducted on any liquid chromatography-mass spectrometry system to explore the binding of any metal or multiple metals simultaneously, underscoring the potential for this method to become an essential strategy for elucidating biological metal-binding molecules.

Categorization of the effects of E. coli LF82 and mutants lacking the chuT and shuU genes on survival, the transcriptome, and metabolome in germ-free honeybee

The precise etiology of inflammatory bowel diseases (IBDs) remains elusive. The Escherichia coli strain LF82 (LF82) is known to be associated with IBD, and we hypothesized that this association may be related to the chuT and shuU genes. Here we constructed a germ-free (GF) honeybee model to investigate the effects of LF82 chuT and shuU genes on the honeybee intestine and their mechanisms. The chuT and shuU gene deletion strains LF82∆chuT and LF82∆shuU were generated by CRISPR-Cas9. These strains, together with nonpathogenic E. coli MG1655 (MG1655) and wildtype LF82, were allowed to colonize the guts of GF honeybees to establish single bacterial colonization models. Intestinal permeability was assessed following the administration of a sterile Brilliant Blue (FCF) solution. Comprehensive transcriptomic and metabolomic analyses of intestinal samples indicated that MG1655 had few disadvantageous effects on honeybees. Conversely, colonization with LF82 and its gene-deletion mutants provoked pronounced activation of genes associated with innate immune pathways, stimulated defensive responses, and induced expression of genes associated with inflammation, oxidative stress, and glycosaminoglycan degradation. Crucially, the LF82∆chuT and LF82∆shuU strains perturbed host heme and iron regulation, as well as tryptophan metabolism. These findings suggest that the deletion of chuT and shuU genes in E. coli LF82 may alleviate intestinal inflammation by partially modulating tryptophan catabolism. Our study proposes that targeting iron uptake mechanisms could be a potential strategy to mitigate the virulence of IBD-associated bacteria.

Exploring the Antibacterial Activity and Cellular Fates of Enterobactin-Drug Conjugates That Target Gram-Negative Bacterial Pathogens

Siderophores are secondary metabolites utilized by bacteria to acquire iron (Fe), an essential transition metal nutrient. Fe levels in the host environment are tightly regulated and can be further restricted to starve invading bacterial pathogens in a host-defense process known as nutritional immunity. To survive and colonize the Fe-limited host environment, bacteria produce siderophores and express cognate siderophore transport machinery. These active transport pathways present an opportunity for selective and efficient drug delivery into bacterial cells, motivating decades of research on synthetic siderophore-antibiotic conjugates (SACs) as a Trojan-horse strategy for the development of targeted antibiotics.Enterobactin (Ent) is a triscatecholate siderophore produced and utilized by many Gram-negative bacteria, including all Escherichia coli and Salmonella species. Within these species, pathogenic strains cause a variety of human diseases including urinary tract infections, gastroenteritis, and sepsis. Infections caused by these Gram-negative pathogens can be difficult to treat because of the impermeability of the outer membrane (OM). This impermeability can be overcome by utilizing siderophores as drug delivery vectors for targeting Gram-negative pathogens. Ent is a promising delivery vector because it undergoes active transport across the OM mediated by the Ent uptake machinery after scavenging Fe(III) from the extracellular environment. Despite the well-elucidated chemistry and biology of Ent, its use for SAC development was hampered by the lack of an appropriate functional group for cargo attachment. Our laboratory addressed this need by designing and synthesizing monofunctionalized Ent scaffolds. Over the past decade, we have used these scaffolds to explore Ent-based SACs with a variety of drug warheads, including β-lactam and fluoroquinolone antibiotics, and Pt(IV) prodrugs. Investigations of the antibacterial activities of these conjugates and their cellular fates have informed our design principles and revealed approaches to achieving enhanced antibacterial potency and pathogen-targeted activity. Collectively, our studies of Ent-drug conjugates have provided discoveries, understanding, and invaluable insights for future design and evaluation of SACs.In this Account, we present the story of our work on Ent-drug conjugates that began about ten years ago with the development of monofunctionalized Ent scaffolds and the design and synthesis of various conjugates based on these scaffolds. We describe the antibacterial activity profiles and uptake pathways of Ent-drug conjugates harboring traditional antibiotics and repurposed platinum anticancer agents as well as studies that address cellular targets and fates. Finally, we discuss other applications of monofunctionalized Ent scaffolds, including a siderophore-based immunization strategy. We intend for this Account to inspire further investigations into the fundamental understanding and translational applications of siderophores and siderophore-drug conjugates.

An adapted method for Cas9-mediated editing reveals the species-specific role of β-glucoside utilization driving competition between Klebsiella species

Cas9-based gene editing tools have revolutionized genetics, enabling the fast and precise manipulation of diverse bacterial species. However, widely applicable genetic tools for non-model gut bacteria are unavailable. Here, we present a two-plasmid Cas9-based system designed for gene deletion and knock-in complementation in three members of the Klebsiella oxytoca species complex (KoSC), which we applied to study the genetic factors underlying the role of these bacteria in competition against Klebsiella pneumoniae. Firstly, the system allowed efficient and precise full-length gene deletion via enhanced lambda Red expression. Furthermore, we tested the efficiency of two independent, functionally validated complementation strategies. Ultimately, the insertion of universal "bookmark" targets during gene deletion subsequently allows the most optimal genetic complementation in K. oxytoca, Klebsiella michiganensis, and Klebsiella grimontii. This approach offers a significant advantage by enabling the use of a single high-efficiency "bookmark" for complementing other loci or strains, eliminating the need for site-specific design. We revealed that the carbohydrate permease CasA is critical in ex vivo assays for K. pneumoniae inhibition by K. oxytoca but is neither sufficient nor required for K. michiganensis and K. grimontii. Thus, the adaptation of state-of-the-art genetic tools to KoSC allows the identification of species-specific functions in microbial competition.

IMPORTANCE

Cas9-based gene editing tools have revolutionized bacterial genetics, yet, their application to non-model gut bacteria is frequently hampered by various limitations. We utilized a two-plasmid Cas9-based system designed for gene deletion in Klebsiella pneumoniae and demonstrate after optimization its utility for gene editing in three members of the Klebsiella oxytoca species complex (KoSC) namely K. oxytoca, Klebsiella michiganensis, and Klebsiella grimontii. We then adapted a recently developed protocol for functional complementation based on universal "bookmark" targets applicable to all tested species. In summary, species-specific adaptation of state-of-the-art genetic tools allows efficient gene deletion and complementation in type strains as well as natural isolates of KoSC members to study microbial interactions.

Intranasal immunization of mice with chimera of Salmonella Typhi protein elicits protective intestinal immunity

Development of safe, highly effective and affordable enteric fever vaccines is a global health priority. Live, oral typhoid vaccines induce strong mucosal immunity and long-term protection, but safety remains a concern. In contrast, efficacy wears off rapidly for injectable, polysaccharide-based vaccines, which elicit poor mucosal response. We previously reported Salmonella Typhi outer membrane protein, T2544 as a potential candidate for bivalent (S. Typhi and S. Paratyphi A) vaccine development. Here, we show that intranasal immunization with a subunit vaccine (chimera of T2544 and cholera toxin B subunit) induced strong systemic and intestinal mucosal immunity and protection from S. Typhi challenge in a mouse model. CTB-T2544 augmented gut-homing receptor expression on lymphocytes that produced Th1 and Th17 cytokines, secretory IgA in stool that inhibited bacterial motility and epithelial attachment, antibody recall response and affinity maturation with increased number of follicular helper T cells and CD4+ central and effector memory cells.

Evaluation of immunogenicity of enterobactin conjugate vaccine for the control of Escherichia coli mastitis in dairy cows

Mastitis is the most common disease of dairy cows that incurs severe economic losses to the dairy industry. Currently, environmental mastitis pathogens are a major problem for most dairy farms. A current commercially available Escherichia coli vaccine does not prevent clinical mastitis and production losses, likely due to antibody accessibility and antigenic variation issues. Therefore, a novel vaccine that prevents clinical disease and production losses is critically needed. Recently a nutritional immunity approach, which restricts bacterial iron uptake by immunologically sequestering conserved iron-binding enterobactin (Ent), has been developed. The objective of this study was to evaluate the immunogenicity of the keyhole limpet hemocyanin-enterobactin (KLH-Ent) conjugate vaccine in dairy cows. Twelve pregnant Holstein dairy cows in their first through third lactations were randomized to the control or vaccine group, with 6 cows per group. The vaccine group received 3 subcutaneous vaccinations of KLH-Ent with adjuvants at drying off (D0), 20 (D21), and 40 (D42) days after drying off. The control group was injected with phosphate-buffered saline (pH 7.4) mixed with the same adjuvants at the same time points. Vaccination effects were assessed over the study period until the end of the first month of lactation. The KLH-Ent vaccine did not cause any systemic adverse reactions or reduction in milk production. Compared with the control group, the vaccine elicited significantly higher levels of serum Ent-specific IgG at calving (C0) and 30 d postcalving (C30), mainly its IgG2 fraction, which was significantly higher at D42, C0, C14, and C30 d, with no significant change in IgG1 levels. Milk Ent-specific IgG and IgG2 levels in the vaccine group were significantly higher on C30. Fecal microbial community structures were similar for both control and vaccine groups on the same day and shifted directionally along the sampling days. In conclusion, the KLH-Ent vaccine successfully triggered strong Ent-specific immune responses in dairy cows without significantly affecting the gut microbiota diversity and health. The results show that Ent conjugate vaccine is a promising nutritional immunity approach in control of E. coli mastitis in dairy cows.

Effects of deferoxamine on intrinsic colistin resistance of Proteus mirabilis

Proteus mirabilis is a common pathogen, which is responsible for urinary tract infections. Iron is a critical element necessary for both humans and pathogens to maintain their biological functions, and iron limitation via chelator agents may be useful in the treatment of infections. The present study aimed to investigate the synergistic interactions between the iron chelator agent deferoxamine (DFO) and the antibacterial drug colistin. The minimum inhibitory concentration (MIC) values of DFO and colistin for P. mirabilis isolates were determined by broth microdilution. The checkerboard technique was used to examine the potential synergy between DFO and colistin. Furthermore, time-kill assays were used for the confirmation of synergy detected by the checkerboard assay, as well as for determining bacteriostatic and bactericidal interactions throughout a 24-h period. As expected, all P. mirabilis isolates were resistant to colistin. DFO did not inhibit P. mirabilis growth when used alone, even at very high doses (10 µg ml-1). Notably, when in combination with DFO, the MIC values of colistin were markedly reduced, and the checkerboard assay results showed synergy between colistin and DFO for all isolates. In addition, in time-kill assays, colistin + DFO exhibited synergistic activity against all strains at most time intervals and concentrations tested. Colistin + DFO showed bactericidal activity at colistin concentrations of 1xMIC and 2xMIC, although a degree of re-growth was observed in one of the strains at 12-24 h. These findings indicated that DFO has the potential for use as an adjunct to colistin through iron sequestration, thus providing synergistic activity to an antibiotic that would not normally be considered a treatment option against P. mirabilis. In vivo experiments in the future may provide useful information on the efficacy of DFO/colistin since these models effectively reflect physiological parameters.

Exploring probiotic effector molecules and their mode of action in gut-immune interactions

Probiotics, live microorganisms that confer health benefits when consumed in adequate amounts, have gained significant attention for their potential therapeutic applications. The beneficial effects of probiotics are believed to stem from their ability to enhance intestinal barrier function, inhibit pathogens, increase beneficial gut microbes, and modulate immune responses. However, clinical studies investigating the effectiveness of probiotics have yielded conflicting results, potentially due to the wide variety of probiotic species and strains used, the challenges in controlling the desired number of live microorganisms, and the complex interactions between bioactive substances within probiotics. Bacterial cell wall components, known as effector molecules, play a crucial role in mediating the interaction between probiotics and host receptors, leading to the activation of signaling pathways that contribute to the health-promoting effects. Previous reviews have extensively covered different probiotic effector molecules, highlighting their impact on immune homeostasis. Understanding how each probiotic component modulates immune activity at the molecular level may enable the prediction of immunological outcomes in future clinical studies. In this review, we present a comprehensive overview of the structural and immunological features of probiotic effector molecules, focusing primarily on Lactobacillus and Bifidobacterium. We also discuss current gaps and limitations in the field and propose directions for future research to enhance our understanding of probiotic-mediated immunomodulation.

Role of iron in the treatment of sepsis

Iron is an important microelement in human and microbial life activities. During the pathophysiological process of sepsis, iron metabolism changes and the body undergoes a series of changes to fight microbial infection. Meanwhile, alterations in iron metabolism during sepsis lead to the development of some diseases, such as transfusion-induced siderosis and anemia. In recent years, several studies have demonstrated the use of iron-chelating agents to fight microbial infections, and new antimicrobial agents have been developed using "Trojan horse" and siderophores immunity. In addition, the use of iron-based nanomaterials as drug delivery systems for gene delivery may be applied to the treatment of sepsis in the future. In this review, we describe the pathophysiological changes in the development and course of sepsis, focusing on the potential of iron in the treatment of sepsis.

Siderophore Immunization Restricted Colonization of Adherent-Invasive Escherichia coli and Ameliorated Experimental Colitis

Inflammatory bowel diseases (IBD) are characterized by chronic inflammation of the gastrointestinal tract and profound alterations to the gut microbiome. Adherent-invasive Escherichia coli (AIEC) is a mucosa-associated pathobiont that colonizes the gut of patients with Crohn's disease, a form of IBD. Because AIEC exacerbates gut inflammation, strategies to reduce the AIEC bloom during colitis are highly desirable. To thrive in the inflamed gut, Enterobacteriaceae acquire the essential metal nutrient iron by producing and releasing siderophores. Here, we implemented an immunization-based strategy to target the siderophores enterobactin and its glucosylated derivative salmochelin to reduce the AIEC bloom in the inflamed gut. Using chemical (dextran sulfate sodium) and genetic (Il10-/- mice) IBD mouse models, we showed that immunization with enterobactin conjugated to the mucosal adjuvant cholera toxin subunit B potently elicited mucosal and serum antibodies against these siderophores. Siderophore-immunized mice exhibited lower AIEC gut colonization, diminished AIEC association with the gut mucosa, and reduced colitis severity. Moreover, Peyer's patches and the colonic lamina propria harbored enterobactin-specific B cells that could be identified by flow cytometry. The beneficial effect of siderophore immunization was primarily B cell-dependent because immunized muMT-/- mice, which lack mature B lymphocytes, were not protected during AIEC infection. Collectively, our study identified siderophores as a potential therapeutic target to reduce AIEC colonization and its association with the gut mucosa, which ultimately may reduce colitis exacerbation. Moreover, this work provides the foundation for developing monoclonal antibodies against siderophores, which could provide a narrow-spectrum strategy to target the AIEC bloom in Crohn's disease patients. IMPORTANCE Adherent-invasive Escherichia coli (AIEC) is abnormally prevalent in patients with ileal Crohn's disease and exacerbates intestinal inflammation, but treatment strategies that selectively target AIEC are unavailable. Iron is an essential micronutrient for most living organisms, and bacterial pathogens have evolved sophisticated strategies to capture iron from the host environment. AIEC produces siderophores, small, secreted molecules with a high affinity for iron. Here, we showed that immunization to elicit antibodies against siderophores promoted a reduction of the AIEC bloom, interfered with AIEC association with the mucosa, and mitigated colitis in experimental mouse models. We also established a flow cytometry-based approach to visualize and isolate siderophore-specific B cells, a prerequisite for engineering monoclonal antibodies against these molecules. Together, this work could lead to a more selective and antibiotic-sparing strategy to target AIEC in Crohn's disease patients.

Dietary phytate primes epithelial antibacterial immunity in the intestine

Although diet has long been associated with susceptibility to infection, the dietary components that regulate host defense remain poorly understood. Here, we demonstrate that consuming rice bran decreases susceptibility to intestinal infection with Citrobacter rodentium, a murine pathogen that is similar to enteropathogenic E. coli infection in humans. Rice bran naturally contains high levels of the substance phytate. Interestingly, phytate supplementation also protected against intestinal infection, and enzymatic metabolism of phytate by commensal bacteria was necessary for phytate-induced host defense. Mechanistically, phytate consumption induced mammalian intestinal epithelial expression of STAT3-regulated antimicrobial pathways and increased phosphorylated STAT3, suggesting that dietary phytate promotes innate defense through epithelial STAT3 activation. Further, phytate regulation of epithelial STAT3 was mediated by the microbiota-sensitive enzyme histone deacetylase 3 (HDAC3). Collectively, these data demonstrate that metabolism of dietary phytate by microbiota decreases intestinal infection and suggests that consuming bran and other phytate-enriched foods may represent an effective dietary strategy for priming host immunity.

Screening of potential antigens from whole proteome and development of multi-epitope vaccine against Rhizopus delemar using immunoinformatics approaches

Mucormycosis is a deadly fungal disease mainly caused by Rhizopus oryzae (strain 99-880), also known as Rhizopus delemar. Previously, mucormycosis occurs in immunocompromised patients of diabetes mellitus, cancer, organ transplant, etc. But there was a drastic increase in mucormycosis cases in the ongoing COVID-19 pandemic. Despite several available therapies and antifungal treatments, the mortality rate of mucormycosis is about more than 50%. Currently, there is no vaccine available in the market for mucormycosis that urgently needs to develop a potential vaccine against mucormycosis with high efficacy. In the present study, we have screened 4 genome-derived predicted antigens (GDPA) through sequential filtration of the whole proteome of R. delemar using different benchmarked bioinformatics tools. These 4 GDPA along with 4 randomly selected experimentally reported antigens (ERA) were sourced for prediction of B- and T- cell epitopes and utilized in designing of two potential multi-epitope vaccine candidates which can induce both innate and adaptive immunity against R. delemar. Besides these, comparative immune simulation studies and in silico cloning were performed using L. lactis as an expression system for their possible uses as oral vaccines. This is the first multi-epitope vaccine designed against R. delemar through systematic pipelined reverse vaccinology and immunoinformatic approaches. Although the wet-lab based experimental validation of designed vaccines is required before testing in the preclinical model, the current study will significantly help in reducing the cost of experimentation as well as improving the efficacy of vaccine therapy against mucormycosis and other pathogenic diseases.Communicated by Ramaswamy H. Sarma.

Laser capture microdissection to study Bacillus cereus iron homeostasis gene expression during Galleria mellonella in vivo gut colonization

Bacillus cereus is a Gram-positive opportunistic pathogen closely related to the entomopathogen, Bacillus thuringiensis, both of which are involved in intestinal infections. Iron is an essential micronutrient for full growth and virulence of pathogens during infection. However, little is known about iron homeostasis during gut infection. Therefore, we aimed to assess the expression of B. cereus genes related to bacterial iron homeostasis, virulence and oxidative stress. The hypothesis is that the expression of such genes would vary between early and later stage colonization in correlation to gut cell damage. To perform the study, a germ-free Galleria mellonella model was set up in order to adapt the use of Laser-capture microdissection (LCM), to select precise areas in the gut lumen from frozen whole larval cryo-sections. Analyses were performed from alive larvae and the expression of targeted genes was assessed byspecific pre-amplification of mRNA followed by quantitative PCR. Firstly, the results reinforce the reliability of LCM, despite a low amount of bacterial RNA recovered. Secondly, bacterial genes involved in iron homeostasis are expressed in the lumen at both 3 and 16 hours post force-feeding. Thirdly, iron gene expression is slightly modulated during gut infection, and lastly, the mRNA of G. mellonella encoding for ferritin and transferrin iron storage and transport are recovered too. Therefore, iron homeostasis should play a role in B. cereus gut colonization. Furthermore, we demonstrate for the first time the value of using LCM for specific in situ gene expression analysis of extracellular bacteria in a whole animal.

Comparative Pathogenomics of Escherichia coli: Polyvalent Vaccine Target Identification through Virulome Analysis

Comparative genomics of bacterial pathogens has been useful for revealing potential virulence factors. Escherichia coli is a significant cause of human morbidity and mortality worldwide but can also exist as a commensal in the human gastrointestinal tract. With many sequenced genomes, it has served as a model organism for comparative genomic studies to understand the link between genetic content and potential for virulence. To date, however, no comprehensive analysis of its complete “virulome” has been performed for the purpose of identifying universal or pathotype-specific targets for vaccine development. Here, we describe the construction of a pathotype database of 107 well-characterized completely sequenced pathogenic and nonpathogenic E. coli strains, which we annotated for major virulence factors (VFs). The data are cross referenced for patterns against pathotype, phylogroup, and sequence type, and the results were verified against all 1,348 complete E. coli chromosomes in the NCBI RefSeq database. Our results demonstrate that phylogroup drives many of the “pathotype-associated” VFs, and ExPEC-associated VFs are found predominantly within the B2/D/F/G phylogenetic clade, suggesting that these phylogroups are better adapted to infect human hosts. Finally, we used this information to propose polyvalent vaccine targets with specificity toward extraintestinal strains, targeting key invasive strategies, including immune evasion (group 2 capsule), iron acquisition (FyuA, IutA, and Sit), adherence (SinH, Afa, Pap, Sfa, and Iha), and toxins (Usp, Sat, Vat, Cdt, Cnf1, and HlyA). While many of these targets have been proposed before, this work is the first to examine their pathotype and phylogroup distribution and how they may be targeted together to prevent disease.

Conjugation to Enterobactin and Salmochelin S4 Enhances the Antimicrobial Activity and Selectivity of β-Lactam Antibiotics against Nontyphoidal Salmonella

The pathogen Salmonella enterica is a leading cause of infection worldwide. Nontyphoidal Salmonella (NTS) serovars typically cause inflammatory diarrhea in healthy individuals, and can cause bacteremia in immunocompromised patients, children, and the elderly. Management of NTS infection poses a challenge because antibiotic treatment prolongs fecal shedding of the pathogen and is thus not recommended for most patients. In recent years, the emergence of antibiotic resistance in NTS has also become a major issue. Thus, new therapeutic strategies to target NTS are needed. Here, we evaluated whether six siderophore-β-lactam conjugates based on enterobactin (Ent) and salmochelin S4 (digulcosylated Ent, DGE) provide antimicrobial activity against the two highly prevalent NTS serovars Typhimurium and Enteritidis by targeting the siderophore receptors FepA and/or IroN. The conjugates showed 10- to 1000-fold lower minimum inhibitory concentrations against both serovars Typhimurium and Enteritidis compared to the parent antibiotics under iron limitation and were recognized and transported by FepA and/or IroN. NTS treated with the Ent/DGE-β-lactam conjugates exhibited aberrant cellular morphologies suggesting inhibition of penicillin-binding proteins, and the conjugates selectively killed NTS in coculture with Staphylococcus aureus. Lastly, the DGE-based conjugates proved to be effective at inhibiting growth of NTS in the presence of the Ent-sequestering protein lipocalin-2. This work describes the successful use of siderophore-antibiotic conjugates against NTS and highlights the opportunity for narrowing the activity spectrum of antibiotics by using Ent and DGE to target enteric bacterial pathogens.

An overview of Salmonella enterica metal homeostasis pathways during infection

Nutritional immunity is a powerful strategy at the core of the battlefield between host survival and pathogen proliferation. A host can prevent pathogens from accessing biological metals such as Mg, Fe, Zn, Mn, Cu, Co or Ni, or actively intoxicate them with metal overload. While the importance of metal homeostasis for the enteric pathogen Salmonella enterica Typhimurium was demonstrated many decades ago, inconsistent results across various mouse models, diverse Salmonella genotypes, and differing infection routes challenge aspects of our understanding of this phenomenon. With expanding access to CRISPR-Cas9 for host genome manipulation, it is now pertinent to re-visit past results in the context of specific mouse models, identify gaps and incongruities in current knowledge landscape of Salmonella homeostasis, and recommend a straight path forward towards a more universal understanding of this historic host-microbe relationship.

Structural insights into the ligand recognition and catalysis of the key aminobutanoyltransferase CntL in staphylopine biosynthesis

Staphylopine (StP) and other nicotianamine-like metallophores are crucial for many pathogens to acquire the transition metals from hosts during invasion. CntL from Staphylococcus aureus (SaCntL) catalyzes the condensation of the 2-aminobutyrate (Ab) moiety of S-adenosylmethionine (SAM) with D-histidine in the biosynthesis of StP. Here, we report the crystal structures of SaCntL in complex with either SAM or two products. The structure of SaCntL consists of an N-terminal four-helix bundle (holding catalytic residue E84) and a C-terminal Rossmann fold (binding the substrates). The sequence connecting the N- and C-terminal domains (N-C linker) in SaCntL was found to undergo conformational alternation between open and closed states. Our structural and biochemical analyses suggested that this intrinsically dynamic interdomain linker forms an additional structural module that plays essential roles in ligand diffusion, recognition, and catalysis. We confirmed that SaCntL stereoselectively carries out the catalysis of D-His but not its enantiomer, L-His, and we found that the N-C linker and active site of SaCntL could accommodate both enantiomers. SaCntL is likely able to bind L-His without catalysis, and as a result, L-His could show inhibitory effects toward SaCntL. These findings provide critical structural and mechanistic insights into CntL, which facilitates a better understanding of the biosynthesis of nicotianamine-like metallophores and the discovery of inhibitors of this process.

Evaluation of the Immunogenic Response of a Novel Enterobactin Conjugate Vaccine in Chickens for the Production of Enterobactin-Specific Egg Yolk Antibodies

Passive immunization with specific egg yolk antibodies (immunoglobulin Y, IgY) is emerging as a promising alternative to antibiotics to control bacterial infections. Recently, we developed a novel conjugate vaccine that could trigger a strong immune response in rabbits directed against enterobactin (Ent), a highly conserved siderophore molecule utilized by different Gram-negative pathogens. However, induction of Ent-specific antibodies appeared to be affected by the choice of animal host and vaccination regimen. It is still unknown if the Ent conjugate vaccine can trigger a specific immune response in layers for the purpose of production of anti-Ent egg yolk IgY. In this study, three chicken vaccination trials with different regimens were performed to determine conditions for efficient production of anti-Ent egg yolk IgY. Purified Ent was conjugated to three carrier proteins, keyhole limpet hemocyanin (KLH), bovine serum albumin (BSA) and CmeC (a subunit vaccine candidate), respectively. Intramuscular immunization of Barred Rock layers with KLH-Ent conjugate four times induced strong immune response against whole conjugate vaccine but the titer of Ent-specific IgY did not change in yolk with only a 4 fold increase detected in serum. In the second trial, three different Ent conjugate vaccines were evaluated in Rhode Island Red pullets with four subcutaneous injections. The KLH-Ent or CmeC-Ent conjugate consistently induced high level of Ent-specific IgY in both serum (up to 2,048 fold) and yolk (up to 1,024 fold) in each individual chicken. However, the Ent-specific immune response was only temporarily and moderately induced using a BSA-Ent vaccination. In the third trial, ten White Leghorn layers were subcutaneously immunized three times with KLH-Ent, leading to consistent and strong immune response against both whole conjugate and the Ent molecule in each chicken; the mean titer of Ent-specific IgY increased approximately 32 and 256 fold in serum and yolk, respectively. Consistent with its potent binding to various Ent derivatives, the Ent-specific egg yolk IgY also inhibited in vitro growth of a representative Escherichia coli strain. Together, this study demonstrated that the novel Ent conjugate vaccine could induce strong, specific, and robust immune response in chickens. The Ent-specific hyperimmune egg yolk IgY has potential for passive immune intervention against Gram-negative infections.

Siderophore natural products as pharmaceutical agents

Siderophore natural products are characterized by an ability to tightly chelate metals. The origins of such compounds are often pathogenic microbes utilizing siderophores as virulence factors during host infection. The mechanism for siderophore formation typically involves the activity of nonribosomal peptide synthetases producing compounds across functional group classifications that include catecholate, phenolate, hydroxamate, and mixed categories. Though siderophore production has been a hallmark of pathogenicity, the evolutionarily-optimized binding abilities of siderophores suggest the possibility of re-directing the compounds towards alternative beneficial applications. In this mini-review, we will first describe siderophore formation origins before discussing alternative applications as pharmaceutical products. In so doing, we will cover examples and applications that include reducing metal overload, targeted antibiotic delivery, cancer treatment, vaccine development, and diagnostics. Included in this analysis will be a discussion on the native production hosts of siderophores and prospects for improvement in compound access through the adoption of heterologous biosynthesis.

Siderophores for medical applications: Imaging, sensors, and therapeutics

Siderophores are low-molecular-weight chelators produced by microorganisms to scavenge iron from the environment and deliver it to cells via specific receptors. Tremendous researches on the molecular basis of siderophore regulation, synthesis, secretion, and uptake have inspired their diverse applications in the medical field. Replacing iron with radionuclides in siderophores, such as the most prominent Ga-68 for positron emission tomography (PET), carves out ways for targeted imaging of infectious diseases and cancers. Additionally, the high affinity of siderophores for metal ions or microorganisms makes them a potent detecting moiety in sensors that can be used for diagnosis. As for therapeutics, the notable Trojan horse-inspired siderophore-antibiotic conjugates demonstrate enhanced toxicity against multi-drug resistant (MDR) pathogens. Besides, siderophores can tackle iron overload diseases and, when combined with moieties such as hydrogels and nanoparticles, a wide spectrum of iron-induced diseases and even cancers. In this review, we briefly outline the related mechanisms, before summarizing the siderophore-based applications in imaging, sensors, and therapeutics.

Metallotherapeutics development in the age of iron-clad bacteria

Drug-resistant infections pose a significant risk to global health as pathogenic bacteria become increasingly difficult to treat. The rapid selection of resistant strains through poor antibiotic stewardship has reduced the number of viable treatments and increased morbidity of infections, especially among the immunocompromised. To circumvent such challenges, new strategies are required to stay ahead of emerging resistance trends, yet research and funding for antibiotic development lags other classes of therapeutics. Though the use of metals in therapeutics has been around for centuries, recent strategies have devoted a great deal of effort into the pathways through which bacteria acquire and utilize iron, which is critical for the establishment of infection. To target iron uptake systems, siderophore-drug conjugates have been developed that hijack siderophore-based iron uptake for delivery of antibiotics. While this strategy has produced several potential leads, the use of siderophores in infection is diminished over time when bacteria adapt to utilize heme as an iron source, leading to a need for the development of porphyrin mimetics as therapeutics. The use of such strategies as well as the inclusion of gallium, a redox-inert iron mimic, are herein reviewed.

Harnessing Iron Acquisition Machinery to Target Enterobacteriaceae

Infections caused by Gram-negative bacteria can be challenging to treat due to the outer membrane permeability barrier and the increasing emergence of antibiotic resistance. During infection, Gram-negative pathogens must acquire iron, an essential nutrient, in the host. Many Gram-negative bacteria utilize sophisticated iron acquisition machineries based on siderophores, small molecules that bind iron with high affinity. In this review, we provide an overview of siderophore-mediated iron acquisition in Enterobacteriaceae and show how these systems provide a foundation for the conceptualization and development of approaches to prevent and/or treat bacterial infections. Differences between the siderophore-based iron uptake machineries of pathogenic Enterobacteriaceae and commensal microbes may lead to the development of selective "Trojan-horse" antimicrobials and immunization strategies that will not harm the host microbiota.

Immunization of Chickens with the Enterobactin Conjugate Vaccine Reduced Campylobacter jejuni Colonization in the Intestine

Campylobacter jejuni is the leading bacterial cause of human enteritis in developed countries. Chicken is the major animal reservoir of C. jejuni and a powerful infection model for human campylobacteriosis. No commercial vaccine against C. jejuni is available to date. The high affinity iron acquisition mediated through enterobactin (Ent), a small siderophore, plays a critical role in the colonization of C. jejuni in the intestine. Recently, an innovative Ent conjugate vaccine has been demonstrated to induce high-level of Ent-specific antibodies in rabbits; the Ent-specific antibodies displayed potent binding ability to Ent and inhibited Ent-dependent growth of C. jejuni. In this study, using specific-pathogen-free (SPF) chickens, we performed three trials to evaluate the immunogenicity of the Ent conjugate vaccine and its efficacy to control C. jejuni colonization in the intestine. The purified Ent was conjugated to the carrier keyhole limpet hemocyanin (KLH). Intramuscular immunization of chickens with the Ent-KLH conjugate for up to three times did not affect the body weight gain, the development of major immune organs and the gut microbiota. In the first two trials, immunizations of chickens with different regimens (two or three times of vaccination) consistently induced strong Ent-specific immune response when compared to control group. Consistent with the high-level of systemic anti-Ent IgG, C. jejuni colonization was significantly reduced by 3-4 log₁₀ units in the cecum in two independent vaccination trials. The third trial demonstrated that single Ent-KLH vaccination is sufficient to elicit high level of systemic Ent-specific antibodies, which could persist for up to eight weeks in chickens. Taken together, the Ent-KLH conjugate vaccine could induce high-level of Ent-specific antibodies in chickens and confer host protection against C. jejuni colonization, which provides a novel strategy for Campylobacter control in poultry and humans.

Antimicrobial resistance in enteric bacteria: current state and next-generation solutions

Antimicrobial resistance is one of the largest threats to global health and imposes substantial burdens in terms of morbidity, mortality, and economic costs. The gut is a key conduit for the genesis and spread of antimicrobial resistance in enteric bacterial pathogens. Distinct bacterial species that cause enteric disease can exist as invasive enteropathogens that immediately evoke gastrointestinal distress, or pathobionts that can arise from established bacterial commensals to inflict dysbiosis and disease. Furthermore, various environmental reservoirs and stressors facilitate the evolution and transmission of resistance. In this review, we present a comprehensive discussion on circulating resistance profiles and gene mobilization strategies of the most problematic species of enteric bacterial pathogens. Importantly, we present emerging approaches toward surveillance of pathogens and their resistance elements as well as promising treatment strategies that can circumvent common resistance mechanisms.

The battle for iron in enteric infections

Iron is an essential element for almost all living organisms, but can be extremely toxic in high concentrations. All organisms must therefore employ homeostatic mechanisms to finely regulate iron uptake, usage and storage in the face of dynamic environmental conditions. The critical step in mammalian systemic iron homeostasis is the fine regulation of dietary iron absorption. However, as the gastrointestinal system is also home to >1014 bacteria, all of which engage in their own programmes of iron homeostasis, the gut represents an anatomical location where the inter-kingdom fight for iron is never-ending. Here, we explore the molecular mechanisms of, and interactions between, host and bacterial iron homeostasis in the gastrointestinal tract. We first detail how mammalian systemic and cellular iron homeostasis influences gastrointestinal iron availability. We then focus on two important human pathogens, Salmonella and Clostridia; despite their differences, they exemplify how a bacterial pathogen must navigate and exploit this web of iron homeostasis interactions to avoid host nutritional immunity and replicate successfully. We then reciprocally explore how iron availability interacts with the gastrointestinal microbiota, and the consequences of this on mammalian physiology and pathogen iron acquisition. Finally, we address how understanding the battle for iron in the gastrointestinal tract might inform clinical practice and inspire new treatments for important diseases.

Enterobactin-specific antibodies inhibit in vitro growth of different gram-negative bacterial pathogens

Enterobactin (Ent)-mediated high affinity iron acquisition is critically important for Gram-negative bacterial pathogens to survive and infect the host. Recently, we reported an efficient method to prepare novel Ent conjugate vaccines for inducing high level of Ent-specific antibodies, which displayed similar bacteriostatic feature as lipocalins, the host innate immune effectors with potent Ent-binding ability. The Ent-specific antibodies also showed a significant advantage over lipocalins by cross-reacting to various Ent derivatives including salmochelins, the glycosylated Ent that can help enteric pathogens evade the siderophore sequestration by host lipocalins. To demonstrate significant potential of the Ent conjugate vaccine for broader applications to prevent and control various Gram-negative infections in human and animal, in this study, we examined inhibitory effect of Ent-specific antibodies on the in vitro growth of three significant Gram-negative pathogens: Escherichia coli (n = 27), Salmonella enterica (n = 8), and Campylobacter spp. (n = 6). The tested strains were diverse with respect to hosts, geographical origins, serotypes, infection sites and siderophore productions. The Ent-specific antibodies significantly suppressed the growth of each tested strain under iron-restricted conditions. For example, the Ent-specific antibodies consistently exerted 2-5 log₁₀ units of growth reduction on most tested avian pathogenic E. coli (9 of 10 strains) isolated in five countries. Despite various dynamic growth responses observed, notably, the Ent-specific antibodies displayed significantly higher magnitude of growth reduction than lipocalin-2 (up to 5 log₁₀ units of difference) on majority of tested E. coli and S. enterica, which is likely due to sequestration of other siderophores (e.g., salmochelins) by the Ent-specific antibodies. Production of a variety of major siderophores by the tested E. coli and S. enterica strains was examined and confirmed by ultra high performance liquid chromatography-high resolution mass spectrometry analysis. Collectively, this study provides critical and compelling in vitro evidence supporting the feasibility of Ent-based immune interventions against several Gram-negative pathogens.

Iron at the host-microbe interface

Iron is an essential micronutrient for nearly all living organisms. In addition to facilitating redox reactions, iron is bound by metalloproteins that participate in a variety of biological processes. As the bioavailability of free iron in host environments is extremely low, iron lies at the center of a battle for nutrients between microbes and their host. Mucosal surfaces such as the respiratory and gastrointestinal tracts are constantly exposed to commensal and pathogenic microorganisms. Whereas a key strategy of mammalian antimicrobial defense is to deprive microbes of iron, pathogens and some commensals have evolved effective strategies to circumvent iron limitation. Here we provide an overview of mechanisms underpinning the tug-of-war for iron between microbes and their host, with a particular focus on mucosal surfaces.

Overview of the Nontyphoidal and Paratyphoidal Salmonella Vaccine Pipeline: Current Status and Future Prospects

Nontyphoidal Salmonella and Salmonella Paratyphi are responsible for significant morbidity and mortality worldwide. To date, no vaccine has been licensed against these organisms. The development of effective vaccines remains an urgent priority. In this review, the rationale for and current status of various vaccine candidates against S. Paratyphi and nontyphoidal Salmonella are presented, with a focus on the research findings from the 2019 International Conference on Typhoid and Other Invasive Salmonelloses. Additionally, other vaccine candidates that are currently undergoing clinical development are highlighted. Future approaches, which may include antigens that are genetically conserved across Salmonella and confer broad, non-serotype-specific protection, are also discussed.

Xenosiderophore Utilization Promotes Bacteroides thetaiotaomicron Resilience during Colitis

During short-lived perturbations, such as inflammation, the gut microbiota exhibits resilience and reverts to its original configuration. Although microbial access to the micronutrient iron is decreased during colitis, pathogens can scavenge iron by using siderophores. How commensal bacteria acquire iron during gut inflammation is incompletely understood. Curiously, the human commensal Bacteroides thetaiotaomicron does not produce siderophores but grows under iron-limiting conditions using enterobacterial siderophores. Using RNA-seq, we identify B. thetaiotaomicron genes that were upregulated during Salmonella-induced gut inflammation and were predicted to be involved in iron uptake. Mutants in the xusABC locus (BT2063-2065) were defective for xenosiderophore-mediated iron uptake in vitro. In the normal mouse gut, the XusABC system was dispensable, while a xusA mutant colonized poorly during colitis. This work identifies xenosiderophore utilization as a critical mechanism for B. thetaiotaomicron to sustain colonization during inflammation and suggests a mechanism of how interphylum iron metabolism contributes to gut microbiota resilience.

Scavenging Bacterial Siderophores with Engineered Lipocalin Proteins as an Alternative Antimicrobial Strategy

Iron acquisition mediated by siderophores, high-affinity chelators for which bacteria have evolved specific synthesis and uptake mechanisms, plays a crucial role in microbiology and in host-pathogen interactions. In the ongoing fight against bacterial infections, this area has attracted biomedical interest. Beyond several approaches to interfere with siderophore-mediated iron uptake from medicinal and immunochemistry, the development of high-affinity protein scavengers that tightly complex the siderophores produced by pathogenic bacteria has appeared as a novel strategy. Such binding proteins have been engineered based on siderocalin-also known as lipocalin 2-an endogenous human scavenger of enterobactin and bacillibactin that controls the systemic spreading of commensal bacteria such as Escherichia coli. By using combinatorial protein design, siderocalin was reshaped to bind several siderophores from Pseudomonas aeruginosa and, in particular, petrobactin from Bacillus anthracis, none of which is recognized by the natural protein. Such engineered versions of siderocalin effectively suppress the growth of corresponding pathogenic bacteria by depriving them of their iron supply and offer the potential to complement antibiotic therapy in situations of acute or persistent infection.

The FeoC [4Fe-4S] Cluster Is Redox-Active and Rapidly Oxygen-Sensitive

The acquisition of iron is essential to establishing virulence among most pathogens. Under acidic and/or anaerobic conditions, most bacteria utilize the widely distributed ferrous iron (Fe²⁺) uptake (Feo) system to import metabolically-required iron. The Feo system is inadequately understood at the atomic, molecular, and mechanistic levels, but we do know it is composed of a main membrane component (FeoB) essential for iron translocation, as well as two small, cytosolic proteins (FeoA and FeoC) hypothesized to function as accessories to this process. FeoC has many hypothetical functions, including that of an iron-responsive transcriptional regulator. Here, we demonstrate for the first time that Escherichia coli FeoC (EcFeoC) binds an [Fe-S] cluster. Using electronic absorption, X-ray absorption, and electron paramagnetic resonance spectroscopies, we extensively characterize the nature of this cluster. Under strictly anaerobic conditions after chemical reconstitution, we demonstrate that EcFeoC binds a redox-active [4Fe-4S]^2+/+ cluster that is rapidly oxygen-sensitive and decays to a [2Fe-2S]²⁺ cluster (t_1/2 ≈ 20 s), similar to the [Fe-S] cluster in the fumarate and nitrate reductase (FNR) transcriptional regulator. We further show that this behavior is nearly identical to the homologous K. pneumoniae FeoC, suggesting a redox-active, oxygen-sensitive [4Fe-4S]²⁺ cofactor is a general phenomenon of cluster-binding FeoCs. Finally, in contrast to FNR, we show that the [4Fe-4S]²⁺ cluster binding to FeoC is associated with modest conformational changes of the polypeptide, but not protein dimerization. We thus posit a working hypothesis in which the cluster-binding FeoCs may function as oxygen-sensitive iron sensors that fine-tune pathogenic ferrous iron acquisition.

Enterobactin-Specific Antibodies Induced by a Novel Enterobactin Conjugate Vaccine

Enterobactin (Ent)-mediated high-affinity iron acquisition is critical for Gram-negative bacteria to survive in the host. Given the bacteriostatic effect of lipocalin resulting from its potent Ent-binding ability, immune intervention directly targeting Ent is promising for iron-dependent pathogen control. Recently, an Ent conjugate vaccine was reported, but it still has several significant weaknesses. In this study, we sought to develop an innovative Ent conjugate vaccine that can induce a high level of antibodies directed against Ent and to provide solid evidence demonstrating siderophore-binding capacity of Ent-specific antibodies. Using a simple method, we successfully conjugated purified Ent to different carriers, including keyhole limpet hemocyanin (KLH), bovine serum albumin, and CmeC, a vaccine candidate for Campylobacter control. Subcutaneous immunization of rabbits with the KLH-Ent conjugate triggered a strong systemic IgG immune response with an up to 16,384-fold increase in IgG titer directed against whole conjugate and an up to 4,096-fold increase in the level of specific anti-Ent IgG. To evaluate the ability of Ent-specific IgG to bind to the Ent derivatives present in vivo, various Ent derivatives were chemically synthesized and a unique enzyme-linked immunosorbent assay method was developed. The Ent-specific IgG also displayed exceptional reactivity to ferric Ent, a linear trimer of Ent, and different salmochelins. Growth assays further demonstrated that the Ent-specific antibodies significantly inhibited Ent-dependent growth of Campylobacter spp. and Escherichia coli Collectively, this study reports an efficient method to prepare a new type of Ent conjugate vaccines for inducing a high level of Ent-specific antibodies, which can bind to various Ent derivatives and display lipocalin-like bacteriostatic features.IMPORTANCE Ent-mediated high-affinity iron acquisition is a universal and critical contributor for Gram-negative pathogens to survive and infect hosts. Published information has supported an innovative immune intervention strategy that directly targets Ent to starve pathogens by limiting the availability of iron to be utilized. Compared to a recently published Ent conjugate, there are three advantages of the vaccine described in this study: ease of preparation, induction of high titer of anti-Ent IgG, and the ability of Ent-specific antibodies to bind various Ent derivatives, including the salmochelins that help enteric pathogens evade sequestration of siderophores by host lipocalins. In addition, the Ent-specific antibodies were demonstrated to function similarly to lipocalin to interfere with the Ent-dependent growth of Campylobacter and E. coli under iron-restricted conditions. This study has significant potential for broader applications to prevent and control various Gram-negative infections in humans and animals.

Dopamine Is a Siderophore-Like Iron Chelator That Promotes Salmonella enterica Serovar Typhimurium Virulence in Mice

We have recently shown that the catecholamine dopamine regulates cellular iron homeostasis in macrophages. As iron is an essential nutrient for microbes, and intracellular iron availability affects the growth of intracellular bacteria, we studied whether dopamine administration impacts the course of Salmonella infections. Dopamine was found to promote the growth of Salmonella both in culture and within bone marrow-derived macrophages, which was dependent on increased bacterial iron acquisition. Dopamine administration to mice infected with Salmonella enterica serovar Typhimurium resulted in significantly increased bacterial burdens in liver and spleen, as well as reduced survival. The promotion of bacterial growth by dopamine was independent of the siderophore-binding host peptide lipocalin-2. Rather, dopamine enhancement of iron uptake requires both the histidine sensor kinase QseC and bacterial iron transporters, in particular SitABCD, and may also involve the increased expression of bacterial iron uptake genes. Deletion or pharmacological blockade of QseC reduced but did not abolish the growth-promoting effects of dopamine. Dopamine also modulated systemic iron homeostasis by increasing hepcidin expression and depleting macrophages of the iron exporter ferroportin, which enhanced intracellular bacterial growth. Salmonella lacking all central iron uptake pathways failed to benefit from dopamine treatment. These observations are potentially relevant to critically ill patients, in whom the pharmacological administration of catecholamines to improve circulatory performance may exacerbate the course of infection with siderophilic bacteria.IMPORTANCE Here we show that dopamine increases bacterial iron incorporation and promotes Salmonella Typhimurium growth both in vitro and in vivo These observations suggest the potential hazards of pharmacological catecholamine administration in patients with bacterial sepsis but also suggest that the inhibition of bacterial iron acquisition might provide a useful approach to antimicrobial therapy.

The Iron Tug-of-War between Bacterial Siderophores and Innate Immunity

Iron is necessary for the survival of almost all aerobic organisms. In the mammalian host, iron is a required cofactor for the assembly of functional iron-sulfur (Fe-S) cluster proteins, heme-binding proteins and ribonucleotide reductases that regulate various functions, including heme synthesis, oxygen transport and DNA synthesis. However, the bioavailability of iron is low due to its insolubility under aerobic conditions. Moreover, the host coordinates a nutritional immune response to restrict the accessibility of iron against potential pathogens. To counter nutritional immunity, most commensal and pathogenic bacteria synthesize and secrete small iron chelators termed siderophores. Siderophores have potent affinity for iron, which allows them to seize the essential metal from the host iron-binding proteins. To safeguard against iron thievery, the host relies upon the innate immune protein, lipocalin 2 (Lcn2), which could sequester catecholate-type siderophores and thus impede bacterial growth. However, certain bacteria are capable of outmaneuvering the host by either producing "stealth" siderophores or by expressing competitive antagonists that bind Lcn2 in lieu of siderophores. In this review, we summarize the mechanisms underlying the complex iron tug-of-war between host and bacteria with an emphasis on how host innate immunity responds to siderophores.

Interbacterial mechanisms of colonization resistance and the strategies pathogens use to overcome them

The communities of bacteria that reside in the intestinal tract are in constant competition within this dynamic and densely colonized environment. At homeostasis, the equilibrium that exists between these species and strains is shaped by their metabolism and also by pathways of active antagonism, which drive competition with related and unrelated strains. Importantly, these normal activities contribute to colonization resistance by the healthy microbiota, which includes the ability to prevent the expansion of potential pathogens. Disruption of the microbiota, resulting from, for example, inflammation or antibiotic use, can reduce colonization resistance. Pathogens that engraft following disruption of the microbiota are often adapted to expand into newly created niches and compete in an altered gut environment. In this review, we examine both the interbacterial mechanisms of colonization resistance and the strategies of pathogenic strains to exploit gaps in colonization resistance.

Pathogens, microbiome and the host: emergence of the ecological Koch's postulates

Even though tremendous progress has been made in the last decades to elucidate the mechanisms of intestinal homeostasis, dysbiosis and disease, we are only at the beginning of understanding the complexity of the gut ecosystem and the underlying interaction networks. We are also only starting to unravel the mechanisms that pathogens have evolved to overcome the barriers imposed by the microbiota and host to exploit the system to their own benefit. Recent work in these domains clearly indicates that the 'traditional Koch's postulates', which state that a given pathogen leads to a distinct disease, are not valid for all 'infectious' diseases, but that a more complete and complex interpretation of Koch's postulates is needed in order to understand and explain them. This review summarises the current understanding of what defines a healthy gut ecosystem and highlights recent progress in uncovering the interplay between the host, its microbiota and invading intestinal pathogens. Based on these recent findings, we propose a new interpretation of Koch's postulates that we term 'ecological Koch's postulates'.

Modulation of the gut microbiota to improve innate resistance

One major benefit from the association of hosts with the complex microbial communities that establish at body surfaces is the resistance to pathogen infection. This protective role of symbiotic microbes is becoming ever more relevant, given the alarming rise of multidrug-resistant pathogens and severe infections in patients following extensive antibiotic treatment. Herein, we highlight some recent mechanistic studies that have provided insights into how the highly dynamic dialogue amongst intestinal bacteria and between intestinal bacteria and their host can contribute to protect the host against pathogens in and outside the gut. We then discuss how delineating the rules of this dialogue can help design strategies to modulate the microbiota and improve host resistance to infections.

Development of a High-Throughput Biochemical Assay to Screen for Inhibitors of Aerobactin Synthetase IucA

Acquiring sufficient quantities of iron to support survival is often a critical limitation for pathogenic bacteria. To meet this demand, bacteria have evolved unique strategies to scavenge iron and circumvent the nutritional immunity exerted by their hosts. One common strategy, which is often a key virulence factor for bacterial pathogens, involves the synthesis, secretion, and reuptake of iron chelators known as siderophores. In vitro and in vivo studies have demonstrated that the siderophore aerobactin is critical for virulence in the hypervirulent pathotype of Klebsiella pneumoniae (hvKP). Given the high rate of multidrug resistance in K. pneumoniae, and in light of the ever-increasing demand for novel Gram-negative therapeutic targets, we identified aerobactin production as a promising antivirulence target in hvKP. Herein, we describe the development of a high-throughput biochemical assay for identifying inhibitors of the aerobactin synthetase IucA. The assay was employed to screen ~110,000 compounds across several commercially available small-molecule libraries. IucA inhibitors with activity at micromolar concentrations were identified in our screening campaigns and confirmed using secondary orthogonal assays. However, the most potent compounds also exhibited some properties commonly observed with promiscuous/nonspecific inhibitors, including incubation time and target enzyme concentration dependence, as well as the potential to antagonize unrelated enzymes.

KCa(H2 O)2 [FeIII (CN)6 ]⋅H2 O Nanoparticles as an Antimicrobial Agent against Staphylococcus aureus

Biocompatible nanoparticles based on a calcium analogue of Prussian blue were designed and synthesized to take advantage of their ability to penetrate the cell membrane in Staphylococcus aureus and to undergo selective ion exchange with intracellular iron to disrupt iron metabolism in such pathogenic bacteria for antibacterial applications. KCa(H₂ O)₂ [Fe^III (CN)₆ ]⋅H₂ O nanoparticles penetrate the bacterial cell membrane and sequester intracellular iron by ion exchange to form insoluble Prussian blue, thus inhibiting bacterial growth.

Bacteriocin biosynthesis contributes to the anti-inflammatory capacities of probiotic Lactobacillus plantarum

Plantaricin EF (PlnEF) is a class IIb bacteriocin produced by Lactobacillus plantarum. We compared L. plantarum NCIMB8826 and LM0419, a plnEFI deletion mutant of that strain lacking plnEF and the gene for the cognate immunity protein plnI, in a 2,4,6-trinitrobenzenesulfonic acid (TNBS) induced mouse model of acute inflammatory bowel disease. Mice fed either L. plantarum NCIMB8826 or LM0419 were not protected against TNBS according to either disease activity or histology (Ameho) scores. Mice consuming NCIMB8826 exhibited intermediate (non-significant) levels of colonic tumour necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) that ranged between the TNBS-treated animals and healthy controls. By comparison, TNF-α and IL-6 quantities were elevated in mice given L. plantarum LM0419 and equivalent to mice given TNBS alone. Both strains survived digestive tract transit in equal numbers and did not result in global changes to the bacterial composition in the intestine according to 16S rRNA gene sequencing either prior to or after TNBS administration. Examination of intestinal taxa showed that mice consuming wild-type L. plantarum, but not LM0419 contained lower proportions of Mucispirillum (Deferribacteres phylum) in the faeces prior to TNBS administration and Parabacteroides (Bacteroidetes phylum) in the caecum after disease induction. Parabacteroides also positively correlated with disease activity and histology scores. These findings suggest a role for PlnEFI production by L. plantarum in benefiting digestive tract health.

Biotechnology of siderophores in high-impact scientific fields

Different aspects of bacterial and fungal siderophore biotechnological applications will be discussed. Areas of application presented include, but are not limited to agriculture, medicine, pharmacology, bioremediation, biodegradation and food industry. In agriculture-related applications, siderophores could be employed to enhance plant growth due to their uptake by rhizobia. Siderophores hindered the presence of plant pathogens in biocontrol strategies. Bioremediation studies on siderophores discuss mostly the mobilization of heavy metals and radionuclides; the emulsifying effects of siderophore-producing microorganisms in oil-contaminated environments are also presented. The different applications found in literature based in medicine and pharmacological approaches range from iron overload to drug delivery systems and, more recently, vaccines. Additional research should be done in siderophore production and their metabolic relevance to have a deeper understanding for future biotechnological advances.

Vaccination against Salmonella Infection: the Mucosal Way

Salmonella enterica subspecies enterica includes several serovars infecting both humans and other animals and leading to typhoid fever or gastroenteritis. The high prevalence of associated morbidity and mortality, together with an increased emergence of multidrug-resistant strains, is a current global health issue that has prompted the development of vaccination strategies that confer protection against most serovars. Currently available systemic vaccine approaches have major limitations, including a reduced effectiveness in young children and a lack of cross-protection among different strains. Having studied host-pathogen interactions, microbiologists and immunologists argue in favor of topical gastrointestinal administration for improvement in vaccine efficacy. Here, recent advances in this field are summarized, including mechanisms of bacterial uptake at the intestinal epithelium, the assessment of protective host immunity, and improved animal models that closely mimic infection in humans. The pros and cons of existing vaccines are presented, along with recent progress made with novel formulations. Finally, new candidate antigens and their relevance in the refined design of anti-Salmonella vaccines are discussed, along with antigen vectorization strategies such as nanoparticles or secretory immunoglobulins, with a focus on potentiating mucosal vaccine efficacy.

Salmonella Typhimurium Diarrhea Reveals Basic Principles of Enteropathogen Infection and Disease-Promoted DNA Exchange

Despite decades of research, efficient therapies for most enteropathogenic bacteria are still lacking. In this review, we focus on Salmonella enterica Typhimurium (S. Typhimurium), a frequent cause of acute, self-limiting food-borne diarrhea and a model that has revealed key principles of enteropathogen infection. We review the steps of gut infection and the mucosal innate-immune defenses limiting pathogen burdens, and we discuss how inflammation boosts gut luminal S. Typhimurium growth. We also discuss how S. Typhimurium-induced inflammation accelerates the transfer of plasmids and phages, which may promote the transmission of antibiotic resistance and facilitate emergence of pathobionts and pathogens with enhanced virulence. The targeted manipulation of the microbiota and vaccination might offer strategies to prevent this evolution. As gut luminal microbes impact various aspects of the host's physiology, improved strategies for preventing enteropathogen infection and disease-inflicted DNA exchange may be of broad interest well beyond the acute infection.

A New Way to Beat Intestinal Pathogens

In the gastrointestinal tract, the tug of war for iron may provide a new way to vaccinate. Recent work shows that immunizing mice with siderophores (small molecules that microbes produce to capture iron) foils pathogen colonization and may instead allow a commensal to expand.