Structural basis for haem piracy from host haemopexin by Haemophilus influenzae

Prevalence of Slam-dependent hemophilins in Gram-negative bacteria

Iron acquisition systems are crucial for pathogen growth and survival in iron-limiting host environments. To overcome nutritional immunity, bacterial pathogens evolved to use diverse mechanisms to acquire iron. Here, we examine a heme acquisition system that utilizes hemophores called hemophilins which are also referred to as HphAs in several Gram-negative bacteria. In this study, we report three new HphA structures from Stenotrophomonas maltophilia, Vibrio harveyi, and Haemophilus parainfluenzae. Structural determination of HphAs revealed an N-terminal clamp-like domain that binds heme and a C-terminal eight-stranded β-barrel domain that shares the same architecture as the Slam-dependent Neisserial surface lipoproteins. The genetic organization of HphAs consists of genes encoding a Slam homolog and a TonB-dependent receptor (TBDR). We investigated the Slam-HphA system in the native organism or the reconstituted system in Escherichia coli cells and found that the efficient secretion of HphA depends on Slam. The TBDR also played an important role in heme uptake and conferred specificity for its cognate HphA. Furthermore, bioinformatic analysis of HphA homologs revealed that HphAs are conserved in the alpha, beta, and gammaproteobacteria. Together, these results show that the Slam-dependent HphA-type hemophores are prevalent in Gram-negative bacteria and further expand the role of Slams in transporting soluble proteins.

IMPORTANCE

This paper describes the structure and function of a family of Slam (Type IX secretion System) secreted hemophores that bacteria use to uptake heme (iron) while establishing an infection. Using structure-based bioinformatics analysis to define the diversity and prevalence of this heme acquisition pathway, we discovered that a large portion of gammaproteobacterial harbors this system. As organisms, including Acinetobacter baumannii, utilize this system to facilitate survival during host invasion, the identification of this heme acquisition system in bacteria species is valuable information and may represent a target for antimicrobials.

Repeat modules and N-linked glycans define structure and antigenicity of a critical enterotoxigenic E. coli adhesin

Enterotoxigenic Escherichia coli (ETEC) cause hundreds of millions of cases of infectious diarrhea annually, predominantly in children from low-middle income regions. Notably, in children, as well as human volunteers challenged with ETEC, diarrheal severity is significantly increased severity in blood group A (bgA) individuals. EtpA, is a secreted glycoprotein adhesin that functions as a blood group A lectin to promote critical interactions between ETEC and blood group A glycans on intestinal epithelia for effective bacterial adhesion and toxin delivery. EtpA is highly immunogenic resulting in robust antibody responses following natural infection and experimental challenge of human volunteers with ETEC. To understand how EtpA directs ETEC-blood group A interactions and stimulates adaptive immunity, we mutated EtpA, mapped its glycosylation by mass-spectrometry (MS), isolated polyclonal (pAbs) and monoclonal antibodies (mAbs) from vaccinated mice and ETEC-infected human volunteers, and determined structures of antibody-EtpA complexes by cryo-electron microscopy. Both bgA and mAbs that inhibited EtpA-bgA interactions and ETEC adhesion, bound to the C-terminal repeat domain highlighting this region as crucial for ETEC pathogen-host interaction. MS analysis uncovered extensive and heterogeneous N-linked glycosylation of EtpA and cryo-EM structures revealed that mAbs directly engage these unique glycan containing epitopes. Finally, electron microscopy-based polyclonal epitope mapping revealed antibodies targeting numerous distinct epitopes on N and C-terminal domains, suggesting that EtpA vaccination generates responses against neutralizing and decoy regions of the molecule. Collectively, we anticipate that these data will inform our general understanding of pathogen-host glycan interactions and adaptive immunity relevant to rational vaccine subunit design.

Structural and biophysical characterization of the secreted, β-helical adhesin EtpA of Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a diarrhoeal pathogen associated with high morbidity and mortality especially among young children in developing countries. At present, there is no vaccine for ETEC. One candidate vaccine antigen, EtpA, is a conserved secreted adhesin that binds to the tips of flagellae to bridge ETEC to host intestinal glycans. EtpA is exported through a Gram-negative, two-partner secretion system (TPSS, type Vb) comprised of the secreted EtpA passenger (TpsA) protein and EtpB (TpsB) transporter that is integrated into the outer bacterial membrane. TpsA proteins share a conserved, N-terminal TPS domain followed by an extensive C-terminal domain with divergent sequence repeats. Two soluble, N-terminal constructs of EtpA were prepared and analysed respectively including residues 67 to 447 (EtpA67-447) and 1 to 606 (EtpA1-606). The crystal structure of EtpA67-447 solved at 1.76 Å resolution revealed a right-handed parallel β-helix with two extra-helical hairpins and an N-terminal β-strand cap. Analyses by circular dichroism spectroscopy confirmed the β-helical fold and indicated high resistance to chemical and thermal denaturation as well as rapid refolding. A theoretical AlphaFold model of full-length EtpA largely concurs with the crystal structure adding an extended β-helical C-terminal domain after an interdomain kink. We propose that robust folding of the TPS domain upon secretion provides a template to extend the N-terminal β-helix into the C-terminal domains of TpsA proteins.

ppe51 Variants Enable Growth of Mycobacterium tuberculosis at Acidic pH by Selectively Promoting Glycerol Uptake

In defined media supplemented with single carbon sources, Mycobacterium tuberculosis (Mtb) exhibits carbon source specific growth restriction. When supplied with glycerol as the sole carbon source at pH 5.7, Mtb establishes a metabolically active state of nonreplicating persistence known as acid growth arrest. We hypothesized that acid growth arrest on glycerol is not a metabolic restriction, but rather an adaptive response. To test this hypothesis, we selected for and identified several Mtb mutants that could grow under these restrictive conditions. All mutations were mapped to the ppe51 gene and resulted in variants with 3 different amino acid substitutions- S211R, E215K, and A228D. Expression of the ppe51 variants in Mtb promoted growth at acidic pH showing that the mutant alleles are sufficient to cause the dominant gain-of-function, Enhanced Acid Growth (EAG) phenotype. Testing growth on other single carbon sources showed the PPE51 variants specifically enhanced growth on glycerol, suggesting PPE51 plays a role in glycerol uptake. Using radiolabeled glycerol, enhanced glycerol uptake was observed in Mtb expressing the PPE51 (S211R) variant, with glycerol overaccumulation in triacylglycerol. Notably, the EAG phenotype is deleterious for growth in macrophages, where the mutants have selectively faster replication and reduced survival in activated macrophages compared to resting macrophages. Recombinant PPE51 protein exhibited differential thermostability in the wild type (WT) or S211R variants in the presence of glycerol, supporting the model that EAG substitutions alter PPE51-glycerol interactions. Together, these findings support that PPE51 variants selectively promote glycerol uptake and that slowed growth at acidic pH is an important adaptive mechanism required for macrophage pathogenesis. IMPORTANCE It is puzzling why Mycobacterium tuberculosis (Mtb) cannot grow on glycerol at acidic pH, as it has a carbon source and oxygen, everything it needs to grow. In this study, we found that Mtb limits uptake of glycerol at acidic pH to restrict its growth and that mutations in ppe51 promote uptake of glycerol at acidic pH and enable growth. That is, Mtb can grow well at acidic pH on glycerol, but has adapted instead to stop growth. Notably, ppe51 variants exhibit enhanced replication and reduced survival in activated macrophages, supporting a role for pH-dependent slowed growth during macrophage pathogenesis.

High-Level Expression of Cell-Surface Signaling System Hxu Enhances Pseudomonas aeruginosa Bloodstream Infection

Bloodstream infections (BSIs) caused by Pseudomonas aeruginosa are associated with a high mortality rate in the clinic. However, the fitness mechanisms responsible for the evolution of virulence factors that facilitate the dissemination of P. aeruginosa to the bloodstream are poorly understood. In this study, a transcriptomic analysis of the BSI-associated P. aeruginosa clinical isolates showed a high-level expression of cell-surface signaling (CSS) system Hxu. Whole-genome sequencing and comparative genomics of these isolates showed that a mutation in rnfE gene was responsible for the elevated expression of the Hxu-CSS pathway. Most importantly, deletion of the hxuIRA gene cluster in a laboratory strain PAO1 reduced its BSI capability while overexpression of the HxuIRA pathway promoted BSI in a murine sepsis model. We further demonstrated that multiple components in the blood plasma, including heme, hemoglobin, the heme-scavenging proteins haptoglobin, and hemopexin, as well as the iron-delivery protein transferrin, could activate the Hxu system. Together, these studies suggested that the Hxu-CSS system was an important signal transduction pathway contributing to the adaptive pathogenesis of P. aeruginosa in BSI.

A zero-sum game or an interactive frame? Iron competition between bacteria and humans in infection war

ABSTRACT

Iron is an essential trace element for both humans and bacteria. It plays a vital role in life, such as in redox reactions and electron transport. Strict regulatory mechanisms are necessary to maintain iron homeostasis because both excess and insufficient iron are harmful to life. Competition for iron is a war between humans and bacteria. To grow, reproduce, colonize, and successfully cause infection, pathogens have evolved various mechanisms for iron uptake from humans, principally Fe 3+ -siderophore and Fe 2+ -heme transport systems. Humans have many innate immune mechanisms that regulate the distribution of iron and inhibit bacterial iron uptake to help resist bacterial invasion and colonization. Meanwhile, researchers have invented detection test strips and coupled antibiotics with siderophores to create tools that take advantage of this battle for iron, to help eliminate pathogens. In this review, we summarize bacterial and human iron metabolism, competition for iron between humans and bacteria, siderophore sensors, antibiotics coupled with siderophores, and related phenomena. We also discuss how competition for iron can be used for diagnosis and treatment of infection in the future.

Identification of a secretory heme-binding protein from Nocardia seriolae involved in cell apoptosis

According to the whole-genome bioinformatics analysis, a heme-binding protein from Nocardia seriolae (HBP) was found. HBP was predicted to be a bacterial secretory protein, located at mitochondrial membrane in eukaryotic cells and have a similar protein structure with the heme-binding protein of Mycobacterium tuberculosis, Rv0203. In this study, HBP was found to be a secretory protein and co-localized with mitochondria in FHM cells. Quantitative analysis of mitochondrial membrane potential value, caspase-3 activity, and transcription level of apoptosis-related genes suggested that overexpression of HBP protein can induce cell apoptosis. In conclusion, HBP was a secretory protein which may target to mitochondria and involve in cell apoptosis in host cells. This research will promote the function study of HBP and deepen the comprehension of the virulence factors and pathogenic mechanisms of N. seriolae.

Genomic erosion and horizontal gene transfer shape functional differences of the ExlA toxin in Pseudomonas spp

Two-partner secretion (TPS) is widespread in the bacterial world. The pore-forming TPS toxin ExlA of Pseudomonas aeruginosa is conserved in pathogenic and environmental Pseudomonas. While P. chlororaphis and P. entomophila displayed ExlA-dependent killing, P. putida did not cause damage to eukaryotic cells. ExlA proteins interacted with epithelial cell membranes; however, only ExlA^Pch induced the cleavage of the adhesive molecule E-cadherin. ExlA proteins participated in insecticidal activity toward the larvae of Galleria mellonella and the fly Drosophila melanogaster. Evolutionary analyses demonstrated that the differences in the C-terminal domains are partly due to horizontal movements of the operon within the genus Pseudomonas. Reconstruction of the evolutionary history revealed the complex horizontal acquisitions. Together, our results provide evidence that conserved TPS toxins in environmental Pseudomonas play a role in bacteria-insect interactions and discrete differences in CTDs may determine their specificity and mode of action toward eukaryotic cells.

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ExlA is a two-partner secreted toxin conserved across Pseudomonas spp.

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Environmental Pseudomonas strains encode ExlA with different cytotoxic activities

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ExlA of environmental Pseudomonas strains play a role in bacteria-insect interactions

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ExlBA operon shows a complex evolutionary history of horizontal gene transfer

Iron-Deficiency in Atopic Diseases: Innate Immune Priming by Allergens and Siderophores

Although iron is one of the most abundant elements on earth, about a third of the world's population are affected by iron deficiency. Main drivers of iron deficiency are beside the chronic lack of dietary iron, a hampered uptake machinery as a result of immune activation. Macrophages are the principal cells distributing iron in the human body with their iron restriction skewing these cells to a more pro-inflammatory state. Consequently, iron deficiency has a pronounced impact on immune cells, favoring Th2-cell survival, immunoglobulin class switching and primes mast cells for degranulation. Iron deficiency during pregnancy increases the risk of atopic diseases in children, while both children and adults with allergy are more likely to have anemia. In contrast, an improved iron status seems to protect against allergy development. Here, the most important interconnections between iron metabolism and allergies, the effect of iron deprivation on distinct immune cell types, as well as the pathophysiology in atopic diseases are summarized. Although the main focus will be humans, we also compare them with innate defense and iron sequestration strategies of microbes, given, particularly, attention to catechol-siderophores. Similarly, the defense and nutritional strategies in plants with their inducible systemic acquired resistance by salicylic acid, which further leads to synthesis of flavonoids as well as pathogenesis-related proteins, will be elaborated as both are very important for understanding the etiology of allergic diseases. Many allergens, such as lipocalins and the pathogenesis-related proteins, are able to bind iron and either deprive or supply iron to immune cells. Thus, a locally induced iron deficiency will result in immune activation and allergic sensitization. However, the same proteins such as the whey protein beta-lactoglobulin can also transport this precious micronutrient to the host immune cells (holoBLG) and hinder their activation, promoting tolerance and protecting against allergy. Since 2019, several clinical trials have also been conducted in allergic subjects using holoBLG as a food for special medical purposes, leading to a reduction in the allergic symptom burden. Supplementation with nutrient-carrying lipocalin proteins can circumvent the mucosal block and nourish selectively immune cells, therefore representing a new dietary and causative approach to compensate for functional iron deficiency in allergy sufferers.

The Small RNA CyaR Activates Translation of the Outer Membrane Haem Receptor chuA in Enterohemorrhagic Escherichia coli

To sense the transition from environment to host, bacteria use a range of environmental cues to control expression of virulence genes. Iron is tightly sequestered in host tissues and in the human pathogen enterohemorrhagic Escherichia coli (EHEC) iron-limitation induces transcription of the outer membrane haem transporter encoded by chuAS. ChuA expression is post-transcriptionally activated at 37°C by a FourU RNA thermometer ensuring that the haem receptor is only expressed under low iron, high temperature conditions that indicate the host. Here we demonstrate that expression of chuA is also independently regulated by the cAMP-responsive small RNA (sRNA) CyaR and transcriptional terminator Rho. These results indicate that chuAS expression is regulated at the transcription initiation, transcript elongation, and translational level. We speculate that additional sensing of the gluconeogenic environment allows further precision in determining when EHEC is at the gastrointestinal epithelium of the host. With previous studies, it appears that the chuAS transcript is controlled by eight regulatory inputs that control expression through six different transcriptional and post-transcriptional mechanisms. The results highlight the ability of regulatory sRNAs to integrate multiple environmental signals into a layered hierarchy of signal input.

ECF Sigma Factor HxuI Is Critical for In Vivo Fitness of Pseudomonas aeruginosa during Infection

The opportunistic pathogen Pseudomonas aeruginosa often adapts to its host environment and causes recurrent nosocomial infections. The extracytoplasmic function (ECF) sigma factor enables bacteria to alter their gene expression in response to host environmental stimuli. Here, we report an ECF sigma factor, HxuI, which is rapidly induced once P. aeruginosa encounters the host. Host stresses such as iron limitation, oxidative stress, low oxygen, and nitric oxide induce the expression of hxuI. By combining RNA-seq and promoter-lacZ reporter fusion analysis, we reveal that HxuI can activate the expression of diverse metabolic and virulence pathways which are critical to P. aeruginosa infections, including iron acquisition, denitrification, pyocyanin synthesis, and bacteriocin production. Most importantly, overexpression of the hxuI in the laboratory strain PAO1 promotes its colonization in both murine lung and subcutaneous infections. Together, our findings show that HxuI, a key player in host stress-response, controls the in vivo adaptability and virulence of P. aeruginosa during infection. IMPORTANCE P. aeruginosa has a strong ability to adapt to diverse environments, making it capable of causing recurrent and multisite infections in clinics. Understanding host adaptive mechanisms plays an important guiding role in the development of new anti-infective agents. Here, we demonstrate that an ECFσ factor of P. aeruginosa response to the host-inflicted stresses, which promotes the bacterial in vivo fitness and pathogenicity. Furthermore, our findings may help explain the emergence of highly transmissible strains of P. aeruginosa and the acute exacerbations during chronic infections.

A novel class of Candida glabrata cell wall proteins with β-helix fold mediates adhesion in clinical isolates

Candida glabrata is an opportunistic pathogenic yeast frequently causing infections in humans. Though it lacks typical virulence factors such as hyphal development, C. glabrata contains a remarkably large and diverse set of putative wall adhesins that is crucial for its success as pathogen. Here, we present an analysis of putative adhesins from the homology clusters V and VI. First, sequence similarity network analysis revealed relationships between cluster V and VI adhesins and S. cerevisiae haze protective factors (Hpf). Crystal structures of A-regions from cluster VI adhesins Awp1 and Awp3b reveal a parallel right-handed β-helix domain that is linked to a C-terminal β-sandwich. Structure solution of the A-region of Awp3b via single wavelength anomalous diffraction phasing revealed the largest known lanthanide cluster with 21 Gd³⁺ ions. Awp1-A and Awp3b-A show structural similarity to pectate lyases but binding to neither carbohydrates nor Ca²⁺ was observed. Phenotypic analysis of awp1Δ, awp3Δ, and awp1,3Δ double mutants did also not confirm their role as adhesins. In contrast, deletion mutants of the cluster V adhesin Awp2 in the hyperadhesive clinical isolate PEU382 demonstrated its importance for adhesion to polystyrene or glass, biofilm formation, cell aggregation and other cell surface-related phenotypes. Together with cluster III and VII adhesins our study shows that C. glabrata CBS138 can rely on a set of 42 Awp1-related adhesins with β-helix/α-crystallin domain architecture for modifying the surface characteristics of its cell wall.

Adhesion to host cells and abiotic, often hydrophobic surfaces, e.g. that of medical equipment like catheters, is an indispensable virulence factor for many pathogenic fungi. Among the latter, the yeast Candida glabrata excels by encoding in its genome large sets of surface-exposed cell wall proteins. Here, we show that in the clinical isolate PEU382 of C. glabrata, hyper-adhesiveness to plastics and the tendency to biofilm formation is conferred by a single adhesin, Awp2. An integrative bioinformatic and structural analysis of this and the related Awp1 and Awp3 adhesins unifies four, so far separately assigned Awp clusters—III, V, VI and VII–into one consisting of 42 Awp1-related adhesins. These adhesins commonly present an N-terminal module consisting of a right-handed β-helix and an α-crystallin domain on the yeast surface and use a calcium-independent mode for adhesion. Their sheer number contrasts to the 20 members of the well characterized Epa and 7 members of the Pwp family of surface proteins. Given these findings we suggest that C. glabrata utilizes just two structurally distinct motifs for colonizing different host niches by adhesion: the β-helix/α-crystallin module of Awp1-related adhesins and the C-type lectin PA14-domain for Epa and Pwp proteins.

Phase Variation in HMW1A Controls a Phenotypic Switch in Haemophilus influenzae Associated with Pathoadaptation during Persistent Infection

Genetic variants arising from within-patient evolution shed light on bacterial adaptation during chronic infection. Contingency loci generate high levels of genetic variation in bacterial genomes, enabling adaptation to the stringent selective pressures exerted by the host. A significant gap in our understanding of phase-variable contingency loci is the extent of their contribution to natural infections. The human-adapted pathogen nontypeable Haemophilus influenzae (NTHi) causes persistent infections, which contribute to underlying disease progression. The phase-variable high-molecular-weight (HMW) adhesins located on the NTHi surface mediate adherence to respiratory epithelial cells and, depending on the allelic variant, can also confer high epithelial invasiveness or hyperinvasion. In this study, we characterize the dynamics of HMW-mediated hyperinvasion in living cells and identify a specific HMW binding domain shared by hyperinvasive NTHi isolates of distinct pathological origins. Moreover, we observed that HMW expression decreased over time by using a longitudinal set of persistent NTHi strains collected from chronic obstructive pulmonary disease (COPD) patients, resulting from increased numbers of simple-sequence repeats (SSRs) downstream of the functional P2_hmw1A promoter, which is the one primarily driving HMW expression. Notably, the increased SSR numbers at the hmw1 promoter region also control a phenotypic switch toward lower bacterial intracellular invasion and higher biofilm formation, likely conferring adaptive advantages during chronic airway infection by NTHi. Overall, we reveal novel molecular mechanisms of NTHi pathoadaptation based on within-patient lifestyle switching controlled by phase variation.

Teleological cooption of Mycobacterium tuberculosis PE/PPE proteins as porins: Role in molecular immigration and emigration

Permeation through bacterial cells for exchange or uptake of biomolecules and ions invariably depend upon the existence of pore-forming proteins (porins) in their outer membrane. Mycobacterium tuberculosis (M. tb) harbours one of the most rigid cell envelopes across bacterial genera and is devoid of the classical porins for solute transport across the cell membrane. Though canonical porins are incompatible with the evolution of permeability barrier, porin like activity has been reported from membrane preparations of pathogenic mycobacteria. This suggests a sophisticated transport mechanism that has been elusive until now, along with the protein family responsible for it. Recent evidence suggests that these slow-growing mycobacteria have co-opted some of PE/PPE family proteins as molecular transport channels, in place of porins, to facilitate uptake of nutrients required to thrive in the restrictive host environment. These reports advocate that PE/PPE proteins, due to their structural ability, have a potential role in importing small molecules to the cell's interior. This mechanism unveils how a successful pathogen overcomes its restrictive membrane's transport limitations for selective uptake of nutrients. If extrapolated to have a role in drug transport, these channels could help understand the emergence of drug resistance. Further, as these proteins are associated with the export of virulence factors, they can be exploited as novel drug targets. There remains, however, an interesting question that as the PE/PPE proteins can allow the 'import' of molecules from outside the cell, is the reverse transport also possible across the M. tb membrane. In this review, we have discussed recent evidence supporting PE/PPE's role as a specific transport channel for selective uptake of small molecule nutrients and, as possible molecular export machinery of M. tb. This newly discovered role as transmembrane channels demands further research on this enigmatic family of proteins to comprehend the pathomechanism of this very smart pathogen.

Modeled Structure of the Cell Envelope Proteinase of Lactococcus lactis

The cell envelope proteinase (CEP) of Lactococcus lactis is a large extracellular protease covalently linked to the peptidoglycan of the cell wall. Strains of L. lactis are typically auxotrophic for several amino acids and in order to grow to high cell densities in milk they need an extracellular protease. The structure of the entire CEP enzyme is difficult to determine experimentally due to the large size and due to the attachment to the cell surface. We here describe the use of a combination of structure prediction tools to create a structural model for the entire CEP enzyme of Lactococcus lactis. The model has implications for how the bacterium interacts with casein micelles during growth in milk, and it has implications regarding the energetics of the proteolytic system. Our model for the CEP indicates that the catalytic triad is activated through a structural change caused by interaction with the substrate. The CEP of L. lactis might become a useful model for the mode of action for enzymes belonging to the large class of S8 proteinases with a PA (protease associated) domain and a downstream fibronectin like domain.

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.

A Glance on the Role of Bacterial Siderophore from the Perspectives of Medical and Biotechnological Approaches

Iron, which is described as the most basic component found in nature, is hard to be assimilated by microorganisms. It has become increasingly complicated to obtain iron from nature as iron (II) in the presence of oxygen oxidized to press (III) oxide and hydroxide, becoming unsolvable at neutral pH. Microorganisms appeared to produce organic molecules known as siderophores in order to overcome this condition. Siderophore's essential function is to connect with iron (II) and make it dissolvable and enable cell absorption. These siderophores, apart from iron particles, have the ability to chelate various other metal particles that have collocated away to focus the use of siderophores on wound care items. There is a severe clash between the host and the bacterial pathogens during infection. By producing siderophores, small ferric iron-binding molecules, microorganisms obtain iron. In response, host immune cells produce lipocalin 2 to prevent bacterial reuptake of siderophores loaded with iron. Some bacteria are thought to produce lipocalin 2-resistant siderophores to counter this risk. The aim of this article is to discuss the recently described roles and applications of bacterial siderophore.

An ECF-type transporter scavenges heme to overcome iron-limitation in Staphylococcus lugdunensis

Energy-coupling factor type transporters (ECF) represent trace nutrient acquisition systems. Substrate binding components of ECF-transporters are membrane proteins with extraordinary affinity, allowing them to scavenge trace amounts of ligand. A number of molecules have been described as substrates of ECF-transporters, but an involvement in iron-acquisition is unknown. Host-induced iron limitation during infection represents an effective mechanism to limit bacterial proliferation. We identified the iron-regulated ECF-transporter Lha in the opportunistic bacterial pathogen Staphylococcus lugdunensis and show that the transporter is specific for heme. The recombinant substrate-specific subunit LhaS accepted heme from diverse host-derived hemoproteins. Using isogenic mutants and recombinant expression of Lha, we demonstrate that its function is independent of the canonical heme acquisition system Isd and allows proliferation on human cells as sources of nutrient iron. Our findings reveal a unique strategy of nutritional heme acquisition and provide the first example of an ECF-transporter involved in overcoming host-induced nutritional limitation.

Moonlighting of Haemophilus influenzae heme acquisition systems contributes to the host airway-pathogen interplay in a coordinated manner

Nutrient iron sequestration is the most significant form of nutritional immunity and causes bacterial pathogens to evolve strategies of host iron scavenging. Cigarette smoking contains iron particulates altering lung and systemic iron homeostasis, which may enhance colonization in the lungs of patients suffering chronic obstructive pulmonary disease (COPD) by opportunistic pathogens such as nontypeable. NTHi is a heme auxotroph, and the NTHi genome contains multiple heme acquisition systems whose role in pulmonary infection requires a global understanding. In this study, we determined the relative contribution to NTHi airway infection of the four heme-acquisition systems HxuCBA, PE, SapABCDFZ, and HbpA-DppBCDF that are located at the bacterial outer membrane or the periplasm. Our computational studies provided plausible 3D models for HbpA, SapA, PE, and HxuA interactions with heme. Generation and characterization of single mutants in the hxuCBA, hpe, sapA, and hbpA genes provided evidence for participation in heme binding-storage and inter-bacterial donation. The hxuA, sapA, hbpA, and hpe genes showed differential expression and responded to heme. Moreover, HxuCBA, PE, SapABCDFZ, and HbpA-DppBCDF presented moonlighting properties related to resistance to antimicrobial peptides or glutathione import, together likely contributing to the NTHi-host airway interplay, as observed upon cultured airway epithelia and in vivo lung infection. The observed multi-functionality was shown to be system-specific, thus limiting redundancy. Together, we provide evidence for heme uptake systems as bacterial factors that act in a coordinated and multi-functional manner to subvert nutritional- and other sources of host innate immunity during NTHi airway infection.

The Role of Iron and Siderophores in Infection, and the Development of Siderophore Antibiotics

Iron is an essential nutrient for bacterial growth, replication, and metabolism. Humans store iron bound to various proteins such as hemoglobin, haptoglobin, transferrin, ferritin, and lactoferrin, limiting the availability of free iron for pathogenic bacteria. However, bacteria have developed various mechanisms to sequester or scavenge iron from the host environment. Iron can be taken up by means of active transport systems that consist of bacterial small molecule siderophores, outer membrane siderophore receptors, the TonB-ExbBD energy-transducing proteins coupling the outer and the inner membranes, and inner membrane transporters. Some bacteria also express outer membrane receptors for iron-binding proteins of the host and extract iron directly from these for uptake. Ultimately, iron is acquired and transported into the bacterial cytoplasm. The siderophores are small molecules produced and released by nearly all bacterial species and are classified according to the chemical nature of their iron-chelating group (ie, catechol, hydroxamate, α-hydroxyl-carboxylate, or mixed types). Siderophore-conjugated antibiotics that exploit such iron-transport systems are under development for the treatment of infections caused by gram-negative bacteria. Despite demonstrating high in vitro potency against pathogenic multidrug-resistant bacteria, further development of several candidates had stopped due to apparent adaptive resistance during exposure, lack of consistent in vivo efficacy, or emergence of side effects in the host. However, cefiderocol, with an optimized structure, has advanced and has been investigated in phase 1 to 3 clinical trials. This article discusses the mechanisms implicated in iron uptake and the challenges associated with the design and utilization of siderophore-mimicking antibiotics.

Pseudomonas aeruginosa possesses three distinct systems for sensing and using the host molecule haem

Pathogens have developed several strategies to obtain iron during infection, including the use of iron-containing molecules from the host. Haem accounts for the vast majority of the iron pool in vertebrates and thus represents an important source of iron for pathogens. Using a proteomic approach, we have identified in this work a previously uncharacterized system, which we name Hxu, that together with the known Has and Phu systems, is used by the human pathogen Pseudomonas aeruginosa to respond to haem. We show that the Has and Hxu systems are functional signal transduction pathways of the cell-surface signalling class and report the mechanism triggering the activation of these signalling systems. Both signalling cascades involve an outer membrane receptor (HasR and HxuA respectively) that upon sensing haem in the extracellular medium produces the activation of an σ^ECF factor in the cytosol. HxuA has a major role in signalling and a minor role in haem acquisition in conditions in which the HasR and PhuR receptors or other sources of iron are present. Remarkably, P. aeruginosa compensates the lack of the HasR receptor by increasing the production of HxuA, which underscores the importance of haem signalling for this pathogen.

Heme Uptake and Utilization by Gram-Negative Bacterial Pathogens

Iron is a transition metal utilized by nearly all forms of life for essential cellular processes, such as DNA synthesis and cellular respiration. During infection by bacterial pathogens, the host utilizes various strategies to sequester iron in a process termed, nutritional immunity. To circumvent these defenses, Gram-negative pathogens have evolved numerous mechanisms to obtain iron from heme. In this review we outline the systems that exist in several Gram-negative pathogens that are associated with heme transport and utilization, beginning with hemolysis and concluding with heme degradation. In addition, Gram-negative pathogens must also closely regulate the intracellular concentrations of iron and heme, since high levels of iron can lead to the generation of toxic reactive oxygen species. As such, we also provide several examples of regulatory pathways that control heme utilization, showing that co-regulation with other cellular processes is complex and often not completely understood.

Iron Acquisition in Mycobacterium tuberculosis

The highly contagious disease tuberculosis (TB) is caused by the bacterium Mycobacterium tuberculosis (Mtb), which has been evolving drug resistance at an alarming rate. Like all human pathogens, Mtb requires iron for growth and virulence. Consequently, Mtb iron transport is an emerging drug target. However, the development of anti-TB drugs aimed at these metabolic pathways has been restricted by the dearth of information on Mtb iron acquisition. In this Review, we describe the multiple strategies utilized by Mtb to acquire ferric iron and heme iron. Mtb iron uptake is a complex process, requiring biosynthesis and subsequent export of Mtb siderophores, followed by ferric iron scavenging and ferric-siderophore import into Mtb. Additionally, Mtb possesses two possible heme uptake pathways and an Mtb-specific mechanism of heme degradation that yields iron and novel heme-degradation products. We conclude with perspectives for potential therapeutics that could directly target Mtb heme and iron uptake machineries. We also highlight how hijacking Mtb heme and iron acquisition pathways for drug import may facilitate drug transport through the notoriously impregnable Mtb cell wall.

The Diversity of Mammalian Hemoproteins and Microbial Heme Scavengers Is Shaped by an Arms Race for Iron Piracy

Iron is an essential micronutrient for most living species. In mammals, hemoglobin (Hb) stores more than two thirds of the body's iron content. In the bloodstream, haptoglobin (Hp) and hemopexin (Hpx) sequester free Hb or heme. Pathogenic microorganisms usually acquire iron from their hosts and have evolved complex systems of iron piracy to circumvent nutritional immunity. Herein, we performed an evolutionary analysis of genes coding for mammalian heme-binding proteins and heme-scavengers in pathogen species. The underlying hypothesis is that these molecules are engaged in a molecular arms race. We show that positive selection drove the evolution of mammalian Hb and Hpx. Positively selected sites in Hb are located at the interaction surface with Neisseria meningitidis heme scavenger HpuA and with Staphylococcus aureus iron-regulated surface determinant B (IsdB). In turn, positively selected sites in HpuA and IsdB are located in the flexible protein regions that contact Hb. A residue in Hb (S45H) was also selected on the Caprinae branch. This site stabilizes the interaction with Trypanosoma brucei hemoglobin-haptoglobin (HbHp) receptor (TbHpHbR), a molecule that also mediates trypanosome lytic factor (TLF) entry. In TbHpHbR, positive selection drove the evolution of a variant (L210S) which allows evasion from TLF but reduces affinity for HbHp. Finally, selected sites in Hpx are located at the interaction surface with the Haemophilus influenzae hemophore HxuA, which in turn displays fast evolving sites at the Hpx-binding interface. These results shed light into host-pathogens conflicts and establish the importance of nutritional immunity as an evolutionary force.

A Colicin M-Type Bacteriocin from Pseudomonas aeruginosa Targeting the HxuC Heme Receptor Requires a Novel Immunity Partner

Pyocins are bacteriocins secreted by Pseudomonas aeruginosa, and they assist in the colonization of different niches. A major subset of these antibacterial proteins adopt a modular organization characteristic of polymorphic toxins. They include a receptor-binding domain, a segment enabling membrane passage, and a toxin module at the carboxy terminus, which eventually kills the target cells. To protect themselves from their own products, bacteriocin-producing strains express an immunity gene concomitantly with the bacteriocin. We show here that a pyocin equipped with a phylogenetically distinct ColM toxin domain, PaeM4, mediates antagonism against a large set of P. aeruginosa isolates. Immunity to PaeM4 is provided by the inner membrane protein PmiC, which is equipped with a transmembrane topology not previously described for the ColM family. Given that strains lacking a pmiC gene are killed by PaeM4, the presence of such an immunity partner likely is a key criterion for escaping cellular death mediated by PaeM4. The presence of a TonB box in PaeM4 and enhanced bacteriocin activity under iron-poor conditions strongly suggested the targeting of a TonB-dependent receptor. Evaluation of PaeM4 activities against TonB-dependent receptor knockout mutants in P. aeruginosa PAO1 revealed that the heme receptor HxuC (PA1302) serves as a PaeM4 target at the cellular surface. Because other ColM-type pyocins may target the ferrichrome receptor FiuA, our results illustrate the versatility in target recognition conferred by the polymorphic nature of ColM-type bacteriocins.IMPORTANCE The antimicrobial armamentarium of a bacterium is a major asset for colonizing competitive environments. Bacteriocins comprise a subset of these compounds. Pyocins are an example of such antibacterial proteins produced by Pseudomonas aeruginosa, killing other P. aeruginosa strains. A large group of these molecules show a modular protein architecture that includes a receptor-binding domain for initial target cell attachment and a killer domain. In this study, we have shown that a novel modular pyocin (PaeM4) that kills target bacteria via interference with peptidoglycan assembly takes advantage of the HxuC heme receptor. Cells can protect themselves from killing by the presence of a dedicated immunity partner, an integral inner membrane protein that adopts a transmembrane topology distinct from that of proteins currently known to provide immunity against such toxin activity. Understanding the receptors with which pyocins interact and how immunity to pyocins is achieved is a pivotal step toward the rational design of bacteriocin cocktails for the treatment of P. aeruginosa infections.

Exolysin Shapes the Virulence of Pseudomonas aeruginosa Clonal Outliers

Bacterial toxins are important weapons of toxicogenic pathogens. Depending on their origin, structure and targets, they show diverse mechanisms of action and effects on eukaryotic cells. Exolysin is a secreted 170 kDa pore-forming toxin employed by clonal outliers of Pseudomonas aeruginosa providing to some strains a hyper-virulent behaviour. This group of strains lacks the major virulence factor used by classical strains, the Type III secretion system. Here, we review the structural features of the toxin, the mechanism of its secretion and the effects of the pore formation on eukaryotic cells.

Covalent bonding of heme to protein prevents heme capture by nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) are Gram‐negative pathogens that contribute to a variety of diseases, including acute otitis media and chronic obstructive pulmonary disease. As NTHi have an absolute requirement for heme during aerobic growth, these bacteria have to scavenge heme from their human hosts. These heme sources can range from free heme to heme bound to proteins, such as hemoglobin. To test the impact of heme structural factors on heme acquisition by NTHi, we prepared a series of heme sources that systematically vary in heme exposure and covalent binding of heme to peptide/protein and tested the ability of NTHi to use these sources to support growth. Results from this study suggest that NTHi can utilize protein‐associated heme only if it is noncovalently attached to the protein.

Two-Partner Secretion: Combining Efficiency and Simplicity in the Secretion of Large Proteins for Bacteria-Host and Bacteria-Bacteria Interactions

Initially identified in pathogenic Gram-negative bacteria, the two-partner secretion (TPS) pathway, also known as Type Vb secretion, mediates the translocation across the outer membrane of large effector proteins involved in interactions between these pathogens and their hosts. More recently, distinct TPS systems have been shown to secrete toxic effector domains that participate in inter-bacterial competition or cooperation. The effects of these systems are based on kin vs. non-kin molecular recognition mediated by specific immunity proteins. With these new toxin-antitoxin systems, the range of TPS effector functions has thus been extended from cytolysis, adhesion, and iron acquisition, to genome maintenance, inter-bacterial killing and inter-bacterial signaling. Basically, a TPS system is made up of two proteins, the secreted TpsA effector protein and its TpsB partner transporter, with possible additional factors such as immunity proteins for protection against cognate toxic effectors. Structural studies have indicated that TpsA proteins mainly form elongated β helices that may be followed by specific functional domains. TpsB proteins belong to the Omp85 superfamily. Open questions remain on the mechanism of protein secretion in the absence of ATP or an electrochemical gradient across the outer membrane. The remarkable dynamics of the TpsB transporters and the progressive folding of their TpsA partners at the bacterial surface in the course of translocation are thought to be key elements driving the secretion process.

PPE Surface Proteins Are Required for Heme Utilization by Mycobacterium tuberculosis

Iron is essential for replication of Mycobacterium tuberculosis, but iron is efficiently sequestered in the human host during infection. Heme constitutes the largest iron reservoir in the human body and is utilized by many bacterial pathogens as an iron source. While heme acquisition is well studied in other bacterial pathogens, little is known in M. tuberculosis. To identify proteins involved in heme utilization by M. tuberculosis, a transposon mutant library was screened for resistance to the toxic heme analog gallium(III)-porphyrin (Ga-PIX). Inactivation of the ppe36, ppe62, and rv0265c genes resulted in resistance to Ga-PIX. Growth experiments using isogenic M. tuberculosis deletion mutants showed that PPE36 is essential for heme utilization by M. tuberculosis, while the functions of PPE62 and Rv0265c are partially redundant. None of the genes restored growth of the heterologous M. tuberculosis mutants, indicating that the proteins encoded by the genes have separate functions. PPE36, PPE62, and Rv0265c bind heme as shown by surface plasmon resonance spectroscopy and are associated with membranes. Both PPE36 and PPE62 proteins are cell surface accessible, while the Rv0265c protein is probably located in the periplasm. PPE36 and PPE62 are, to our knowledge, the first proline-proline-glutamate (PPE) proteins of M. tuberculosis that bind small molecules and are involved in nutrient acquisition. The absence of a virulence defect of the ppe36 deletion mutant indicates that the different iron acquisition pathways of M. tuberculosis may substitute for each other during growth and persistence in mice. The emerging model of heme utilization by M. tuberculosis as derived from this study is substantially different from those of other bacteria.

Tuberculosis is caused by Mycobacterium tuberculosis and is a devastating disease affecting eight million people each year. Iron is an essential nutrient for replication of M. tuberculosis in the human host. More than 70% of iron in the human body is bound in heme. Not surprisingly, many bacterial pathogens, including M. tuberculosis, are able to acquire iron from heme. However, the mechanism of heme uptake by M. tuberculosis is poorly understood. We have identified two novel surface proteins that bind heme and are required for heme utilization by M. tuberculosis. These findings constitute a major advancement of our understanding of iron acquisition by M. tuberculosis and show that M. tuberculosis has evolved heme uptake systems different from the paradigms established by other bacteria.

Addressing recent docking challenges: A hybrid strategy to integrate template-based and free protein-protein docking

Protein-protein docking is an important computational tool for predicting protein-protein interactions. With the rapid development of proteomics projects, more and more experimental binding information ranging from mutagenesis data to three-dimensional structures of protein complexes are becoming available. Therefore, how to appropriately incorporate the biological information into traditional ab initio docking has been an important issue and challenge in the field of protein-protein docking. To address these challenges, we have developed a Hybrid DOCKing protocol of template-based and template-free approaches, referred to as HDOCK. The basic procedure of HDOCK is to model the structures of individual components based on the template complex by a template-based method if a template is available; otherwise, the component structures will be modeled based on monomer proteins by regular homology modeling. Then, the complex structure of the component models is predicted by traditional protein-protein docking. With the HDOCK protocol, we have participated in the CPARI experiment for rounds 28-35. Out of the 25 CASP-CAPRI targets for oligomer modeling, our HDOCK protocol predicted correct models for 16 targets, ranking one of the top algorithms in this challenge. Our docking method also made correct predictions on other CAPRI challenges such as protein-peptide binding for 6 out of 8 targets and water predictions for 2 out of 2 targets. The advantage of our hybrid docking approach over pure template-based docking was further confirmed by a comparative evaluation on 20 CASP-CAPRI targets. Proteins 2017; 85:497-512. © 2016 Wiley Periodicals, Inc.

Diverse structural approaches to haem appropriation by pathogenic bacteria

The critical need for iron presents a challenge for pathogenic bacteria that must survive in an environment bereft of accessible iron due to a natural low bioavailability and their host's nutritional immunity. Appropriating haem, either direct from host haemoproteins or by secreting haem-scavenging haemophores, is one way pathogenic bacteria can overcome this challenge. After capturing their target, haem appropriation systems must remove haem from a high-affinity binding site (on the host haemoprotein or bacterial haemophore) and transfer it to a binding site of lower affinity on a bacterial receptor. Structural information is now available to show how, using a combination of induced structural changes and steric clashes, bacteria are able to extract haem from haemophores, haemopexin and haemoglobin. This review focuses on structural descriptions of these bacterial haem acquisition systems, summarising how they bind haem and their target haemoproteins with particularly emphasis on the mechanism of haem extraction.

Modeling protein-protein and protein-peptide complexes: CAPRI 6th edition

We present the sixth report evaluating the performance of methods for predicting the atomic resolution structures of protein complexes offered as targets to the community-wide initiative on the Critical Assessment of Predicted Interactions (CAPRI). The evaluation is based on a total of 20,670 predicted models for 8 protein-peptide complexes, a novel category of targets in CAPRI, and 12 protein-protein targets in CAPRI prediction Rounds held during the years 2013-2016. For two of the protein-protein targets, the focus was on the prediction of side-chain conformation and positions of interfacial water molecules. Seven of the protein-protein targets were particularly challenging owing to their multicomponent nature, to conformational changes at the binding site, or to a combination of both. Encouragingly, the very large multiprotein complex with the nucleosome was correctly predicted, and correct models were submitted for the protein-peptide targets, but not for some of the challenging protein-protein targets. Models of acceptable quality or better were obtained for 14 of the 20 targets, including medium quality models for 13 targets and high quality models for 8 targets, indicating tangible progress of present-day computational methods in modeling protein complexes with increased accuracy. Our evaluation suggests that the progress stems from better integration of different modeling tools with docking procedures, as well as the use of more sophisticated evolutionary information to score models. Nonetheless, adequate modeling of conformational flexibility in interacting proteins remains an important area with a crucial need for improvement. Proteins 2017; 85:359-377. © 2016 Wiley Periodicals, Inc.

Performance of MDockPP in CAPRI rounds 28-29 and 31-35 including the prediction of water-mediated interactions

Protein-protein interactions are either through direct contacts between two binding partners or mediated by structural waters. Both direct contacts and water-mediated interactions are crucial to the formation of a protein-protein complex. During the recent CAPRI rounds, a novel parallel searching strategy for predicting water-mediated interactions is introduced into our protein-protein docking method, MDockPP. Briefly, a FFT-based docking algorithm is employed in generating putative binding modes, and an iteratively derived statistical potential-based scoring function, ITScorePP, in conjunction with biological information is used to assess and rank the binding modes. Up to 10 binding modes are selected as the initial protein-protein complex structures for MD simulations in explicit solvent. Water molecules near the interface are clustered based on the snapshots extracted from independent equilibrated trajectories. Then, protein-ligand docking is employed for a parallel search for water molecules near the protein-protein interface. The water molecules generated by ligand docking and the clustered water molecules generated by MD simulations are merged, referred to as the predicted structural water molecules. Here, we report the performance of this protocol for CAPRI rounds 28-29 and 31-35 containing 20 valid docking targets and 11 scoring targets. In the docking experiments, we predicted correct binding modes for nine targets, including one high-accuracy, two medium-accuracy, and six acceptable predictions. Regarding the two targets for the prediction of water-mediated interactions, we achieved models ranked as "excellent" in accordance with the CAPRI evaluation criteria; one of these two targets is considered as a difficult target for structural water prediction. Proteins 2017; 85:424-434. © 2016 Wiley Periodicals, Inc.

pyDock scoring for the new modeling challenges in docking: Protein-peptide, homo-multimers, and domain-domain interactions

The sixth CAPRI edition included new modeling challenges, such as the prediction of protein-peptide complexes, and the modeling of homo-oligomers and domain-domain interactions as part of the first joint CASP-CAPRI experiment. Other non-standard targets included the prediction of interfacial water positions and the modeling of the interactions between proteins and nucleic acids. We have participated in all proposed targets of this CAPRI edition both as predictors and as scorers, with new protocols to efficiently use our docking and scoring scheme pyDock in a large variety of scenarios. In addition, we have participated for the first time in the servers section, with our recently developed webserver, pyDockWeb. Excluding the CASP-CAPRI cases, we submitted acceptable models (or better) for 7 out of the 18 evaluated targets as predictors, 4 out of the 11 targets as scorers, and 6 out of the 18 targets as servers. The overall success rates were below those in past CAPRI editions. This shows the challenging nature of this last edition, with many difficult targets for which no participant submitted a single acceptable model. Interestingly, we submitted acceptable models for 83% of the evaluated protein-peptide targets. As for the 25 cases of the CASP-CAPRI experiment, in which we used a larger variety of modeling techniques (template-based, symmetry restraints, literature information, etc.), we submitted acceptable models for 56% of the targets. In summary, this CAPRI edition showed that pyDock scheme can be efficiently adapted to the increasing variety of problems that the protein interactions field is currently facing. Proteins 2017; 85:487-496. © 2016 Wiley Periodicals, Inc.

Lessons from (co-)evolution in the docking of proteins and peptides for CAPRI Rounds 28-35

Computational protein-protein docking is of great importance for understanding protein interactions at the structural level. Critical assessment of prediction of interactions (CAPRI) experiments provide the protein docking community with a unique opportunity to blindly test methods based on real-life cases and help accelerate methodology development. For CAPRI Rounds 28-35, we used an automatic docking pipeline integrating the coarse-grained co-evolution-based potential InterEvScore. This score was developed to exploit the information contained in the multiple sequence alignments of binding partners and selectively recognize co-evolved interfaces. Together with Zdock/Frodock for rigid-body docking, SOAP-PP for atomic potential and Rosetta applications for structural refinement, this pipeline reached high performance on a majority of targets. For protein-peptide docking and interfacial water position predictions, we also explored different means of taking evolutionary information into account. Overall, our group ranked 1^st by correctly predicting 10 targets, composed of 1 High, 7 Medium and 2 Acceptable predictions. Excellent and Outstanding levels of accuracy were reached for each of the two water prediction targets, respectively. Altogether, in 15 out of 18 targets in total, evolutionary information, either through co-evolution or conservation analyses, could provide key constraints to guide modeling towards the most likely assemblies. These results open promising perspectives regarding the way evolutionary information can be valuable to improve docking prediction accuracy. Proteins 2017; 85:378-390. © 2016 Wiley Periodicals, Inc.