Assmilation of iron by pathogenic Neisseria spp

Tripartite ATP-Independent Periplasmic (TRAP) Transporters and Tripartite Tricarboxylate Transporters (TTT): From Uptake to Pathogenicity

The ability to efficiently scavenge nutrients in the host is essential for the viability of any pathogen. All catabolic pathways must begin with the transport of substrate from the environment through the cytoplasmic membrane, a role executed by membrane transporters. Although several classes of cytoplasmic membrane transporters are described, high-affinity uptake of substrates occurs through Solute Binding-Protein (SBP) dependent systems. Three families of SBP dependant transporters are known; the primary ATP-binding cassette (ABC) transporters, and the secondary Tripartite ATP-independent periplasmic (TRAP) transporters and Tripartite Tricarboxylate Transporters (TTT). Far less well understood than the ABC family, the TRAP transporters are found to be abundant among bacteria from marine environments, and the TTT transporters are the most abundant family of proteins in many species of β-proteobacteria. In this review, recent knowledge about these families is covered, with emphasis on their physiological and structural mechanisms, relating to several examples of relevant uptake systems in pathogenicity and colonization, using the SiaPQM sialic acid uptake system from Haemophilus influenzae and the TctCBA citrate uptake system of Salmonella typhimurium as the prototypes for the TRAP and TTT transporters, respectively. High-throughput analysis of SBPs has recently expanded considerably the range of putative substrates known for TRAP transporters, while the repertoire for the TTT family has yet to be fully explored but both types of systems most commonly transport carboxylates. Specialized spectroscopic techniques and site-directed mutagenesis have enriched our knowledge of the way TRAP binding proteins capture their substrate, while structural comparisons show conserved regions for substrate coordination in both families. Genomic and protein sequence analyses show TTT SBP genes are strikingly overrepresented in some bacteria, especially in the β-proteobacteria and some α-proteobacteria. The reasons for this are not clear but might be related to a role for these proteins in signaling rather than transport.

Strategies of Intracellular Pathogens for Obtaining Iron from the Environment

Most microorganisms are destroyed by the host tissues through processes that usually involve phagocytosis and lysosomal disruption. However, some organisms, called intracellular pathogens, are capable of avoiding destruction by growing inside macrophages or other cells. During infection with intracellular pathogenic microorganisms, the element iron is required by both the host cell and the pathogen that inhabits the host cell. This minireview focuses on how intracellular pathogens use multiple strategies to obtain nutritional iron from the intracellular environment in order to use this element for replication. Additionally, the implications of these mechanisms for iron acquisition in the pathogen-host relationship are discussed.

Pyrophosphate-mediated iron acquisition from transferrin in Neisseria meningitidis does not require TonB activity

The ability to acquire iron from various sources has been demonstrated to be a major determinant in the pathogenesis of Neisseria meningitidis. Outside the cells, iron is bound to transferrin in serum, or to lactoferrin in mucosal secretions. Meningococci can extract iron from iron-loaded human transferrin by the TbpA/TbpB outer membrane complex. Moreover, N. meningitidis expresses the LbpA/LbpB outer membrane complex, which can extract iron from iron-loaded human lactoferrin. Iron transport through the outer membrane requires energy provided by the ExbB-ExbD-TonB complex. After transportation through the outer membrane, iron is bound by periplasmic protein FbpA and is addressed to the FbpBC inner membrane transporter. Iron-complexing compounds like citrate and pyrophosphate have been shown to support meningococcal growth ex vivo. The use of iron pyrophosphate as an iron source by N. meningitidis was previously described, but has not been investigated. Pyrophosphate was shown to participate in iron transfer from transferrin to ferritin. In this report, we investigated the use of ferric pyrophosphate as an iron source by N. meningitidis both ex vivo and in a mouse model. We showed that pyrophosphate was able to sustain N. meningitidis growth when desferal was used as an iron chelator. Addition of a pyrophosphate analogue to bacterial suspension at millimolar concentrations supported N. meningitidis survival in the mouse model. Finally, we show that pyrophosphate enabled TonB-independent ex vivo use of iron-loaded human or bovine transferrin as an iron source by N. meningitidis. Our data suggest that, in addition to acquiring iron through sophisticated systems, N. meningitidis is able to use simple strategies to acquire iron from a wide range of sources so as to sustain bacterial survival.

FbpA--a bacterial transferrin with more to offer

BACKGROUND

Gram negative bacteria require iron for growth and virulence. It has been shown that certain pathogenic bacteria such as Neisseria gonorrhoeae possess a periplasmic protein called ferric binding protein (FbpA), which is a node in the transport of iron from the cell exterior to the cytosol.

SCOPE OF REVIEW

The relevant literature is reviewed which establishes the molecular mechanism of FbpA mediated iron transport across the periplasm to the inner membrane.

MAJOR CONCLUSIONS

Here we establish that FbpA may be considered a bacterial transferrin on structural and functional grounds. Data are presented which suggest a continuum whereby FbpA may be considered as a naked iron carrier, as well as a Fe-chelate carrier, and finally a member of the larger family of periplasmic binding proteins.

GENERAL SIGNIFICANCE

An investigation of the molecular mechanisms of action of FbpA as a member of the transferrin super family enhances our understanding of bacterial mechanisms for acquisition of the essential nutrient iron, as well as the modes of action of human transferrin, and may provide approaches to the control of pathogenic diseases. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Replication of Neisseria meningitidis within epithelial cells requires TonB-dependent acquisition of host cell iron

Neisseria meningitidis (meningococcus [MC]) is able to enter and replicate within epithelial cells. Iron, an essential nutrient for nearly all organisms, is an important determinant in the ability of MC to cause disease; however, its role in MC intracellular replication has not been investigated. We analyzed the growth of MC within the A431 human epithelial cell line and the dependence of this growth on iron uptake. We present evidence here that chelation of iron from infected tissue culture cells with Desferal strongly inhibited intracellular replication of wild-type (wt) MC. We also provide genetic evidence that iron must be acquired by MC from the host cell in order for it to replicate. An hmbR mutant that is unable to use hemoglobin iron and could not grow in tissue culture media without iron supplementation replicated more rapidly within epithelial cells than its wt parent strain. An fbpA mutant that is unable to utilize human transferrin iron or lactoferrin iron replicated normally within cells. In contrast, a tonB mutant could not replicate intracellularly unless infected cultures were supplemented with ferric nitrate. Taken together, these findings strongly suggest that MC intracellular replication requires TonB-dependent uptake of a novel host cell iron source.

Effect of siderophores, catecholamines, and catechol compounds on Listeria spp. Growth in iron-complexed medium

Almost all bacteria require iron for growth and virulence expression. However, Listeria spp. do not produce any siderophore for iron acquisition. Representative strains of each of the six species of Listeria were examined for their ability to use various compounds as iron suppliers in iron-restricted medium. Here we show that L. monocytogenes, L. innocua, L. ivanovii, L. welshimeri, L. seeligeri, and L. grayi were able to use exogenous siderophores and various catechol ligands, including catecholamines, to overcome growth inhibition induced by tropolone, an iron chelating agent. In contrast, no growth promoting effect was observed with normetanephrine or 4-hydroxy-3-methoxyphenylglycol-piperazine salt, which indicates that the o-diphenol function of the ligand must be free to allow iron acquisition. Furthermore, we demonstrate that catecholamines do not act through specific bacterial receptors, because no difference in growth stimulation was observed between [+]- and [-]-norepinephrine. These results show that utilization of a variety of catechol compounds to acquire iron is a general phenomenon in the genus Listeria.

Molecular characterization of FrpB, the 70-kilodalton iron-regulated outer membrane protein of Neisseria meningitidis

The structural gene encoding the 70-kDa outer membrane protein FrpB of Neisseria meningitidis was cloned and sequenced. A mutant lacking FrpB was constructed. No difference in iron utilization between the mutant and the parental strain was observed. A minor effect of the mutation on serum resistance was observed. A topology model for FrpB in the outer membrane is proposed.

Identification of an iron-regulated outer membrane protein of Neisseria meningitidis involved in the utilization of hemoglobin complexed to haptoglobin

Hemoglobin complexed to the plasma protein haptoglobin can be used by Neisseria meningitidis as a source of iron to support growth in vitro. An N meningitidis mutant, DNM2E4, was generated by insertion of the mini-Tn3erm transposon into the gene coding for an 85-kDa iron-regulated outer membrane protein. Membrane proteins prepared from DNM2E4 were identical to those of the wild-type strain except that the 85-kDa protein was not produced. This mutant was unable to use hemoglobin-haptoglobin complexes as an iron source to support growth and was also impaired in the utilization of free hemoglobin. The mutant failed to bind free hemoglobin, hemoglobin-haptoglobin complexes, or apo-haptoglobin in a solid-phase dot blot assay. The 85-kDa protein was affinity purified when hemoglobin-haptoglobin complexes were used as a ligand but was not purified when free hemoglobin was used. We hypothesize that the 85-kDa iron-regulated protein is the hemoglobin-haptoglobin receptor and designate this protein Hpu (for hemoglobin-haptoglobin utilization).

Vibrio cholerae iron transport systems: roles of heme and siderophore iron transport in virulence and identification of a gene associated with multiple iron transport systems

Vibrio cholerae iron transport mutants were tested for their ability to cause disease in an infant mouse model. The mice were challenged with either the wild-type strain, a vibriobactin synthesis mutant, a heme utilization mutant, or double mutants containing both the vibriobactin synthesis defect and the heme utilization defect. When mice were challenged with 10(7) bacteria, the ability of the double mutant to survive in the intestines was greatly reduced and that of the heme utilization mutant was slightly reduced compared with that of the wild type or the vibriobactin synthesis mutant. When the inoculum size was reduced 10-fold, all of the iron transport mutants failed to colonize the intestines and failed to cause diarrhea in the mice, whereas the wild-type strain was not cleared and elicited a diarrheal response. These data indicate that disruption of either the heme utilization or the vibriobactin uptake system reduces the ability of V. cholerae to cause disease. One of the heme utilization mutants, DHH1, was found to be defective also in utilization of vibriobactin and ferrichrome, mimicking the Escherichia coli TonB- phenotype. This mutant was the least virulent of the iron transport mutants tested. Transformation of DHH1 with the recombinant plasmid pHUT4 restored the abilities to use hemin, vibriobactin, and ferrichrome as iron sources, suggesting that pHUT4 encodes a gene(s) involved globally in the iron transport systems. Hybridization of Vibrio DNA with the V. cholerae heme utilization genes demonstrated the presence of DNA homologous to the genes encoding the outer membrane protein HutA and the inner membrane protein HutB in all the V. cholerae strains tested. The probe containing hutA, but not that containing hutB, also hybridized to DNA from Vibrio parahaemolyticus.

Molecular characterization of the 98-kilodalton iron-regulated outer membrane protein of Neisseria meningitidis

When grown under iron limitation, Neisseria meningitidis expresses several additional outer membrane proteins (OMPs), which were studied to assess their vaccine potential. Two monoclonal antibodies were obtained against a 98-kDa OMP of strain 2996 (B:2b:P1.2). Cross-reactivity studies revealed that the two antibodies reacted with 44 and 42 of 74 meningococcal strains, respectively. The antibodies did not block the binding of transferrin or lactoferrin to intact cells. The structural gene for the protein, tentatively designated iroA, was isolated and sequenced. Computer analysis revealed homology to the ferric siderophore receptors in the outer membrane of Escherichia coli and to gonococcal transferrin-binding protein 1 (TbpA). The high degree of cross-reactivity and the results of Southern blot analyses, which showed that the iroA gene is also present in strains that did not react with the monoclonal antibodies, suggest that the 98-kDa OMP is well conserved among meningococci and that it is a suitable vaccine candidate. However, the antibodies were not bactericidal in an in vitro assay with human complement.

Identification and cloning of a fur homolog from Neisseria gonorrhoeae

The promoter region of the major iron-regulated protein of Neisseria gonorrhoeae, Fbp, has two regions that exhibit homology with the Escherichia coli consensus Fur-binding sequences. Gel retardation assays suggested that purified E. coli Fur bound to two sites within the Fbp promoter. The presence of a gonococcal Fur homolog was suggested by Southern hybridization under conditions of low stringency, which revealed a DNA locus that exhibited homology to the E. coli fur gene. Oligonucleotides derived from the conserved regions of fur genes of extremely diverse bacteria were used to amplify a 140-bp fragment of a putative gonococcal fur gene. This fragment was used to identify clones containing the entire gonococcal fur gene. After sequencing the gonococcal fur gene and its promoter region, we found that gonococcal Fur exhibited 50% identity with E. coli Fur at the amino acid level; however, it complemented two E. coli Fur- mutants. The presence of a Fur homolog in N. gonorrhoeae suggests that Fur-regulated genes are widely distributed among extremely diverse bacteria.

Role of iron in regulation of virulence genes

The abilities of bacterial pathogens to adapt to the environment within the host are essential to their virulence. Microorganisms have adapted to the iron limitation present in mammalian hosts by evolving diverse mechanisms for the assimilation of iron sufficient for growth. In addition, many bacterial pathogens have used the low concentration of iron present in the host as an important signal to enhance the expression of a wide variety of bacterial toxins and other virulence determinants. The molecular basis of coordinate regulation by iron has been most thoroughly studied in Escherichia coli. In this organism, coordinate regulation of gene expression by iron depends on the regulatory gene, fur. Regulation of gene expression by iron in a number of pathogenic organisms is coordinated by proteins homologous to the Fur protein of E. coli. Additional regulatory proteins may be superimposed on the Fur repressor to provide the fine-tuning necessary for the precise regulation of individual virulence genes in response to iron and other environmental signals. Studies of the mechanisms of regulation of iron acquisition systems and virulence determinants by iron should lead to a better understanding of the adaptive response of bacteria to the low-iron environment of the host and its importance in virulence.

Transferrins and heme-compounds as iron sources for pathogenic bacteria

The low concentration of free iron in body fluids creates bacteriostatic conditions for many microorganisms and is therefore an important defense factor of the body against invading bacteria. Pathogenic bacteria have developed several mechanisms for acquiring iron from the host. Siderophore-mediated iron uptake involves the synthesis of low molecular weight iron chelators called siderophores which compete with the host iron-binding glycoproteins lactoferrin (LF) and transferrin (TF) for iron. Other ways to induce iron uptake, without the mediation of siderophores, are the possession of outer membrane protein receptors that actually recognize the complex of TF or LF with iron, resulting in the internalization of this metal, and the use of heme-compounds released into the circulation after lysis of erythrocytes. In this review, the nonsiderophore-mediated iron-uptake systems used by certain pathogenic bacteria are emphasized. The possible contribution of these iron-uptake systems to the virulence of pathogens is also discussed.

Molecular cloning, partial purification, and characterization of a haemin-binding lipoprotein from Haemophilus influenzae type b

A library of genomic DNA fragments from Haemophilus influenzae type b (Hib) DL42 was constructed in plasmid pBR322, transformed into Escherichia coli strain RR1, and screened for recombinant clones with haemin-binding activity by plating onto haemin-containing agar. Expression of haemin-binding activity by clones correlated with the expression of a protein with an apparent molecular weight of 51,000 (51K) that was also recognized by anti-Hib strain DL42 serum in immunoblots. One recombinant clone, designated pHM2, with the smallest DNA insert (3.62 kb) was characterized further. Ethanol inhibition of expression of pHM2 in minicells revealed that the 51K protein was the result of a processing event involving a larger precursor. E. coli RR1(pHM2) adsorbed haemin in liquid suspensions as well as from solid media. Subcloning of a 2.6 kb fragment of pHM2 into a shuttle vector permitted the construction of a recombinant Hib clone, DL42(pHM1002), which overexpressed the 51K haemin-binding protein. This 51K protein appears to be peripherally associated with the inner, and possibly outer, membranes of Hib. Affinity chromatography on haemin-agarose was utilized to purify the haemin-binding protein from both E. coli RR1(pHM2) and Hib DL42(pHM1002) to near homogeneity. The use of the antibiotic globomycin in a minicell expression system and radioimmunoprecipitation analysis of Hib proteins intrinsically radiolabelled with [3H]-palmitate indicated that the 51K haemin-binding protein is a lipoprotein.

Competition between Neisseria gonorrhoeae and Staphylococcus epidermidis during iron-limited or replete continuous culture

Neisseria gonorrhoeae strain P9-2 was grown in iron-limited or replete continuous culture at a dilution rate of 0.05 h-1, in the presence and absence of Staphylococcus epidermidis. Gonococci maintained expression of pili (P+) and the transparent colony phenotype in pure culture during transitions of iron- and cystine-limited growth. They competed well with staphylococci during iron-limited co-culture and comprised greater than 95% of the population. Transition to cystine-limited growth allowed the staphylococcus to predominate but the gonococcus did not wash out. Furthermore, the gonococcal opaque colony phenotype (O+), indicating synthesis of outer membrane proteins II, was now expressed. Restoration of iron limitation returned the co-culture to its original composition but with P+O+ gonococci dominating. These results suggest that environments might exist in Man where gonococci can compete successfully with normal indigenous bacteria during infection.

A pleiotropic iron-uptake mutant of Neisseria meningitidis lacks a 70-kilodalton iron-regulated protein

We isolated an iron-uptake mutant of Neisseria meningitidis M986-NCV-1 that was severely limited in the ability to use several sources of iron in the form of Fe3+. This mutant, FAM11, grew poorly or not at all with human transferrin (TF) or lactoferrin (LF) as the sole iron source in a defined medium but grew as well as wild-type meningococci with hemin or hemoglobin. Uptake of 55Fe bound to TF, LF, dicitrate complexes, aerobactin, or nitrilotriacetate was reduced to 0 to 4% of the wild-type level. FAM11 did not produce an iron-repressible outer membrane protein (FeRP) of 70 kilodaltons (kDa) found in membranes of iron-stressed M986-NCV-1. Western blot (immunoblot) analysis using rabbit antiserum against this protein revealed that at least 17 of 18 meningococcal and 10 of 14 gonococcal strains produced an FeRP of ca. 70 kDa. The 70-kDa FeRP was shown to be surface exposed by radioimmunoprecipitation with human immune sera. These data suggest that the 70-kDa FeRP is somehow involved in Fe uptake from TF and LF. However, we were unable to transform the iron-uptake phenotype from FAM11 into wild-type meningococci to confirm this. Revertants of FAM11 that grew with TF and LF did not regain the ability to make the 70-kDa FeRP but also did not completely regain the Fe-uptake phenotype of M986-NCV-1.

Iron uptake from lactoferrin and transferrin by Neisseria gonorrhoeae

The major iron (Fe) sources available to Neisseria gonorrhoeae in the human host are probably transferrin (TF) and lactoferrin (LF). Although a number of studies have examined Fe uptake by Neisseria meningitidis, no comparable studies have been done on Fe uptake by the gonococcus from TF and LF. We found that, like meningococci, gonococci removed Fe from TF and LF in an energy-dependent manner; uptake was repressed by Fe and did not proceed by a siderophore-mediated uptake system. Unlike published studies examining meningococcal Fe uptake from TF, our study showed that gonococcal Fe uptake from both TF and LF was highly efficient; uptake saturated at 1 microM protein, and growth with 5% saturated TF and LF occurred at maximal rates when the protein was present in appreciable concentrations. We conclude that the availability of protein-bound Fe probably does not limit growth of N. gonorrhoeae in the human body.

Human immunoglobulin G antibody response to the major gonococcal iron-regulated protein

In humans, gonococcal infection occurs in environments limited with respect to free iron. Neisseria gonorrhoeae produces increased quantities of iron-regulated membrane proteins when grown under in vitro conditions which restrict the availability of free iron. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot (immunoblot) techniques, we studied the reactivity of specific antibodies to the 37-kilodalton (kDa) major iron-regulated protein (MIRP) of gonococci grown under iron-limiting conditions. Antibodies reactive with the 37-kDa MIRP were distinguished from those reactive with protein I by using purified 37-kDa MIRP or gonococcal protein preparations. Acute-phase sera from patients with disseminated gonococcal infection (DGI) reacted strongly to both the 37-kDa MIRP and protein I. Acute sera from nine patients with uncomplicated gonorrhea did not exhibit strong reactivity with the 37-kDa MIRP and were indistinguishable from five control sera. When compared with acute-phase sera, convalescent-phase sera from patients with DGI failed to demonstrate increased reactivity, whereas convalescent-phase sera from one of nine patients with uncomplicated gonorrhea developed reactivity to the 37-kDa MIRP. These data indicate that (i) the 37-kDa MIRP is expressed and antigenic in vivo and (ii) humans with DGI consistently develop a systemic antibody response to the 37-kDa MIRP.

Effects of serum carrier proteins on the growth of pathogenic neisseriae with heme-bound iron

The pathogenic neisseriae can use free heme and hemoglobin as an essential source of iron (Fe) for growth in vitro, but it is unknown whether they can utilize heme bound to human hemopexin or to human serum albumin, or hemoglobin bound to haptoglobin. We found that neither Neisseria meningitidis nor Neisseria gonorrhoeae used bound heme, but bound hemoglobin was used as an Fe source by two meningococcal strains and one gonococcal strain. A second gonococcal strain, previously shown to use free hemoglobin poorly or not at all, also did not grow with hemoglobin-haptoglobin complex as an Fe source. These observations suggest that hemoglobin might act as an Fe source in vivo for many pathogenic neisseriae even when in complexed (bound) form, but heme probably would not support growth in vivo if bound to serum carrier proteins.

Purification and characterization of the major iron-regulated protein expressed by pathogenic Neisseriae

This report describes a method to purify the major iron-regulated protein (MIRP) expressed by N. gonorrhoeae and N. meningitidis. This purification procedure involves maximal expression of the MIRP by growing the organisms on iron-limited media; cellular disruption by sonication followed by centrifugal fractionation; selective solubilization of the MIRP with the cationic detergent hexadecyltrimethylammonium bromide; cation-exchange chromatography in the presence of this detergent; and gel filtration chromatography. The MIRP purified by this technique migrates as a single band when analyzed by SDS-PAGE. The purified MIRP displayed an unusually basic isoelectric point, this value being greater than 9.35. Further biochemical analysis revealed the highly conserved nature of this protein isolated from the two pathogenic species of the genus Neisseria. For example, the amino acid composition of the meningococcal and gonococcal MIRPs were nearly identical and amino terminal sequence analysis showed that both shared the identical primary sequence through residue 48. Surprisingly, the first five NH2-terminal residues of the MIRPs exhibited homology with the first five residues of the gonococcal porin, protein I. Purified preparations of the MIRP exhibited a characteristic pink color reminiscent of the basic iron-binding protein lactoferrin. This observation coupled with the property of iron-regulation prompted us to analyze purified MIRP for iron-content. Approximately 0.5 mol iron per 1 mol of MIRP was detected. This study is the first to show that iron is associated with the MIRP, a property that may implicate this protein as playing a direct role in neisserial iron assimilation. While the precise function of the MIRP is not known, the availability of this protein in pure and biologically relevant quantities will allow further studies to elucidate its pathobiologic function.

Distribution of an antigenically related iron-regulated protein among the Neisseria spp

Several iron-regulated proteins of Neisseria gonorrhoeae have been reported. One of these, a 37,000-molecular-weight protein (37K protein), appears to be common to all gonococcal isolates. Recently, the occurrence of a similar protein has also been noted in N. meningitidis. The gonococcal 37K protein has been purified and used to produce both rabbit monospecific antiserum and murine monoclonal antibodies. Using these antibody reagents, we analyzed 57 strains from nine species of Neisseria and the closely related organism Branhamella catarrhalis for the presence of proteins antigenically related to the gonococcal 37K protein. Strains grown on medium with low iron content were probed for antigenic reactivity by Western blot techniques and an enzyme-linked immunosorbent assay. Proteins which cross-reacted with the rabbit monospecific antiserum were designated as AgR-37K proteins. The data indicated that the AgR-37K proteins were conserved among the 40 strains of N. gonorrhoeae, N. meningitidis, N. lactamica, and N. cinerea tested. Seventeen strains from other species of Neisseria and Branhamella did not express AgR-37K proteins with the exception of one N. subflava isolate. All AgR-37K proteins appeared to be regulated by the amount of available iron in the growth medium. Murine monoclonal antibodies were used to probe the antigenic heterogeneity of the AgR-37K proteins from different Neisseria spp. Two of seven monoclonal antibodies were broadly cross-reactive, recognizing the AgR-37K proteins from all species examined. The remaining five monoclonal antibodies were more discriminating, recognizing the AgR-37K proteins from certain species. The antigenic conservation of these AgR-37K proteins, particularly among the pathogenic members of the genus Neisseria, may imply that these proteins serve a common function in pathogenicity.

Response of Neisseria gonorrhoeae to iron limitation: alterations in expression of membrane proteins without apparent siderophore production

For acquisition of iron, an essential nutrient, most microorganisms produce siderophores (low-molecular-weight iron-chelating compounds) and membrane proteins to serve as receptors for the iron-siderophore complexes. The gonococcus does not appear to produce a siderophore, since the quantity of siderophore detected by bioassays of culture supernatants from strains F62 and FA19 was never greater than the amount present in the uninoculated medium. Iron limitation of the laboratory strains F62 and FA19 and 12 recent clinical isolates resulted in the expression of several iron-repressible membrane proteins. The expression of proteins in the apparent molecular weight range of 70,000 to 100,000 was strain dependent. All strains expressed 36,000-dalton (36K) and 19.5K proteins. FA19 and F62 were also grown in medium containing iron sources commonly encountered in vivo (i.e., transferrin, lactoferrin, hemoglobin, or hemin). Comparison of growth rates indicates that transferrin and lactoferrin were more readily utilized as iron sources than hemin and hemoglobin were. Expression of the iron-repressible proteins varied depending upon the iron source. Fewer iron-repressible proteins were observed when cells were supplied with transferrin or lactoferrin than when the cultures were grown with either hemin or hemoglobin. The 36K protein was expressed with all four iron sources.

[Modification of virulence and immunogenicity of Pasteurella multocida by iron in vitro]

The virulence of Pasteurella multocida strains in experimental infections of mice has been shown to depend on the iron contents of the cultivation media. Frequent passages of bacteria on iron-deficient growth media result in drastic decrease of virulence. In the case of one strain also immunogenicity was lowered. The results with nutritional media differing in their iron contents show that iron probably exhibits a regulatory effect on the production of a not yet identified virulence.

The meningococcus and mechanisms of pathogenicity

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Siderophore production by pathogenic Neisseria spp

Previous studies have established the importance of iron acquisition to the growth and virulence of Neisseria meningitidis and Neisseria gonorrhoeae. Although preliminary evidence that the Neisseria spp. produce siderophores has been presented, the exact mechanism of iron acquisition has remained obscure. Siderophore production by N. gonorrhoeae and N. meningitidis was induced in two different low-iron media. The iron-reactive siderophores, "gonobactin" and "meningobactin," were partially purified by ion exchange chromatography followed by extraction with phenol-chloroform-ether or by gel filtration. The compounds were of low molecular weight, their synthesis was repressed by iron in the medium, and they appeared to be hydroxamic acids since they were stimulatory for Arthrobacter flavescens JG-9 (a hydroxamate auxotroph) and gave a positive Csáky reaction for bound hydroxylamine. In the iron form, the compounds had an absorption maximum of approximately 420 nm. Although meningobactin stimulated growth of the gonococcus in low-iron media and vice versa, the homologous activity was more marked, indicating that the compounds, though similar, were probably not identical. As determined by A. flavescens assay the meningococcus produced three to five times more siderophore than did the gonococcus; however, the amount of siderophore present in the culture fluids of even the meningococcus was 100- to 1,000-fold lower than the concentration of hydroxamate siderophores reported to be produced by Bacillus megaterium or Aerobacter aerogenes. Virulent, colony type 1 N. gonorrhoeae produced significantly more gonobactin than did the avirulent colony type 3 gonococci.

Effect of siderophores on virulence of Neisseria gonorrhoeae

The virulence of Neisseria gonorrhoeae for chicken embryos can be modified in a predictable manner by the addition of microbial siderophores to the inoculum. "Meningobactin" and "gonobactin," siderophores isolated from iron-limited cultures of meningococci and gonococci, respectively, enhance the virulence of the relatively avirulent colony type 3 (T3) organisms, but have essentially no effect on the virulence of T1 organisms. Both of these compounds were found previously to stimulate in vitro growth of the pathogenic Neisseria spp. under conditions made iron limiting by the addition of conalbumin, the transferrin counterpart of chickens. Similarly, ferrated schizokinen and arthrobactin, both dihydroxamate siderophores which stimulated growth in iron-limited conditions in vitro, also enhanced virulence of T3 organisms, whereas desferrioxamine B mesylate (Desferal), a trihydroxamate previously shown to be inhibitory in vitro, decreased the virulence of the T1 colony form. This was due to the iron-binding function of the molecule, as the iron-saturated form, ferrioxamine B mesylate, did not affect virulence. An additional trihydroxamate siderophore, ferrichrome A, which was inactive on Neisseria spp. in either the deferri- or ferrated forms in vitro, likewise did not affect virulence in the chicken embryo model. The neisserial siderophores were more effective than the other microbial siderophores in enhancing virulence of T3 gonococci. The results add to the evidence that the ability to acquire iron is an important determinant of virulence.