Porphyrin-based compounds exert antibacterial action against the sexually transmitted pathogens Neisseria gonorrhoeae and Haemophilus ducreyi

A series of porphyrin based compounds without (nMP) or with (MP) metals were found to have potent bactericidal action in vitro against the sexually transmitted pathogens Neisseria gonorrhoeae and Haemophilus ducreyi. nMP and MP did not show bactericidal activity against five species of lactobacilli. An MP containing gallium had the capacity to block a gonococcal infection in a murine vaginal model, indicating that its development as a topical microbicide to block sexually transmitted bacterial infections is warranted. In contrast to other bacterial species, loss of the gonococcal haemoglobin uptake system encoded by hpuB or energy supplied through the TonB-ExbB-ExbD system did not significantly affect levels of MP-susceptibility in gonococci. In contrast, mutations in gonococci that inactivate the mtrCDE-encoded efflux pump were found to enhance gonococcal susceptibility to nMPs and MPs while over-production of this efflux pump decreased levels of gonococcal susceptibility to these compounds.

Identification of functionally important regions of a haemoglobin receptor from Neisseria meningitidis

The HmbR outer-membrane receptor enables Neisseria meningitidis to use haemoglobin (Hb) as a source of iron. This protein functions by binding Hb, removing haem from it, and releasing the haem into the periplasm. Functionally important HmbR receptor domains were discerned using a series of HmbR deletions and site-directed mutations. Mutations exhibiting similar defective phenotypes in N. meningitidis fell into two groups. The first group of mutations affected Hb binding and were located in putative extracellular loops (L) L2 (amino acid residues (aa) 192-230) and L3 (aa 254-284). The second group of mutations resulted in a failure to utilize Hb but proficiency in Hb binding was retained. These mutations localized to the putative extracellular loops L6 (aa 420-462) and L7 (aa 486-516). A highly conserved protein motif found in all haem/Hb receptors, within putative extracellular loop L7 of HmbR, is essential for Hb utilization but not required for Hb binding. This finding suggests a mechanistic involvement of this motif in haem removal from Hb. In addition, an amino-terminal deletion in the putative cork-like domain of HmbR affected Hb usage but not Hb binding. This result supports a role of the cork domain in utilization steps that are subsequent to Hb binding.

Homologues of neisserial heme oxygenase in gram-negative bacteria: degradation of heme by the product of the pigA gene of Pseudomonas aeruginosa

The oxidative cleavage of heme to release iron is a mechanism by which some bacterial pathogens can utilize heme as an iron source. The pigA gene of Pseudomonas aeruginosa is shown to encode a heme oxygenase protein, which was identified in the genome sequence by its significant homology (37%) with HemO of Neisseria meningitidis. When the gene encoding the neisserial heme oxygenase, hemO, was replaced with pigA, we demonstrated that pigA could functionally replace hemO and allow for heme utilization by neisseriae. Furthermore, when pigA was disrupted by cassette mutagenesis in P. aeruginosa, heme utilization was defective in iron-poor media supplemented with heme. This defect could be restored both by the addition of exogenous FeSO4, indicating that the mutant did not have a defect in iron metabolism, and by in trans complementation with pigA from a plasmid with an inducible promoter. The PigA protein was purified by ion-exchange chromotography. The UV-visible spectrum of PigA reconstituted with heme showed characteristics previously reported for other bacterial and mammalian heme oxygenases. The heme-PigA complex could be converted to ferric biliverdin in the presence of ascorbate, demonstrating the need for an exogenous reductant. Acidification and high-performance liquid chromatography analysis of the ascorbate reduction products identified a major product of biliverdin IX-beta. This differs from the previously characterized heme oxygenases in which biliverdin IX-alpha is the typical product. We conclude that PigA is a heme oxygenase and may represent a class of these enzymes with novel regiospecificity.

Mismatch repair and the regulation of phase variation in Neisseria meningitidis

Neisseria meningitidis controls the expression of several genes involved in host adaptation by a process known as phase variation. The phase variation frequency of haemoglobin (Hb) receptors among clinical isolates of serogroups A, B and C differed drastically, ranging from approximately 10(-6) to 10(-2) cfu-1. Frequencies of phase variation are a genetic trait of a particular strain, as two unlinked Hb receptors, hpuAB and hmbR, phase varied with similar frequencies within a given isolate. Based on these frequencies, six Neisserial clinical isolates could be grouped into three distinct classes; slow, medium and fast. An increase in phase variation frequency was accompanied by high rates of spontaneous mutation to rifampicin and nalidixic acid resistance in one medium and one fast strain. The remaining three medium strains displayed elevated levels of phase variation without increases in overall mutability, as they possessed low rates of spontaneous mutation to drug resistance. The mismatch repair system of N. meningitidis was found to play an important role in determining the overall mutability of the clinical isolates. Inactivation of mismatch repair in any strain, regardless of its original phenotype, increased mutability to a level seen in the fast strain. Insertional inactivation of mutS and mutL in the slow strain led to 500- and 250-fold increases in hmbR switching frequency respectively. Concurrently, the frequency of spontaneous point mutations of mutS and mutL mutants from the slow strain was increased 20- to 30-fold to the level seen in the high strain. The status of Dam methylation did not correlate with either the phase variation frequency of Hb receptors or the general mutability of Neisserial strains. Analysis of an expanded set of isolates identified defects in mismatch repair as the genetic basis for strains displaying both the fast Hb switching and high mutation rate phenotypes. In conclusion, elevated frequencies of phase variation were accompanied by increased overall mutability in some N. meningitidis isolates including strains shown to be mismatch repair defective. Other isolates have evolved mechanisms that seem to affect only the switching frequency of phase-variable genes without an accompanied increased accumulation of spontaneous mutations.

Crystal structure of heme oxygenase from the gram-negative pathogen Neisseria meningitidis and a comparison with mammalian heme oxygenase-1

We report the crystal structure of heme oxygenase from the pathogenic bacterium Neisseria meningitidis at 1.5 A and compare and contrast it with known structures of heme oxygenase-1 from mammalian sources. Both the bacterial and mammalian enzymes share the same overall fold, with a histidine contributing a ligand to the proximal side of the heme iron and a kinked alpha-helix defining the distal pocket. The distal helix differs noticeably in both sequence and conformation, and the distal pocket of the Neisseria enzyme is substantially smaller than in the mammalian enzyme. Key glycine residues provide the flexibility for the helical kink, allow close contact of the helix backbone with the heme, and may interact directly with heme ligands.

Antimicrobial properties of porphyrins

A large number of natural and synthetic porphyrins of diverse chemical compositions and characteristics can be isolated from nature or synthesised in the laboratory. Antimicrobial and antiviral activities of porphyrins are based on their ability to catalyse peroxidase and oxidase reactions, absorb photons and generate reactive oxygen species (ROS) and partition into lipids of bacterial membranes. Light-dependent, photodynamic activity of natural and synthetic porphyrins and pthalocyanines against Gram-positive and Gram-negative bacteria has been well demonstrated. Some non-iron metalloporphyrins (MPs) possess a powerful light-independent antimicrobial activity that is based on the ability of these compounds to increase the sensitivity of bacteria to ROS or directly produce ROS. MPs mimic haem in their molecular structure and are actively accumulated by bacteria via high affinity haem-uptake systems. The same uptake systems can be used to deliver antibiotic-porphyrin and antibacterial peptide-porphyrin conjugates. Haemin, the most well known natural porphyrin, possesses a significant antibacterial activity that is augmented by the presence of physiological concentrations of hydrogen peroxide or a reducing agent. Natural and synthetic porphyrins have relatively low toxicity in vitro and in vivo. The ability for numerous chemical modifications and the large number of different mechanisms by which porphyrins affect microbial and viral pathogens place porphyrins into a group of compounds with an outstanding potential for discovery of novel agents, procedures and materials active against pathogenic microorganisms.

Degradation of heme in gram-negative bacteria: the product of the hemO gene of Neisseriae is a heme oxygenase

A full-length heme oxygenase gene from the gram-negative pathogen Neisseria meningitidis was cloned and expressed in Escherichia coli. Expression of the enzyme yielded soluble catalytically active protein and caused accumulation of biliverdin within the E. coli cells. The purified HemO forms a 1:1 complex with heme and has a heme protein spectrum similar to that previously reported for the purified heme oxygenase (HmuO) from the gram-positive pathogen Corynebacterium diphtheriae and for eukaryotic heme oxygenases. The overall sequence identity between HemO and these heme oxygenases is, however, low. In the presence of ascorbate or the human NADPH cytochrome P450 reductase system, the heme-HemO complex is converted to ferric-biliverdin IXalpha and carbon monoxide as the final products. Homologs of the hemO gene were identified and characterized in six commensal Neisseria isolates, Neisseria lactamica, Neisseria subflava, Neisseria flava, Neisseria polysacchareae, Neisseria kochii, and Neisseria cinerea. All HemO orthologs shared between 95 and 98% identity in amino acid sequences with functionally important residues being completely conserved. This is the first heme oxygenase identified in a gram-negative pathogen. The identification of HemO as a heme oxygenase provides further evidence that oxidative cleavage of the heme is the mechanism by which some bacteria acquire iron for further use.

Bacterial heme sources: the role of heme, hemoprotein receptors and hemophores

The major mechanisms by which Gram-negative bacteria acquire heme from host heme-carrier proteins involve either direct binding to specific outer membrane receptors or release of bacterial hemophores that take up heme from host heme carriers and shuttle it back to specific receptors. The ability to interact with and remove heme from carrier proteins distinguishes heme from conceptually similar siderophore and vitamin B12 receptors. Recent genetic, biochemical and crystallization studies have started to unravel the mechanism and molecular interactions between heme-carrier proteins and components of bacterial heme assimilation systems.

Use of heme compounds as iron sources by pathogenic neisseriae requires the product of the hemO gene

Heme compounds are an important source of iron for neisseriae. We have identified a neisserial gene, hemO, that is essential for heme, hemoglobin (Hb), and haptoglobin-Hb utilization. The hemO gene is located 178 bp upstream of the hmbR Hb receptor gene in Neisseria meningitidis isolates. The product of the hemO gene is homologous to enzymes that degrade heme; 21% of its amino acid residues are identical, and 44% are similar, to those of the human heme oxygenase-1. DNA sequences homologous to hemO were ubiquitous in commensal and pathogenic neisseriae. HemO genetic knockout strains of Neisseria gonorrhoeae and N. meningitidis were unable to use any heme source, while the assimilation of transferrin-iron and iron-citrate complexes was unaffected. A phenotypic characterization of a conditional hemO mutant, constructed by inserting an isopropyl-beta-D-thiogalactopyranoside (IPTG)-regulated promoter upstream of the ribosomal binding site of hemO, confirmed the indispensability of the HemO protein in heme utilization. The expression of HemO also protected N. meningitidis cells against heme toxicity. hemO mutants were still able to transport heme into the cell, since both heme and Hb could complement an N. meningitidis hemA hemO double mutant for growth. The expression of the HmbR receptor was reduced significantly by the inactivation of the hemO gene, suggesting that hemO and hmbR are transcriptionally linked. The expression of the unlinked Hb receptor, HpuAB, was not altered. Comparison of the polypeptide patterns of the wild type and the hemO mutant led to detection of six protein spots with an altered expression pattern, suggesting a more general role of HemO in the regulation of gene expression in Neisseriae.

Use of heme-protein complexes by the Yersinia enterocolitica HemR receptor: histidine residues are essential for receptor function

The abilities of two bacterial active heme transporters, HmbR of Neisseria meningitidis and HemR of Yersinia enterocolitica, to use different heme sources were compared. While HmbR-expressing cells used only hemoglobin (Hb) and heme, HemR-expressing bacteria were able to grow on Hb, heme, myoglobin, hemopexin, catalase, human and bovine serum albumin-heme, and haptoglobin-hemoglobin complexes as sources of iron. Expression of functional HemR allowed Escherichia coli cells to respond to heme-containing peptides, microperoxidases MP-8, MP-9, and MP-11, suggesting the ability of HemR to transport heme covalently linked to other molecules. Comparison of HemR with other heme receptors identified several highly conserved histidine residues as well as two conserved amino acid motifs, the FRAP and NPNL boxes. A site-directed mutagenesis approach was used to investigate the roles of His128, His192, His352, and His461 residues in HemR function. The HemR receptor with histidine changed to lysine at position 128 (HemR(H128K)), HemR(H461L), HemR(H461A), and HemR(H128A,H461A) mutant receptors were unable to use Hb, human serum albumin-heme, and myoglobin as sources of porphyrin and iron. Utilization of free heme was also severely affected, with some residual heme uptake in cells expressing HemR(H128K), HemR(H461A), and HemR(H461L). Conversely, the HemR(H192T), HemR(H352A), HemR(H352K), and HemR(H192T,H352K) mutant receptors were fully functional. All mutant HemR proteins were expressed in the outer membrane at levels similar to that of the wild-type HemR receptor. Nonfunctional HemRs were able to bind heme- and Hb-agarose. A hypothetical model of the HemR function in which two conserved histidine residues, His128 and His461, participate in the transport of heme through the receptor pore is postulated.

The 17-gene ethanolamine (eut) operon of Salmonella typhimurium encodes five homologues of carboxysome shell proteins

The eut operon of Salmonella typhimurium encodes proteins involved in the cobalamin-dependent degradation of ethanolamine. Previous genetic analysis revealed six eut genes that are needed for aerobic use of ethanolamine; one (eutR), encodes a positive regulator which mediates induction of the operon by vitamin B12 plus ethanolamine. The DNA sequence of the eut operon included 17 genes, suggesting a more complex pathway than that revealed genetically. We have correlated an open reading frame in the sequence with each of the previously identified genes. Nonpolar insertion and deletion mutations made with the Tn10-derived transposable element T-POP showed that at least 10 of the 11 previously undetected eut genes have no Eut phenotype under the conditions tested. Of the dispensable eut genes, five encode apparent homologues of proteins that serve (in other organisms) as shell proteins of the carboxysome. This bacterial organelle, found in photosynthetic and sulfur-oxidizing bacteria, may contribute to CO2 fixation by concentrating CO2 and excluding oxygen. The presence of these homologues in the eut operon of Salmonella suggests that CO2 fixation may be a feature of ethanolamine catabolism in Salmonella.

Iron acquisition systems in the pathogenic Neisseria

Pathogenic neisseriae have a repertoire of high-affinity iron uptake systems to facilitate acquisition of this essential element in the human host. They possess surface receptor proteins that directly bind the extracellular host iron-binding proteins transferrin and lactoferrin. Alternatively, they have siderophore receptors capable of scavenging iron when exogenous siderophores are present. Released intracellular haem iron present in the form of haemoglobin, haemoglobin-haptoglobin or free haem can be used directly as a source of iron for growth through direct binding by specific surface receptors. Although these receptors may vary in complexity and composition, the key protein involved in the transport of iron (as iron, haem or iron-siderophore) across the outer membrane is a TonB-dependent receptor with an overall structure presumably similar to that determined recently for Escherichia coli FhuA or FepA. The receptors are potentially ideal vaccine targets in view of their critical role in survival in the host. Preliminary pilot studies indicate that transferrin receptor-based vaccines may be protective in humans.

HmbR, a hemoglobin-binding outer membrane protein of Neisseria meningitidis, undergoes phase variation

Neisseria meningitidis uses hemoglobin (Hb) as an iron source via two TonB-dependent outer membrane receptors, HmbR and HpuB. Analysis of 25 epidemiologically unrelated clinical isolates from serogroups A, B, C, and Y revealed that 64% strains possessed both Hb receptor genes. Examination of the hmbR expression pattern in strains in which the hpuB gene was genetically inactivated revealed two distinct Hb utilization phenotypes. Five strains retained the ability to grow as a confluent lawn, while seven grew only as single colonies around Hb discs. The single-colony phenotype observed for some hpuB mutants is suggestive of phase variation of hmbR. The length of the poly(G) tract starting at position +1164 of hmbR absolutely correlated with the two Hb utilization phenotypes. All five strains that grew as confluent lawns around Hb discs possessed either 9 or 12 consecutive G residues. All seven strains that grew as single colonies around Hb discs had poly(G) tracts of a length other than 9 or 12. These single-colony variants that arose around the Hb discs had poly(G) tracts with either 9 or 12 consecutive G residues restoring the hmbR reading frame. Inactivation of hmbR in these strains resulted in a loss of Hb utilization, demonstrating that the change in the hmbR gene was responsible for the phenotypic switch. The switching rates from hmbR phase off to phase on were approximately 5 x 10(-4) in four serogroup C strains, 2 x 10(-2) in the serogroup A isolate, and 7 x 10(-6) in the serogroup B isolate.

Non-iron metalloporphyrins: potent antibacterial compounds that exploit haem/Hb uptake systems of pathogenic bacteria

A new group of potent antibacterial compounds, non-iron metalloporphyrins (MPs), is described. MPs possess a strong antibacterial activity against Gram-positive bacteria, Gram-negative bacteria and mycobacteria. Anaerobically grown bacteria and microorganisms that do not respire and/or express haem uptake systems were resistant to MPs. Antibacterial activity of MPs was not affected by known antibiotic resistance mechanisms operating in bacteria. The most potent MP against Y. enterocolitica, methicillin-resistant S. aureus and M. smegmatis was gallium protoporphyrin IX (Ga-PPIX). When tested alone, Ga ions and metal-free porphyrins had approximately 100-fold higher minimum inhibitory concentration (MIC) values for these organisms. Ga-PPIX was not degraded by MP-sensitive bacteria, indicating that the whole molecule is responsible for antibacterial activity. MPs are antibacterial 'Trojan horses', as they exploit haem transport systems of Gram-negative bacteria as portals of entry into the cell. Bacterial mutants in superoxide dismutases, catalases and stationary-phase sigma factors were hypersensitive to Ga-PPIX. The extreme sensitivity of sod mutants to MPs and the requirement for active respiration for MP activity suggests that these compounds stimulate the production of reactive oxygen radicals in bacteria. Ga-PPIX was not toxic to primary human fibroblasts, several established cell lines and experimental animals at concentrations > 100-fold higher than the MIC for sensitive bacteria.

The identification of exported proteins with gene fusions to invasin

Exported proteins are integral to understanding the biology of bacterial organisms. They have special significance in pathogenesis research because they can mediate critical interactions between pathogens and eukaryotic cell surfaces. Further, they frequently serve as targets for vaccines and diagnostic tests. The commonly used genetic assays for identifying exported proteins use fusions to alkaline phosphatase or beta-lactamase. These systems are not ideal for identifying outer membrane proteins because they identify a large number of inner membrane proteins as well. We addressed this problem by developing a gene fusion system that preferentially identifies proteins that contain cleavable signal sequences and are released from the inner membrane. This system selects fusions that restore outer membrane localization to an amino terminal-truncated Yersinia pseudotuberculosis invasin derivative. In the present study, a variety of Salmonella typhimurium proteins that localize beyond the inner membrane were identified with gene fusions to this invasin derivative. Previously undescribed proteins identified include ones that share homology with components of fimbrial operons, multiple drug resistance efflux pumps and a haemolysin. All of the positive clones analysed contain cleavable signal sequences. Moreover, over 40% of the genes identified encode putative outer membrane proteins. This system has several features that may make it especially useful in the study of genetically intractable organisms.

Cloning and characterization of the Neisseria meningitidis rfaC gene encoding alpha-1,5 heptosyltransferase I

We cloned and characterized the Neisseria meningitidis rfaC gene which encodes an enzyme, alpha-1,5 heptosyltransferase I, involved in the synthesis of the deep-core of the lipooligosaccharide. The N. meningitidis rfaC mutant, obtained by an allelic exchange, produced lipooligosaccharide which migrated faster in sodium dodecyl sulfate-polyacrylamide gel electrophoresis than the lipooligosaccharide isolated from the wild-type N. meningitidis. The N. meningitidis rfaC mutant was not affected by growth in a rich microbiological medium and did not show any defect in adhesion to epithelial cell lines. Conversely, the rfaC mutant was attenuated in the infant rat model of meningococcemia, thus confirming the importance of intact lipooligosaccharide in the virulence of N. meningitidis.

Neisseria meningitidis tonB, exbB, and exbD genes: Ton-dependent utilization of protein-bound iron in Neisseriae

We have recently cloned and characterized the hemoglobin (Hb) receptor gene, hmbR, from Neisseria meningitidis. To identify additional proteins that are involved in Hb utilization, the N. meningitidis Hb utilization system was reconstituted in Escherichia coli. Five cosmids from N. meningitidis DNA library enabled a heme-requiring (hemA), HmbR-expressing mutant of E. coli to use Hb as both porphyrin and iron source. Nucleotide sequence analysis of DNA fragments subcloned from the Hb-complementing cosmids identified four open reading frames, three of them homologous to Pseudomonas putida, E. coli, and Haemophilus influenzae exbB, exbD, and tonB genes. The N. meningitidis TonB protein is 28.8 to 33.6% identical to other gram-negative TonB proteins, while the N. meningitidis ExbD protein shares between 23.3 and 34.3% identical amino acids with other ExbD and TolR proteins. The N. meningitidis ExbB protein was 24.7 to 36.1% homologous with other gram-negative ExbB and TolQ proteins. Complementation studies indicated that the neisserial Ton system cannot interact with the E. coli FhuA TonB-dependent outer membrane receptor. The N. meningitidis tonB mutant was unable to use Hb, Hb-haptoglobin complexes, transferrin, and lactoferrin as iron sources. Insertion of an antibiotic cassette in the 3' end of the exbD gene produced a leaky phenotype. Efficient usage of heme by N. meningitidis tonB and exbD mutants suggests the existence of a Ton-independent heme utilization mechanism. E. coli complementation studies and the analysis of N. meningitidis hmbR and hpu mutants suggested the existence of another Hb utilization mechanism in this organism.

Identification of a new iron regulated locus of Salmonella typhi

In order to identify genes belonging to the Fur regulon of Salmonella typhi which are absent from Escherichia coli K-12, a plasmid gene bank consisting of 4000 independent clones was screened for Fur regulated promoters using the Fur titration assay (FURTA). DNA probes generated from FURTA positive plasmids were then used for hybridization with chromosomal DNA from S. typhi, Salmonella typhimurium and E. coli. Using these techniques we identified an iron regulated locus present in S. typhi and S. typhimurium but not in E. coli. Further cloning and nucleotide sequence analysis identified two open reading frames, termed iroBC, organized in a typical operon structure. The genes iroBC were located at 4 and 57 centisomes on the physical maps of Salmonella typhi and S. typhimurium, respectively. This region of the S. typhimurium chromosome contains a large DNA loop which is absent from the corresponding area of the E. coli chromosome. Finally, we developed a new method for generation of single copy transcriptional fusions. A suicide vector was constructed, which allows for the generation of chromosomal fusions to the promoterless E. coli lacZYA genes. By integration of this construct at the iro locus we could establish iron responsive expression of iroBC.

Contribution of TonB- and Feo-mediated iron uptake to growth of Salmonella typhimurium in the mouse

We examined the role of iron(II) and iron(III) uptake, mediated by FeoB and TonB, respectively, in infection of the mouse by Salmonella typhimurium. The S. typhimurium feoB gene, encoding a homolog of an Escherichia coli cytoplasmic membrane iron(II) permease, was cloned, and a mutant was generated by allelic exchange. In addition, an S. typhimurium tonB mutant was constructed. Together these two mutations inactivate all known iron uptake systems of S. typhimurium. We examined the abilities of these mutants to grow in vitro and in different compartments of the host. Mutants in feoB were outcompeted by the wild type during mixed colonization of the mouse intestine, but the feoB mutation did not attenuate S. typhimurium for oral or intraperitoneal infection of mice. The tonB mutation attenuated S. typhimurium for infection of mice by the intragastric route but not the intraperitoneal route, and the mutant was recovered in lower numbers from the Peyer's patches and mesenteric lymph nodes than the wild type. These results indicate that TonB-mediated iron uptake contributes to colonization of the Peyer's patches and mesenteric lymph nodes but not the liver and spleen of the mouse. The tonB feoB double mutant, given intraperitoneally, was able to infect the liver and spleen at wild-type doses, indicating that additional iron acquisition systems are used during growth at systemic sites of infection.

HmbR outer membrane receptors of pathogenic Neisseria spp.: iron-regulated, hemoglobin-binding proteins with a high level of primary structure conservation

We have recently cloned and characterized the hemoglobin receptor gene from Neisseria meningitidis serogroup C. N. meningitidis cells expressing HmbR protein were able to bind biotinylated hemoglobin, and the binding was specifically inhibited by unlabeled hemoglobin and not heme. The HmbR-mediated hemoglobin binding activity of N. meningitidis cells was shown to be iron regulated. The presence of hemoglobin but not heme in the growth medium stimulated HmbR-mediated hemoglobin binding activity. The efficiency of utilization of different hemoglobins by the HmbR-expressing N. meningitidis cells was shown to be species specific; human hemoglobin was the best source of iron, followed by horse, rat, turkey, dog, mouse, and sheep hemoglobins, The phenotypic characterization of HmbR mutants of some clinical strains of N. meningitidis suggested the existence of two unrelated hemoglobin receptors. The HmbR-unrelated hemoglobin receptor was shown to be identical to Hpu, the hemoglobin-haptoglobin receptor of N. meningitidis. The Hpu-dependent hemoglobin utilization system was not able to distinguish between different sources of hemoglobin; all animal hemoglobins were utilized equally well. HmbR-like genes are also present in N. meningitidis serogroups A and B, Neisseria gonorrhoeae MS11 and FA19, Neisseria perflava, and Neisseria polysaccharea. The hemoglobin receptor genes from N. meningitidis serogroups A and B and N. gonorrhoeae MS11 were cloned, and their nucleotide sequences were determined. The nucleotide sequence identity ranged between 86.5% (for N. meningitidis serogroup B hmbR and MS11 hmbR) and 93.4% (for N. meningitidis serogroup B hmbR and N. meningitidis serogroup C hmbR). The deduced amino acid sequences of these neisserial hemoglobin receptors were also highly related, with overall 84.7% conserved amino acid residues. A stop codon was found in the hmbR gene of N. gonorrhoeae MS11. This strain was still able to use hemoglobin and hemoglobin-haptoglobin complexes as iron sources, indicating that some gonococci may express only the HmbR-independent hemoglobin utilization system.

Macrophage killing is an essential virulence mechanism of Salmonella typhimurium

Phagocytic cells are a critical line of defense against infection. The ability of a pathogen to survive and even replicate within phagocytic cells is a potent method of evading the defense mechanisms of the host. A number of pathogens survive within macrophages after phagocytosis and this contributes to their virulence. Salmonella is one of these pathogens. Here we report that 6-14 hr after Salmonella enters the macrophage and replicates, it resides in large vacuoles and causes the destruction of these cells. Furthermore, we identified four independently isolated MudJ-lacZ insertion mutants that no longer cause the formation of these vacuoles or kill the macrophages. All four insertions were located in the ompR/envZ regulon. These findings suggest that killing and escape from macrophages may be as important steps in Salmonella pathogenesis as are survival and replication in these host cells.

Fur regulon of Salmonella typhimurium: identification of new iron-regulated genes

In order to identify genes belonging to the Fur regulon of Salmonella typhimurium, a bank of 10,000 independent S. typhimurium MudJ insertion mutants was screened for lacZ fusions regulated by the iron response regulator Fur. In parallel, a plasmid gene bank of S. typhimurium consisting of 10,000 independent clones was screened for Fur-regulated promoters or iron binding proteins by the Fur titration assay (FURTA). Fur-regulated MudJ insertions and Fur-regulated promoters were mapped. In addition, iron-regulated promoter activities of transcriptional fusions from MudJ insertions and FURTA-positive clones were quantified. The nucleotide sequences of 11 FURTA-positive plasmids and of short fragments of DNA flanking three MudJ insertions were determined. By these methods we identified 14 Fur-regulated genes of S. typhimurium. For 11 of these genes, Fur-regulated homologs have been described in Escherichia coli or Yersinia enterocolitica, including fhuA,fhuB,fepA,fes,fepD,p43,entB,fur ,foxA,hemP, and fhuE. In addition, we identified three genes with homologs in other bacteria which have not previously been shown to be Fur regulated.

Functional domains of the Escherichia coli ferric uptake regulator protein (Fur)

The functions of N- and C-terminal domains of the Fur repressor of Escherichia coli in promoter recognition and dimerization were studied. We investigated the ability of fusion proteins containing the N- or C-terminal domain of Fur to dimerize and to repress a Fur-regulated lacZ fusion gene. The N-terminal domain, when fused to the C-terminal domain of the repressor CI857, repressed a Fur-regulated lacZ fusion. However, the Fur-CI857 fusion was unable to complement the growth defect of an E. coli fur mutant on fumarate and succinate. The C-terminal domain of Fur, when fused to the N-terminus of CI857, repressed a lambda Pr-regulated lacZ fusion, indicating dimerization of the chimeric protein, which is a prerequisite for CI activity. Both fusion proteins were fully active under both iron-rich and iron-poor growth conditions. We conclude that the N-terminal domain of Fur is involved in recognition of the Fur-responsive promoter and the C-terminus mediates oligomerization of the repressor.

Identification of the hopG gene, a component of Escherichia coli K-12 type II export system, and its conservation among different pathogenic Escherichia coli and Shigella isolates

The Escherichia coli K-12 gene coding for a component of a type II export system was identified and characterized. The HopG protein contains a typical prepilin peptidase cleavage site and has a high degree of homology with proteins PulG, OutG, and ExeG, which are components of type II secretion systems from Klebsiella pneumoniae, Erwinia carotovora, and Aeromonas hydrophila.

Ethanolamine utilization in Salmonella typhimurium: nucleotide sequence, protein expression, and mutational analysis of the cchA cchB eutE eutJ eutG eutH gene cluster

A fragment of the Salmonella typhimurium ethanolamine utilization operon was cloned and characterized. The 6.3-kb nucleotide sequence encoded six complete open reading frames, termed cchA, cchB, eutE, eutJ, eutG, and eutH. In addition, the nucleotide sequences of two incomplete open reading frames, termed eutX and eutI, were also determined. Comparison of the deduced amino acid sequences and entries in the GenBank database indicated that eutI encodes a phosphate acetyltransferase-like enzyme. The deduced amino acid sequences of the EutE and EutG proteins revealed a significant degree of homology with the Escherichia coli alcohol dehydrogenase AdhE sequence. Mutations in eutE or eutG completely abolished the ability of mutants to utilize ethanolamine as a carbon source and reduced the ability to utilize ethanolamine as a nitrogen source. The product of eutE is most probably an acetaldehyde dehydrogenase catalyzing the conversion of acetaldehyde into acetyl coenzyme A. The product of the eutG gene, an uncommon iron-containing alcohol dehydrogenase, may protect the cell from unconverted acetaldehyde by converting it into an alcohol. The deduced amino acid sequence of cchA resembles that of carboxysome shell proteins from Thiobacillus neapolitanus and Synechococcus sp. as well as that of the PduA product from S. typhimurium. CchA and CchB proteins may be involved in the formation of an intracellular microcompartment responsible for the metabolism of ethanolamine. The hydrophobic protein encoded by the eutH gene possesses some characteristics of bacterial permeases and might therefore be involved in the transport of ethanolamine. Ethanolamine-utilization mutants were slightly attenuated in a mouse model of S. typhimurium infection, indicating that ethanolamine may be an important source of nitrogen and carbon for S. typhimurium in vivo.

The Neisseria meningitidis haemoglobin receptor: its role in iron utilization and virulence

The Neisseria meningitidis haemoglobin receptor gene, hmbR, was cloned by complementation in a porphyrin-requiring Escherichia coli mutant. hmbR encodes an 89.5 kDa outer membrane protein which shares amino acid homology with the TonB-dependent receptors of Gram-negative bacteria. HmbR had the highest similarity to Neisseria transferrin and lactoferrin receptors. The utilization of haemoglobin as an iron source required internalization of the haemin moiety by the cell. The mechanism of haemin internalization via the haemoglobin receptor was TonB-dependent in E. coli. A N. meningitidis hmbR mutant was unable to use haemoglobin but could still use haemin as a sole iron source. The existence of a second N. meningitidis receptor gene, specific for haemin, was shown by the isolation of cosmids which did not hybridize with the hmbR probe, but which were able to complement an E. coli hemA aroB mutant on haemin-supplemented plates. The N. meningitidis hmbR mutant was attenuated in an infant rat model for meningococcal infection, indicating that haemoglobin utilization is important for N. meningitidis virulence.

Transport of haemin across the cytoplasmic membrane through a haemin-specific periplasmic binding-protein-dependent transport system in Yersinia enterocolitica

The Yersinia enterocolitica O:8 periplasmic binding-protein-dependent transport (PBT) system for haemin was cloned and characterized. It consisted of four proteins: the periplasmic haemin-binding protein HemT, the haemin permease protein HemU, the ATP-binding hydrophilic protein HemV and the putative haemin-degrading protein HemS. Y. enterocolitica strains mutated in hemU or hemV genes were unable to use haemin as an iron source whereas those mutated in the hemT gene were able to use haemin as an iron source. As Escherichia coli strains expressing only the haemin outer membrane receptor protein HemR from Y. enterocolitica were capable of using haemin as an iron source the existence of an E. coli K-12 haemin-specific PBT system is postulated. The first gene in the Y. enterocolitica haemin-specific PBT system encoded a protein, HemS, which is probably involved in the degradation of haemin in the cytoplasm. The presence of the hemS gene was necessary to prevent haemin toxicity in E. coli strains that accumulate large amounts of haemin in the cytoplasm. We propose a model of haemin utilization in Y. enterocolitica in which HemT, HemU and HemV proteins transport haemin into the cytoplasm where it is degraded by HemS thereby liberating the iron.

Salmonella typhimurium loci involved in survival within macrophages

A set of Tn10 mutants of Salmonella typhimurium which have a diminished capacity to survive in murine macrophages and decreased virulence in mice has been described previously. In this study, we characterized 30 of these mutants and determined map locations of Tn10 insertions for 23 of these strains. In addition, short fragments of transposon-flanking DNA were cloned, and the nucleotide sequence was determined for 23 mutants. Seven mutants carried transposon insertions in known genes, representing six loci: htrA, prc, purD, fliD, nagA, and smpB. The possible roles of these genes in Salmonella virulence are discussed. One insertion was found to be in an unknown gene which shared homology with the open reading frames Bv' and Bv located in the pin inversion system of Shigella boydii. In one mutant, Tn10 was found to be inserted in a gene with significant homology to adhE of Escherichia coli and Clostridium acetobutylicum. The map location and degree of homology indicate that the Salmonella gene encodes a related, but different, dehydrogenase. In 14 of the mutants analyzed, Tn10 was inserted into genes which had no significant homologies to entries in the DNA and protein data bases. In conclusion, 16 insertions define loci, termed ims for impaired macrophage survival, which have not yet been described in S. typhimurium but have been shown previously to be necessary for full virulence in mice. Although most ims loci are distributed randomly throughout the genome, a cluster was found between 75 and 78 min on the Salmonella chromosome.

Fur regulon in gram-negative bacteria. Identification and characterization of new iron-regulated Escherichia coli genes by a fur titration assay

A highly sensitive genetic screen for the detection of cloned genes coding for iron-regulated and iron-storage/binding proteins was developed. The Fur titration assay (FURTA) enabled identification of cloned iron-regulated genes from different Gram-positive and Gram-negative bacteria such as: Bacillus subtilis, Escherichia coli, Pantoea agglomerans, Pseudomonas putida, Salmonella typhimurium, Serratia marcescens and Yersinia enterocolitica. An ordered E. coli cosmid library was screened for either new genes containing Fur-box nucleotide sequences or genes coding for iron-storage/binding proteins. Among 150 cosmids covering approximately 85% of the E. coli genome, 24 cosmids were identified as positive by FURTA. Nine of them contained new E. coli Fur-regulated genes and/or iron-storage/binding genes since they mapped at loci different to any of the known Fur-box containing genes. A new E. coli gene encoding a 8.7 kDa high-potential iron-sulfur-like protein was identified, cloned and sequenced. The Fur titration assay was also used to probe in vivo interaction between Fur repressor and different synthetic plasmid-located Fur-boxes. Non-optimal base-pairs in one half of the Fur-box nucleotide sequence led to a dramatic decrease of Fur repressor affinity. A synthetic Fur-box with changes in both Fur-box halves was no longer bound by the Fur repressor complex in vivo.

Molecular cloning of bacterial DNA in vivo using a transposable R6K ori and a P1vir phage

A new method of cloning in vivo using the P1vir phage and transposon Tn5-rpsL oriR6K was developed. The method relies upon recircularization of transducing DNA containing a transposon insertion in a recombination-deficient strain of Escherichia coli K-12 and subsequent stable replication of the recircularized DNA. Using this method, we were able to clone in vivo the chromosomal region located between approximately 7.1 and 9.2 min on the E. coli K-12 map in a 95-kb plasmid.

Purification of yersiniabactin: a siderophore and possible virulence factor of Yersinia enterocolitica

HPLC analysis revealed that Yersinia enterocolitica WA-C produced two substances under iron-limiting conditions one of which was identified as 2,3-dihydroxybenzoyl-L-serine. The other compound had iron-complexing activity and was called yersiniabactin. The fur mutant H1852 was shown to produce yersiniabactin constitutively in an iron-independent manner. Yersiniabactin was isolated by ethyl acetate extraction from the spent medium of H1852, size-fractionation chromatography and preparative HPLC. A catechol function was demonstrated with different chemical assays and by UV-visible spectroscopy. The molecular mass of yersiniabactin was determined to be 482 Da. Purified yersiniabactin stimulated growth of Y. enterocolitica and Escherichia coli phi under iron-limiting conditions and apparently served as an iron carrier. Transport of 55Fe-yersiniabactin was TonB-dependent, indicating a receptor-mediated uptake across the outer membrane. A pesticin-resistant mutant missing the receptor protein FyuA was unable to transport and use yersiniabactin as a siderophore.

Virulence of Yersinia enterocolitica is closely associated with siderophore production, expression of an iron-repressible outer membrane polypeptide of 65,000 Da and pesticin sensitivity

Iron-repressible outer membrane proteins (Irp) and siderophore production of Yersinia enterocolitica, serotype 08, were subjected to analysis. Here four Irps of apparent molecular weights of 62,000, 65,000, 74,000 and 75,000 could be identified which were expressed constitutively by a fur mutant. Production of a novel catechol-containing siderophore (denoted yersiniabactin) was detected by siderophore-indicator agar (chrome azurol S) and feeding experiments. Growth support by yersiniabactin under iron-restricted conditions was TonB- and Irp65-dependent and correlated with pesticin-sensitivity of Yersinia enterocolitica and Escherichia coli O. From these results we conclude that Irp65 of Y. enterocolitica functions as yersiniabactin receptor (FyuA) and as pesticin receptor. By immunoblotting using rabbit antibodies against Irp65 and chrome azurol S-agar, we were able to demonstrate that all tested mouse-lethal Y. enterocolitica and Yersinia pseudotuberculosis strains of different serotypes express siderophores and Irp65. Moreover, the anti-Irp65 rabbit serum did not cross-react with the known iron-repressible high-molecular-weight proteins (HMWPs). Evidently, the mouse lethality trait in enteropathogenic Yersinia spp. is closely associated with a novel iron-uptake system, comprising the production of a siderophore and a siderophore receptor of apparent molecular mass 65,000 Da.

Survey on newly characterized iron uptake systems of Yersinia enterocolitica

Iron assimilation systems have been shown to be important for virulence in Yersinia enterocolitica. In principle, iron transport in Y. enterocolitica is similarly organized as in Escherichia coli although some differences exist in the siderophores used. A TonB function was identified which in a tonB mutant was complemented by the E. coli tonB gene. Three outer membrane receptors for siderophores were cloned and sequenced: FoxA for ferrioxamine B and E, FcuA for ferrichrome and HemR for heme uptake. In addition, two receptors were identified by mutants: CccA for catechol cephalosporins and FyuA for yersiniabactin, the siderophore produced by virulent yersiniae. In addition, the FyuA protein is assumed to be the pesticin receptor.

Escherichia coli K-12 ferrous iron uptake mutants are impaired in their ability to colonize the mouse intestine

The streptomycin-treated mouse colonization model was used to investigate the role of the Fe2+ uptake system (Feo) of Escherichia coli K12 in the colonization of the mouse intestine. Mutants impaired in the uptake of Fe2+ ions were shown to be deficient also in their colonization ability. Both enterochelin-producing and enterochelin-nonproducing Escherichia coli feo mutants were unable to colonize the mouse intestine. These results demonstrated that Fe(II) is an essential source of iron for E. coli grown in the intestine.

Hemin uptake system of Yersinia enterocolitica: similarities with other TonB-dependent systems in gram-negative bacteria

The hemin receptor HemR of Yersinia enterocolitica was identified as a 78 kDa iron regulated outer membrane protein. Cells devoid of the HemR receptor as well as cells mutated in the tonB gene were unable to take up hemin as an iron source. The hemin uptake operon from Y. enterocolitica was cloned in Escherichia coli K12 and was shown to encode four proteins: HemP (6.5 kDa), HemR (78 kDa), HemS (42 kDa) and HemT (27 kDa). When expressed in E.coli hemA aroB, a plasmid carrying genes for HemP and HemR allowed growth on hemin as a porphyrin source. Presence of genes for HemP, HemR and HemS was necessary to allow E.coli hemA aroB cells to use hemin as an iron source. The nucleotide sequence of the hemR gene and its promoter region was determined and the amino acid sequence of the HemR receptor deduced. HemR has a signal peptide of 28 amino acids and a typical TonB box at its amino-terminus. Upstream of the first gene in the operon (hemP), a well conserved Fur box was found which is in accordance with the iron-regulated expression of HemR.