Salmonella typhi iron uptake mutants are attenuated in mice

Iron Acquisition Systems of Gram-negative Bacterial Pathogens Define TonB-Dependent Pathways to Novel Antibiotics

Iron is an indispensable metabolic cofactor in both pro- and eukaryotes, which engenders a natural competition for the metal between bacterial pathogens and their human or animal hosts. Bacteria secrete siderophores that extract Fe3+ from tissues, fluids, cells, and proteins; the ligand gated porins of the Gram-negative bacterial outer membrane actively acquire the resulting ferric siderophores, as well as other iron-containing molecules like heme. Conversely, eukaryotic hosts combat bacterial iron scavenging by sequestering Fe3+ in binding proteins and ferritin. The variety of iron uptake systems in Gram-negative bacterial pathogens illustrates a range of chemical and biochemical mechanisms that facilitate microbial pathogenesis. This document attempts to summarize and understand these processes, to guide discovery of immunological or chemical interventions that may thwart infectious disease.

Genomic insights into evolution of pathogenicity and resistance of multidrug-resistant Raoultella ornithinolytica WM1

Raoultella ornithinolytica is a poorly understood opportunistic pathogen, and the underlying mechanisms of its multidrug resistance and pathogenicity have not yet been comprehensively investigated. The multidrug-resistant (MDR) strain WM1 was isolated from the blood of a male patient in Tianjin, China, in 2018. Here, we describe the complete genome and provide a genomic analysis of R. ornithinolytica WM1. The isolate was resistant to all tested antimicrobials except amikacin, tobramycin, and tigecycline. Two plasmids, pWM1-1 (IncHI5) and pWM1-2 (IncR), carried multidrug-resistance regions. A large antimicrobial resistance island region resided on pWM1-1 and exhibited mosaic structures resulting from the acquisition of complex integrations of variable regions, including genes conferring resistance to multiple classes of antimicrobials. Moreover, WM1 possessed virulence-related elements that encode several virulence factors, including type I fimbriae, Escherichia coli common pilus, type II and VI secretion systems, yersiniabactin, enterobactin, and surface polysaccharide, indicating pathogenic potential. Furthermore, the core genome phylogeny and pan-genome analyses revealed extensive genetic diversity. Our analysis indicates the need for stringent infection control, antimicrobial stewardship, periodic resistance monitoring, and rational medication to address potential threats posed by MDR R. ornithinolytica strains.

Conformational rearrangements in the N-domain of Escherichia coli FepA during ferric enterobactin transport

The Escherichia coli outer membrane receptor FepA transports ferric enterobactin (FeEnt) by an energy- and TonB-dependent, but otherwise a mechanistically undetermined process involving its internal 150-residue N-terminal globular domain (N-domain). We genetically introduced pairs of Cys residues in different regions of the FepA tertiary structure, with the potential to form disulfide bonds. These included Cys pairs on adjacent β-strands of the N-domain (intra-N) and Cys pairs that bridged the external surface of the N-domain to the interior of the C-terminal transmembrane β-barrel (inter-N-C). We characterized FeEnt uptake by these mutants with siderophore nutrition tests, [⁵⁹Fe]Ent binding and uptake experiments, and fluorescence decoy sensor assays. The three methods consistently showed that the intra-N disulfide bonds, which restrict conformational motion within the N-domain, prevented FeEnt uptake, whereas most inter-N-C disulfide bonds did not prevent FeEnt uptake. These outcomes indicate that conformational rearrangements must occur in the N terminus of FepA during FeEnt transport. They also argue against disengagement of the N-domain out of the channel as a rigid body and suggest instead that it remains within the transmembrane pore as FeEnt enters the periplasm.

EntE, EntS and TolC synergistically contributed to the pathogenesis of APEC strain E058

Extraintestinal pathogenic Escherichia coli (ExPEC) shows an enhanced ability to cause infection outside the intestinal tract. Avian pathogenic E. coli (APEC), one type of ExPEC, causes avian colibacillosis, a disease of significant economic importance to poultry producers worldwide that is characterized by systemic infection. Some ExPEC strains as well as other pathogenic enterobacteria produce enterobactin, a catecholate siderophore used to sequester iron during infection. Here, we showed that disruption of enterobactin efflux via outer membrane protein TolC significantly decreased the pathogenicity of APEC strain E058. Furthermore, colonization and persistence assays performed using a chicken infection model showed that the ΔtolC mutant was obviously attenuated (p˂0.001). In contrast, disruption of enterobactin synthesis gene entE and/or the inner membrane transporter gene entS had little effect on pathogenicity. Analysis of growth kinetics revealed a significant reduction in the growth of triple mutant strain E058ΔentEΔentSΔtolC in iron-deficient medium compared with the wild-type strain (p˂0.001), while no growth impairment was noted for the E058ΔtolC mutant in either Luria-Bertani broth or iron-deficient medium. The E058ΔentEΔentSΔtolC mutant also showed significantly decreased virulence compared with single mutant strain E058ΔtolC. Low-copy complementation of strains E058ΔtolC and E058ΔentEΔentSΔtolC with plasmid-borne tolC restored virulence to wild-type levels in the chicken infection model. Macrophage infection assays showed that ingestion of E058ΔtolC by macrophage cell line HD11 cells was reduced compared with ingestion of the E058ΔentEΔentSΔtolC mutant. However, no significant differences were observed between the mutants and the wild-type in a chicken serum resistance assay. Together, these results suggest that EntE, EntS and TolC synergistically contributed to the pathogenesis of APEC strain E058 in an iron-deficient environment.

An update on the transport and metabolism of iron in Listeria monocytogenes: the role of proteins involved in pathogenicity

Listeria monocytogenes is a Gram-positive bacterium that causes a rare but severe human disease with high mortality rate. The microorganism is widespread in the natural environment where it shows a saprophytic lifestyle. In the human body it infects many different cell types, where it lives intracellularly, however it may also temporarily live extracellularly. The ability to survive and grow in such diverse niches suggests that this bacterium has a wide range of mechanisms for both the acquisition of various sources of iron and effective management of this microelement. In this review, data about the mechanisms of transport, metabolism and regulation of iron, including recent findings in these areas, are summarized with focus on the importance of these mechanisms for the virulence of L. monocytogenes. These data indicate the key role of haem transport and maintenance of intracellular iron homeostasis for the pathogenesis of L. monocytogenes. Furthermore, some of the proteins involved in iron homeostasis like Fri and FrvA seem to deserve special attention due to their potential use in the development of new therapeutic antilisterial strategies.

Salmonella typhi sternal wound infection

Samonella typhi usually causes gastrointestinal infections. Few reports in the literature described skin and soft tissue infections related to Salmonella species, especially in immunocompetent patients. Our case exhibited sternal abscess growing Salmonella typhi.

Contribution of siderophore systems to growth and urinary tract colonization of asymptomatic bacteriuria Escherichia coli

The molecular mechanisms that define asymptomatic bacteriuria (ABU) Escherichia coli colonization of the human urinary tract remain to be properly elucidated. Here, we utilize ABU E. coli strain 83972 as a model to dissect the contribution of siderophores to iron acquisition, growth, fitness, and colonization of the urinary tract. We show that E. coli 83972 produces enterobactin, salmochelin, aerobactin, and yersiniabactin and examine the role of these systems using mutants defective in siderophore biosynthesis and uptake. Enterobactin and aerobactin contributed most to total siderophore activity and growth in defined iron-deficient medium. No siderophores were detected in an 83972 quadruple mutant deficient in all four siderophore biosynthesis pathways; this mutant did not grow in defined iron-deficient medium but grew in iron-limited pooled human urine due to iron uptake via the FecA ferric citrate receptor. In a mixed 1:1 growth assay with strain 83972, there was no fitness disadvantage of the 83972 quadruple biosynthetic mutant, demonstrating its capacity to act as a "cheater" and utilize siderophores produced by the wild-type strain for iron uptake. An 83972 enterobactin/salmochelin double receptor mutant was outcompeted by 83972 in human urine and the mouse urinary tract, indicating a role for catecholate receptors in urinary tract colonization.

The Ins and Outs of siderophore mediated iron uptake by extra-intestinal pathogenic Escherichia coli

Extraintestinal pathogenic E. coli (ExPEC) are responsible for many infectious diseases in livestock, such as airsacculitis in poultry, acute mastitis in dairy animals and neonatal septicaemia and urinary tract infections (UTI) in pigs and cattle. In their animal hosts, ExPEC have to cope with low iron availability. By using different strategies, ExPEC strains are able to retrieve iron sequestered by host proteins. One of these strategies is the use of siderophores, which are small secreted molecules with high affinity for iron. ExPEC are known to synthesize up to four different types of siderophores: enterobactin, salmochelins, yersiniabactin and aerobactin. Steps required for iron acquisition by siderophores include (1) siderophore synthesis in the cytoplasm, (2) siderophore secretion, (3) ferri-siderophore reception, (4) ferri-siderophore internalization and (5) iron release in the cytoplasm. Each siderophore has specific properties and may be differentially regulated to provide different advantages, potentially allowing ExPEC to adapt to different environmental conditions or to overcome host innate immunity. Iron acquisition by siderophores plays a significant role in ExPEC virulence and, as it requires outer membrane receptors, it constitutes an interesting target for the development of vaccines that could be used to limit the number of infectious diseases due to ExPEC in livestock.

Secretion, but not overall synthesis, of catecholate siderophores contributes to virulence of extraintestinal pathogenic Escherichia coli

Extraintestinal pathogenic Escherichia coli (ExPEC) use siderophores to sequester iron during infection. Enterobactin and salmochelins are catecholate siderophores produced by some ExPEC strains and other pathogenic enterobacteria. Siderophore export and synthesis mutants of avian ExPEC strain χ7122 were tested in a chicken infection model. In single-strain infections, siderophore-negative (ΔentDΔiuc), ΔentS and ΔentSΔiroC export mutants were attenuated in tissues and blood, whereas the ΔiroC export mutant was only attenuated in blood. Interestingly, the ΔentD mutant, producing only aerobactin, retained full virulence, and loss of entD in the ΔentSΔiroC mutant restored virulence. LC-MS/MS quantification of siderophores in export mutants demonstrated that loss of entS impaired enterobactin and mono-glucosylated enterobactin secretion, whereas loss of iroC impaired di- and tri-glucosylated enterobactin secretion. Loss of entS and/or iroC resulted in intracellular accumulation and increased secretion of siderophore monomers. Catecholate siderophore export mutants also demonstrated decreased fitness in a co-challenge infection model. By contrast, catecholate siderophore synthesis mutants (ΔentD and ΔiroB) competed as well as the wild-type strain. Results establish that EntS and IroC mediate specific export of catecholate siderophores and the role of these exporters for ExPEC virulence is contingent on enterobactin synthesis, which is not required when other siderophores like aerobactin are functional.

Serum transferrin saturation increase is associated with decrease of antibacterial activity of serum in patients with HFE-related genetic hemochromatosis

OBJECTIVES

Patients with type 1 (HFE-related) genetic hemochromatosis are usually excluded from blood donation on the basis that this disease may facilitate bacterial infections. The aim of the present study was to evaluate the serum antibacterial effect against Salmonella enterica Typhimurium LT2 in relation to iron status.

METHODS

Serum samples were collected in 26 iron-overloaded (homozygous C282Y mutation) and 35 iron-depleted hemochromatosis patients and 33 healthy control subjects. The antibacterial activity of sera and iron parameters were tested for each patient.

RESULTS

Serum from normal controls had an antibacterial effect against Salmonella Typhimurium LT2. The antibacterial effect decreased from the 1:2 to the 1:8 dilution and was always significantly lower in the iron-overloaded group. In both control and iron-depleted patients, a positive correlation was found between the decrease of antibacterial effect and the increase of both serum iron and transferrin saturation.

CONCLUSIONS

These results (a) support the view that chronic iron overload decreases serum antibacterial effect against Salmonella enterica Typhimurium LT2, (b) favor the interest of including, besides serum ferritinemia, serum transferrin saturation levels as a further criterion for iron-depletive treatment efficacy, and (c) provide an argument for not discouraging the use of blood from iron-depleted hemochromatosis patients for transfusion.

The CorA Mg2+ channel is required for the virulence of Salmonella enterica serovar typhimurium

CorA is the primary Mg(2+) channel in Salmonella enterica serovar Typhimurium. A corA mutant is attenuated in mice and defective for invasion of and replication within epithelial cells. Microarray studies show that several virulence effectors are repressed in a corA mutant strain, which ultimately manifests itself as a decrease in virulence.

Iron acquisition systems for ferric hydroxamates, haemin and haemoglobin in Listeria monocytogenes

Listeria monocytogenes is a Gram-positive bacterium that causes severe opportunistic infections in humans and animals. We biochemically characterized, for the first time, the iron uptake processes of this facultative intracellular pathogen, and identified the genetic loci encoding two of its membrane iron transporters. Strain EGD-e used iron complexes of hydroxamates (ferrichrome and ferrichrome A, ferrioxamine B), catecholates (ferric enterobactin, ferric corynebactin) and eukaryotic binding proteins (transferrin, lactoferrin, ferritin, haemoglobin). Quantitative determinations showed 10-100-fold lower affinity for ferric siderophores (Km approximately 1-10 nM) than Gram-negative bacteria, and generally lower uptake rates. Vmax for [59Fe]-enterobactin (0.15 pMol per 10(9) cells per minute) was 400-fold lower than that of Escherichia coli. For [59Fe]-corynebactin, Vmax was also low (1.2 pMol per 10(9) cells per minute), but EGD-e transported [59Fe]-apoferrichrome similarly to E. coli (Vmax=24 pMol per 10(9) cells per minute). L. monocytogenes encodes potential Fur-regulated iron transporters at 2.031 Mb (the fur-fhu region), 2.184 Mb (the feo region), 2.27 Mb (the srtB region) and 2.499 Mb (designated hupDGC region). Chromosomal deletions in the fur-fhu and hupDGC regions diminished iron uptake from ferric hydroxamates and haemin/haemoglobin respectively. In the former locus, deletion of fhuD (lmo1959) or fhuC (lmo1960) strongly reduced [59Fe]-apoferrichrome uptake. Deletion of hupC (lmo2429) eliminated the uptake of haemin and haemoglobin, and decreased the virulence of L. monocytogenes 50-fold in mice. Elimination of srtB region genes (Deltalmo2185, Deltalmo2186, Deltalmo2183), both sortase structural genes (DeltasrtB, DeltasrtA, DeltasrtAB), fur and feoB did not impair iron transport. However, deletion of bacterioferritin (Deltafri, lmo943; 0.97 Mb) decreased growth and altered iron uptake: Vmax of [59Fe]-corynebactin transport tripled in this strain, whereas that of [59Fe]-apoferrichrome decreased 20-fold.

The role of iron in infections with intracellular bacteria

The requirement for iron as a critical component for cellular processes has long been appreciated. During infection with intracellular bacteria, iron is required by both the host cell and the pathogen that inhabits the host cell. Macrophages require iron as a cofactor for the execution of important antimicrobial effector mechanisms, including the NADPH dependent oxidative burst and the production of nitrogen radicals catalysed by the inducible nitric oxide synthase. On the other side of the equation, intracellular bacteria such as Salmonella typhimurium and Mycobacterium tuberculosis have an obligate requirement for iron to support their growth and survival inside cells. This brief report summarises the background to our work on iron modulation in infections with these two organisms and highlights key observations on how modulation of host iron status disturbs the equilibrium between host and pathogen and can determine the outcome of infection.

Environmental cues and gene expression in Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans

Microorganisms typically adapt to environmental cues by turning on and off the expression of virulence genes which, in turn, allows for optimal growth and survival within different environmental niches. This adaptation strategy includes sensing and responding to changes in nutrients, pH, temperature, oxygen tension, redox potential, microbial flora, and osmolarity. For a bacterium to adhere to, penetrate, replicate in, and colonize host cells, it is critical that virulence genes are expressed during certain periods of the infection process. Thus, throughout the different stages of an infection, different sets of virulence factors are turned on and off in response to different environmental signals, allowing the bacterium to effectively adapt to its varying niche. In this review, we focus on the regulation of virulence gene expression in two pathogens which have been implicated as major etiological agents in adult and juvenile periodontal diseases: Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans. Understanding the mechanisms of virulence gene expression in response to the local environment of the host will provide crucial information in the development of effective treatments targeted at eradication of these periodontal disease pathogens.

Exchangeability of N termini in the ligand-gated porins of Escherichia coli

The ferric siderophore transporters of the Gram-negative bacterial outer membrane manifest a unique architecture: Their N termini fold into a globular domain that lodges within, and physically obstructs, a transmembrane porin beta-barrel formed by their C termini. We exchanged and deleted the N termini of two such siderophore receptors, FepA and FhuA, which recognize and transport ferric enterobactin and ferrichrome, respectively. The resultant chimeric proteins and empty beta-barrels avidly bound appropriate ligands, including iron complexes, protein toxins, and viruses. Thus, the ability to recognize and discriminate these molecules fully originates in the transmembrane beta-barrel domain. Both the hybrid and the deletion proteins also transported the ferric siderophore that they bound. The FepA constructs showed less transport activity than wild type receptor protein, but the FhuA constructs functioned with turnover numbers that were equivalent to wild type. The mutant proteins displayed the full range of transport functionalities, despite their aberrant or missing N termini, confirming (Braun, M., Killmann, H., and Braun, V. (1999) Mol. Microbiol. 33, 1037-1049) that the globular domain within the pore is dispensable to the siderophore internalization reaction, and when present, acts without specificity during solute uptake. These and other data suggest a transport process in which siderophore receptors undergo multiple conformational states that ultimately expel the N terminus from the channel concomitant with solute internalization.

Restricted growth of ent(-) and tonB mutants of Salmonella enterica serovar Typhi in human Mono Mac 6 monocytic cells

Monocytes and macrophages are an important host defense in humans infected with Salmonella enterica serovar Typhi. Bacterial ability to survive in these cells is therefore a crucial virulence characteristic of this pathogen. In this study, we demonstrate that growth of a Salmonella enterica serovar Typhi enterochelin synthesis mutant and a tonB mutant in the human monocyte cell line Mono Mac 6 is restricted compared to that of the parental wild-type Ty2 strain. These results suggest that enterochelin- and TonB-mediated iron uptake plays a role in S. enterica serovar Typhi pathogenesis, and also suggest that mutations in iron uptake may attenuate S. enterica serovar Typhi strains for human beings.

Discovery of a nonclassical siderophore, legiobactin, produced by strains of Legionella pneumophila

The mechanisms by which Legionella pneumophila, a facultative intracellular parasite and the agent of Legionnaires' disease, acquires iron are largely unexplained. Several earlier studies indicated that L. pneumophila does not elaborate siderophores. However, we now present evidence that supernatants from L. pneumophila cultures can contain a nonproteinaceous, high-affinity iron chelator. More specifically, when aerobically grown in a low-iron, chemically defined medium (CDM), L. pneumophila secretes a substance that is reactive in the chrome azurol S (CAS) assay. Importantly, the siderophore-like activity was only observed when the CDM cultures were inoculated to relatively high density with bacteria that had been grown overnight to log or early stationary phase in CDM or buffered yeast extract. Inocula derived from late-stationary-phase cultures, despite ultimately growing, consistently failed to result in the elaboration of siderophore-like activity. The Legionella CAS reactivity was detected in the culture supernatants of the serogroup 1 strains 130b and Philadelphia-1, as well as those from representatives of other serogroups and other Legionella species. The CAS-reactive substance was resistant to boiling and protease treatment and was associated with the <1-kDa supernatant fraction. As would also be expected for a siderophore, the addition of 0.5 or 2.0 microM iron to the cultures repressed the expression of the CAS-reactive substance. Interestingly, the supernatants were negative in the Arnow, Csáky, and Rioux assays, indicating that the Legionella siderophore was not a classic catecholate or hydroxamate and, hence, might have a novel structure. We have designated the L. pneumophila siderophore legiobactin.

Effect of mutS and recD mutations on Salmonella virulence

Hybrid derivatives of closely related bacteria may be used to dissect strain-specific functions that contribute to virulence within a host. However, mismatches between DNA sequences are a potent barrier to recombination. Recipients with mutS and recD mutations overcome this barrier, allowing construction of genetic hybrids. To determine whether Salmonella hybrids constructed in a mutS recD host can be used to study virulence, we assayed the effect of mutS and recD mutations on the virulence of Salmonella typhimurium 14028s in mice. Mutants defective in either mutS or recD do not affect the time course or the 50% lethal dose (LD(50)) of the infection. In contrast, the inactivation of both mutS and recD results in a synthetic phenotype which substantially increases the time required to cause a lethal infection without changing the LD(50). This phenotype results from an inability of mutS recD double mutants to rapidly adapt to purine-limiting conditions present within macrophages. Although the disease progression is slower, S. typhimurium mutS recD mutants retain the ability to cause lethal infections, and, thus, hybrids constructed in mutS recD hosts may permit the analysis of virulence factors in a surrogate animal model.

Pathogenesis of Mycobacterium avium subspecies paratuberculosis infections in ruminants: still more questions than answers

Mycobacterium avium subspecies paratuberculosis is the etiologic agent of paratuberculosis (Johnes disease), a chronic enteritis in ruminants, which is one of the most widespread bacterial diseases of domestic animals, causing enormous economic losses worldwide. Though the disease was first described more than a century ago, the biology of the infecting organism and the mechanisms of its interactions with the host still remain a mystery. In this review, recent advances made on pathogenesis of paratuberculosis are summarized and future challenges are discussed.

Genetic and physiologic characterization of ferric/cupric reductase constitutive mutants of Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic yeast that causes meningitis in immunocompromised patients. Because iron acquisition is critical for growth of a pathogen in a host, we studied the regulation of the ferric reductase and ferrous uptake system of this organism. We isolated 18 mutants, representing four independent loci, with dysregulated ferric reductase. The mutant strains had >10-fold higher than wild-type WT reductase activity in the presence of iron. Two of the strains also had >7-fold higher than WT iron uptake in the presence of iron but were not markedly iron sensitive. Both were sensitive to the oxidative stresses associated with superoxide and hydrogen peroxide. One strain exhibited only 23% of the WT level of iron uptake in the absence of iron and grew poorly without iron supplementation of the medium, phenotypes consistent with an iron transport deficiency; it was sensitive to superoxide but not to hydrogen peroxide. The fourth strain had high reductase activity but normal iron uptake; it was not very sensitive to oxidative stress. We also demonstrated that the ferric reductase was regulated by copper and could act as a cupric reductase. Sensitivity to oxidants may be related to iron acquisition by a variety of mechanisms and may model the interaction of the yeast with the immune system.

Identification and characterization of a novel extracellular ferric reductase from Mycobacterium paratuberculosis

A novel extracellular mycobacterial enzyme was identified in the ruminant pathogen Mycobacterium paratuberculosis. The enzyme was capable of mobilizing iron from different sources such as ferric ammonium citrate, ferritin, and transferrin by reduction of the metal. The purified reductase had a calculated Mr of 17,000, was sensitive to proteinase K treatment, and had an isoelectric point of pH 9. Analysis of the amino acid composition revealed glycine, serine, asparagine (or aspartic acid), and glutamine (or glutamic acid) as the most frequently occurring residues. Enzymatic activity was highest at 37 degrees C and between pH 5 and 10. The calculated Km and Vmax for ferric ammonium citrate were 0.213 mM and 0.345 mM min(-1) mg(-1), respectively. Using a specific antireductase antibody in immunoelectron microscopy, we were able to detect the enzyme associated with intracellular mycobacteria in naturally M. paratuberculosis-infected bovine tissue. We prepose that the reductase of M. paratuberculosis represents an alternative strategy of mycobacteria to mobilize ferric iron and discuss its potential role in bacterial evasion of intracellular defense mechanisms.

Characterization of lipoprotein IRP1 from Corynebacterium diphtheriae, which is regulated by the diphtheria toxin repressor (DtxR) and iron

The Corynebacterium diphtheriae irp1 gene is negatively regulated by DtxR and iron. The nucleotide sequence of irp1 revealed that it has homology with genes involved in iron acquisition. Expression of the irp1 gene showed that it encodes a lipoprotein (IRP1) with a predicted size of 38 kDa. Northern blot experiments indicated that transcription from the irp1 promoter is repressed in high-iron medium and suggested that irp1 is part of an iron-regulated operon.

Transcription of the Corynebacterium diphtheriae hmuO gene is regulated by iron and heme

The hmuO gene is required for the utilization of heme and hemoglobin as iron sources by Corynebacterium diphtheriae. The product of hmuO has homology to eukaryotic heme oxygenases which are involved in the degradation of heme and the release of iron. To investigate the mechanism of hmuO regulation, a promoterless lacZ gene present on the promoter-probe vector pCM502 was placed under transcriptional control of the hmuO promoter. In C. diphtheriae C7, optimal expression from the hmuO promoter was obtained only in the presence of heme or hemoglobin under low-iron conditions. Expression of hmuO in high-iron medium containing heme was repressed five- to sixfold from that seen under low-iron conditions in the presence of heme. Transcription from the hmuO promoter in the absence of heme or hemoglobin was fully repressed in high-iron medium and was expressed at very low levels in iron-depleted conditions. Expression studies with tile hmuO-lacZ fusion construct in C7hm723, a dtxR mutant of C7, and in a hmuO mutant of C. diphtheriae HC1 provided further evidence that transcription of the hmuO promoter is repressed by DtxR and iron and activated by heme. In Escherichia coli, the hmuO promoter was expressed at very low levels under all conditions examined. Gel mobility shift assays and DNase I footprinting experiments indicated that DtxR binds in a metal-dependent manner to a sequence that overlaps the putative hmuO promoter. Total cellular RNA isolated from C. diphtheriae was used to identify the transcriptional start site for the hmuO gene. Northern blot analysis suggested that the hmuO mRNA was monocistronic and that transcription was heme inducible.

Utilization of host iron sources by Corynebacterium diphtheriae: identification of a gene whose product is homologous to eukaryotic heme oxygenases and is required for acquisition of iron from heme and hemoglobin

Corynebacterium diphtheriae was examined for the ability to utilize various host compounds as iron sources. C. diphtheriae C7(-) acquired iron from heme, hemoglobin, and transferrin. A siderophore uptake mutant of strain C7 was unable to utilize transferrin but was unaffected in acquisition of iron from heme and hemoglobin, which suggests that C. diphtheriae possesses a novel mechanism for utilizing heme and hemoglobin as iron sources. Mutants of C. diphtheriae and Corynebacterium ulcerans that are defective in acquiring iron from heme and hemoglobin were isolated following chemical mutagenesis and streptonigrin enrichment. A recombinant clone, pCD293, obtained from a C7(-) genomic plasmid library complemented several of the C. ulcerans mutants and three of the C. diphtheriae mutants. The nucleotide sequence of the gene (hmuO) required for complementation was determined and shown to encode a protein with a predicted mass of 24,123 Da. Sequence analysis revealed that HmuO has 33% identity and 70% similarity with the human heme oxygenase enzyme HO-1. Heme oxygenases, which have been well characterized in eukaryotes but have not been identified in prokaryotes, are involved in the oxidation of heme and subsequent release of iron from the heme moiety. It is proposed that the HmuO protein is essential for the utilization of heme as an iron source by C. diphtheriae and that the heme oxygenase activity of HmuO is involved in the release of iron from heme. This is the first report of a bacterial gene whose product has homology to heme oxygenases.

Iron supplying systems of Salmonella in diagnostics, epidemiology and infection

Well-known and newly characterised mechanisms, both endogenous and exogenous, for the uptake of iron by Salmonella are outlined, and their possible roles at various stages in infection are discussed. The contributions of a detailed understanding of iron supplying systems to techniques for diagnosis, epidemiology and disease management are described.