Siderophore production and membrane alterations by Bordetella pertussis in response to iron starvation

Role and regulation of heme iron acquisition in gram-negative pathogens

Bacteria that reside in animal tissues and/or cells must acquire iron from their host. However, almost all of the host iron is sequestered in iron-containing compounds and proteins, the majority of which is found within heme molecules. Thus, likely iron sources for bacterial pathogens (and non-pathogenic symbionts) are free heme and heme-containing proteins. Furthermore, the cellular location of the bacterial within the host (intra or extracellular) influences the amount and nature of the iron containing compounds available for transport. The low level of free iron in the host, coupled with the presence of numerous different heme sources, has resulted in a wide range of high-affinity iron acquisition strategies within bacteria. However, since excess iron and heme are toxic to bacteria, expression of these acquisition systems is highly regulated. Precise expression in the correct host environment at the appropriate times enables heme iron acquisitions systems to contribute to the growth of bacterial pathogens within the host. This mini-review will highlight some of the recent findings in these areas for gram-negative pathogens.

Involvement of multiple distinct Bordetella receptor proteins in the utilization of iron liberated from transferrin by host catecholamine stress hormones

Bordetella bronchiseptica is a pathogen that can acquire iron using its native alcaligin siderophore system, but can also use the catechol xenosiderophore enterobactin via the BfeA outer membrane receptor. Transcription of bfeA is positively controlled by a regulator that requires induction by enterobactin. Catecholamine hormones also induce bfeA transcription and B. bronchiseptica can use the catecholamine noradrenaline for growth on transferrin. In this study, B. bronchiseptica was shown to use catecholamines to obtain iron from both transferrin and lactoferrin in the absence of siderophore. In the presence of siderophore, noradrenaline augmented transferrin utilization by B. bronchiseptica, as well as siderophore function in vitro. Genetic analysis identified BfrA, BfrD and BfrE as TonB-dependent outer membrane catecholamine receptors. The BfeA enterobactin receptor was found to not be involved directly in catecholamine utilization; however, the BfrA, BfrD and BfrE catecholamine receptors could serve as receptors for enterobactin and its degradation product 2,3-dihydroxybenzoic acid. Thus, there is a functional link between enterobactin-dependent and catecholamine-dependent transferrin utilization. This investigation characterizes a new B. bronchiseptica mechanism for iron uptake from transferrin that uses host stress hormones that not only deliver iron directly to catecholamine receptors, but also potentiate siderophore activity by acting as iron shuttles.

Identification of a new protective antigen of Bordetella pertussis

Antigenic proteins whose expression is induced under iron starvation, an environmental condition that bacterial pathogens have to face during colonization, might be potential candidates for improved vaccine. By mean of immune proteomics we identified novel antigens of Bordetella pertussis maximally expressed under iron limitation. Among them, Bp1152 (named as IRP1-3) showed a particularly strong reaction with human IgG purified from pooled sera of pertussis-infected individuals. Computer analysis showed IRP1-3 as a dimeric membrane protein potentially involved in iron uptake. Experimental data revealed the surface-exposure of this protein and showed its increase under iron starvation to be independent of bacterial virulence phase. Immunization of mice with the recombinant IRP1-3 resulted in a strong antibody response. These antibodies not only recognized the native protein on bacterial surface but also promote effective bacterial phagocytosis by human PMN, a key protecting activity against this pathogen. Accordingly, IRP1-3 proved protective against B. pertussis infection in mouse model. Expression of IRP1-3 was found conserved among clinical isolates of B. pertussis and positively regulated by iron starvation in these strains. Taken together these results suggest that this protein might be an interesting novel vaccine candidate.

Transcriptional profiling of the iron starvation response in Bordetella pertussis provides new insights into siderophore utilization and virulence gene expression

Serological studies of patients with pertussis and the identification of antigenic Bordetella pertussis proteins support the hypothesis that B. pertussis perceives an iron starvation cue and expresses multiple iron source utilization systems in its natural human host environment. Furthermore, previous studies using a murine respiratory tract infection model showed that several of these B. pertussis iron systems are required for colonization and persistence and are differentially expressed over the course of infection. The present study examined genome-wide changes in B. pertussis gene transcript abundance in response to iron starvation in vitro. In addition to known iron source utilization genes, we identified a previously uncharacterized iron-repressed cytoplasmic membrane transporter system, fbpABC, that is required for the utilization of multiple structurally distinct siderophores including alcaligin, enterobactin, ferrichrome, and desferrioxamine B. Expression of type III secretion system genes was also found to be upregulated during iron starvation in both B. pertussis strain Tohama I and Bordetella bronchiseptica strain RB50. In a survey of type III secretion system protein production by an assortment of B. pertussis laboratory-adapted and low-passage clinical isolate strains, iron limitation increased the production and secretion of the type III secretion system-specific translocation apparatus tip protein Bsp22 in all Bvg-proficient strains. These results indicate that iron starvation in the infected host is an important environmental cue influencing not only Bordetella iron transport gene expression but also the expression of other important virulence-associated genes.

Impact of alcaligin siderophore utilization on in vivo growth of Bordetella pertussis

Bordetella pertussis, the causative agent of human whooping cough, or pertussis, is an obligate human pathogen with diverse high-affinity transport systems for the assimilation of iron, a biometal that is essential for growth. Under iron starvation stress conditions, B. pertussis produces the siderophore alcaligin. The alcaligin siderophore gene cluster, consisting of the alcABCDERS and fauA genes, encodes activities required for alcaligin biosynthesis, the export of the siderophore from the cell, the uptake of the ferric alcaligin complex across the outer membrane, and the transcriptional activation of alcaligin system genes by an autogenous mechanism involving alcaligin sensing. The fauA gene encodes a 79-kDa TonB-dependent outer membrane receptor protein required for the uptake and utilization of ferric alcaligin as an iron source. In this study, using mixed-infection competition experiments in a mouse respiratory model, inactivation of the B. pertussis ferric alcaligin receptor protein was found to have a profound impact on in vivo growth and survival of a fauA mutant compared with a coinfecting wild-type strain. The attenuating effect of fauA inactivation was evident early in the course of the infection, suggesting that the contribution of ferric alcaligin transport to the ecological fitness of B. pertussis may be important for adaptation to iron-restricted host conditions that exist at the initial stages of infection. Alcaligin-mediated iron acquisition by B. pertussis may be critical for successful host colonization and establishment of infection.

Profiling theBordetellapertussisProteome during Iron Starvation

Regulation of gene expression in response to local iron concentration is commonly observed in bacterial pathogens that face this nutrient limitation during host infection. In this study, a proteomic approach was used to analyze the differential protein expression of Bordetella pertussis under iron limitation. Whole cell lysates (WCL) and outer membrane fractions of bacteria grown either under iron-starvation or iron-excess conditions were analyzed by two-dimensional (2-D) gel electrophoresis. Statistical analysis revealed 36 proteins displaying differential expression, 9 with higher expression under iron-excess and 27 with increased expression under iron-starvation. These proteins were subjected to tryptic digestion and MALDI-TOF MS. Apart from those previously reported, we identified new low-iron-induced proteins that might help to explain the increased virulence of this phenotype. Additionally, we found evidence that at least one of the identified proteins, solely expressed under iron starvation, is highly immunogenic in infected individuals.

Bordetella iron transport and virulence

Bordetella pertussis, Bordetella parapertussis, and Bordetella bronchiseptica are pathogens with a complex iron starvation stress response important for adaptation to nutrient limitation and flux in the mammalian host environment. The iron starvation stress response is globally regulated by the Fur repressor using ferrous iron as the co-repressor. Expression of iron transport system genes of Bordetella is coordinated by priority regulation mechanisms that involve iron source sensing. Iron source sensing is mediated by distinct transcriptional activators that are responsive to the cognate iron source acting as the inducer.

Heme-responsive transcriptional activation of Bordetella bhu genes

Bordetella pertussis and Bordetella bronchiseptica, gram-negative respiratory pathogens of mammals, possess a heme iron utilization system encoded by the bhuRSTUV genes. Preliminary evidence suggested that expression of the BhuR heme receptor was stimulated by the presence of heme under iron-limiting conditions. The hurIR (heme uptake regulator) genes were previously identified upstream of the bhuRSTUV gene cluster and are predicted to encode homologs of members of the iron starvation subfamily of extracytoplasmic function (ECF) regulators. In this study, B. pertussis and B. bronchiseptica DeltahurI mutants, predicted to lack an ECF sigma factor, were constructed and found to be deficient in the utilization of hemin and hemoglobin. Genetic complementation of DeltahurI strains with plasmid-borne hurI restored wild-type levels of heme utilization. B. bronchiseptica DeltahurI mutant BRM23 was defective in heme-responsive production of the BhuR heme receptor; hurI in trans restored heme-inducible BhuR expression to the mutant and resulted in BhuR overproduction. Transcriptional analyses with bhuR-lacZ fusion plasmids confirmed that bhuR transcription was activated in iron-starved cells in response to heme compounds. Heme-responsive bhuR transcription was not observed in mutant BRM23, indicating that hurI is required for positive regulation of bhu gene expression. Furthermore, bhuR was required for heme-inducible bhu gene activation, supporting the hypothesis that positive regulation of bhuRSTUV occurs by a surface signaling mechanism involving the heme-iron receptor BhuR.

The Bordetella bhu locus is required for heme iron utilization

Bordetella pertussis and Bordetella bronchiseptica are capable of obtaining iron from hemin and hemoglobin. Genes encoding a putative bacterial heme iron acquisition system (bhu, for Bordetella heme utilization) were identified in a B. pertussis genomic sequence database, and the corresponding DNA was isolated from a virulent strain of B. pertussis. A B. pertussis bhuR mutant, predicted to lack the heme outer membrane receptor, was generated by allelic exchange. In contrast to the wild-type strain, bhuR mutant PM5 was incapable of acquiring iron from hemin and hemoglobin; genetic complementation of PM5 with the cloned bhuRSTUV genes restored heme utilization to wild-type levels. In parallel studies, B. bronchiseptica bhu sequences were also identified and a B. bronchiseptica bhuR mutant was constructed and confirmed to be defective in heme iron acquisition. The wild-type B. bronchiseptica parent strain grown under low-iron conditions produced the presumptive BhuR protein, which was absent in the bhuR mutant. Furthermore, production of BhuR by iron-starved B. bronchiseptica was markedly enhanced by culture in hemin-supplemented medium, suggesting that these organisms sense and respond to heme in the environment. Analysis of the genetic region upstream of the bhu cluster identified open reading frames predicted to encode homologs of the Escherichia coli ferric citrate uptake regulators FecI and FecR. These putative Bordetella regulators may mediate heme-responsive positive transcriptional control of the bhu genes.

Reduced virulence of a Bordetella bronchiseptica siderophore mutant in neonatal swine

One means by which Bordetella bronchiseptica scavenges iron is through production of the siderophore alcaligin. A nonrevertible alcaligin mutant derived from the virulent strain 4609, designated DBB25, was constructed by insertion of a kanamycin resistance gene into alcA, one of the genes essential for alcaligin biosynthesis. The virulence of the alcA mutant in colostrum-deprived, caesarean-delivered piglets was compared with that of the parent strain in two experiments. At 1 week of age, piglets were inoculated with phosphate-buffered saline, 4609, or DBB25. Two piglets in each group were euthanatized on day 10 postinfection. The remainder were euthanatized at 21 days postinfection. Clinical signs, including fever, coughing, and sneezing, were present in both groups. Nasal washes performed 7, 14, and 21 days postinoculation demonstrated that strain DBB25 colonized the nasal cavity but did so at levels that were significantly less than those achieved by strain 4609. Analysis of colonization based on the number of CFU per gram of tissue recovered from the turbinate, trachea, and lung also demonstrated significant differences between DBB25 and 4609, at both day 10 and day 21 postinfection. Mild to moderate turbinate atrophy was apparent in pigs inoculated with strain 4609, while turbinates of those infected with strain DBB25 developed no or mild atrophy. We conclude from these results that siderophore production by B. bronchiseptica is not essential for colonization of swine but is required for maximal virulence. B. bronchiseptica mutants with nonrevertible defects in genes required for alcaligin synthesis may be candidates for evaluation as attenuated, live vaccine strains in conventionally reared pigs.

Transcriptional activation of Bordetella alcaligin siderophore genes requires the AlcR regulator with alcaligin as inducer

Genetic and biochemical studies have established that Fur and iron mediate repression of Bordetella alcaligin siderophore system (alc) genes under iron-replete nutritional growth conditions. In this study, transcriptional analyses using Bordetella chromosomal alc-lacZ operon fusions determined that maximal alc gene transcriptional activity under iron starvation stress conditions is dependent on the presence of alcaligin siderophore. Mutational analysis and genetic complementation confirmed that alcaligin-responsive transcriptional activation of Bordetella alcaligin system genes is dependent on AlcR, a Fur-regulated AraC-like positive transcriptional regulator encoded within the alcaligin gene cluster. AlcR-mediated transcriptional activation is remarkably sensitive to inducer, occurring at extremely low alcaligin concentrations. This positive autogenous control circuit involving alcaligin siderophore as the inducer for AlcR-mediated transcriptional activation of alcaligin siderophore biosynthesis and transport genes coordinates environmental and intracellular signals for maximal expression of these genes under conditions in which the presence of alcaligin in the environment is perceived.

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.

Essential role of the iron-regulated outer membrane receptor FauA in alcaligin siderophore-mediated iron uptake in Bordetella species

Phenotypic analysis using heterologous host systems localized putative Bordetella pertussis ferric alcaligin transport genes and Fur-binding sequences to a 3.8-kb genetic region downstream from the alcR regulator gene. Nucleotide sequencing identified a TonB-dependent receptor family homolog gene, fauA, predicted to encode a polypeptide with high amino acid sequence similarity with known bacterial ferric siderophore receptors. In Escherichia coli, the fauA genes of both B. pertussis and Bordetella bronchiseptica directed the production of a 79-kDa polypeptide, approximating the predicted size of the mature FauA protein. B. bronchiseptica fauA insertion mutant BRM17 was unable to utilize ferric alcaligin, and in complementation analyses ferric alcaligin utilization was restored to this mutant by supplying the wild-type fauA gene in trans. Mutant BRM18, carrying a nonpolar in-frame fauA deletion mutation, was defective in ferric alcaligin utilization and (55)Fe-ferric alcaligin uptake and no longer produced a 79-kDa iron-regulated outer membrane protein. In complementation analyses, BRM18 merodiploids bearing the wild-type fauA gene in trans regained ferric alcaligin siderophore transport and utilization functions and produced the 79-kDa protein. Analysis of a plasmid-borne fauA-lacZ operon fusion confirmed that fauA is subject to iron regulation at the transcriptional level and that cis-acting transcriptional control elements mediating fauA iron repressibility reside within the 3.8-kb PstI fauA DNA region. Moreover, expression of the fauA-lacZ fusion gene under iron starvation conditions was shown to be alcR dependent. FauA is a 79-kDa iron-regulated outer membrane receptor protein required for transport and utilization of ferric alcaligin siderophore complexes by Bordetella species.

Two iron-regulated putative ferric siderophore receptor genes in Bordetella bronchiseptica and Bordetella pertussis

Two iron-regulated genes with deduced homology to TonB-dependent ferric siderophore receptors were cloned from Bordetella bronchiseptica by screening a library of TnphoA insertion mutants. bfrB and bfrC were iron-repressed in B. bronchiseptica by a Fur-dependent mechanism, and were expressed from promoters overlapped by potential Fur-binding sites. Both genes were highly conserved among Bordetella species and were also iron-regulated in Bordetella pertussis. bfrB and bfrC mutants of both species and a bfrB-bfrC double mutant of B. bronchiseptica had no discernible defects in utilization of known iron sources for Bordetella.

Identification of AlcR, an AraC-type regulator of alcaligin siderophore synthesis in Bordetella bronchiseptica and Bordetella pertussis

A Fur titration assay was used to isolate DNA fragments bearing putative Fur binding sites (FBS) from a partial Bordetella bronchiseptica genomic DNA library. A recombinant plasmid bearing a 3.5-kb DNA insert was further studied. Successive deletions in the cloned fragment enabled us to map a putative FBS at about 2 kb from one end. Sequence analysis revealed the presence of an FBS upstream from a new gene encoding an AraC-type transcriptional regulator. The deduced protein displays similarity to PchR, an activator of pyochelin siderophore and ferripyochelin receptor synthesis in Pseudomonas aeruginosa. Homologous genes in Bordetella pertussis and Bordetella parapertussis were PCR amplified, and sequence comparisons indicated a very high conservation in the three species. The B. pertussis and B. bronchiseptica chromosomal genes were inactivated by allelic exchange. Under low-iron growth conditions, the mutants did not secrete the alcaligin siderophore and lacked AlcC, an alcaligin biosynthetic enzyme. Alcaligin production was restored after transformation with a plasmid bearing the wild-type gene. On the basis of its role in regulation of alcaligin biosynthesis, the new gene was designated alcR. Additional sequence determination showed that alcR is located about 2 kb downstream from the alcABC operon and is transcribed in the same orientation. Two tightly linked open reading frames, alcD and alcE, were identified between alcC and alcR. AlcE is a putative iron-sulfur protein; AlcD shows no homology with the proteins in the database. The production of major virulence factors and colonization in the mouse respiratory infection model are AlcR independent.

Identification, characterization, and immunogenicity of the lactoferrin-binding protein from Helicobacter pylori

Iron acquisition plays an important role in bacterial virulence. Different studies have been initiated to define the mechanism by which Helicobacter pylori acquires iron. We had previously demonstrated that human lactoferrin (HLf) supported full growth of the bacteria in media lacking other iron sources. The ability of H. pylori to use HLf as an iron source had been found to be dependent on cell-to-protein contact. Since lactoferrin has been found in significant amounts in human stomach resection specimens from patients with superficial or atrophic gastritis, the iron uptake of H. pylori via a specific HLf receptor may play a major role in the virulence of H. pylori infection. In this study, by using affinity chromatography with biotinylated HLf and streptavidin-agarose, we identified a 70-kDa lactoferrin-binding protein (Lbp) from outer membrane proteins of H. pylori. This Lbp was only present when H. pylori was grown in an iron-starved medium, suggesting that it serves in iron uptake. Direct binding assays with increasing concentrations of biotinylated HLf demonstrated that the lactoferrin interaction with the outer membrane of H. pylori grown in iron-restricted medium was saturable. Competitive binding experiments with bovine and human lactoferrin and with transferrin of horse, bovine, and human origin indicated that this Lbp appeared highly specific for HLf. A number of other studies have focused on the importance of transferrin and lactoferrin receptors in pathogenic bacteria and their specificity with the host species. This observation might explain the very strict human specificity of H. pylori.

Identification and characterization of iron-regulated Bordetella pertussis alcaligin siderophore biosynthesis genes

Bordetella bronchiseptica mutants BRM1, BRM6, and BRM9 fail to produce the native dihydroxamate siderophore alcaligin. A 4.5-kb BamHI-Smal Bordetella pertussis genomic DNA fragment carried multiple genes required to restore alcaligin production to these siderophore-deficient mutants. Phenotypic complementation analysis using subclones of the 4.5-kb genomic region demonstrated that the closely linked BRM1 and BRM9 mutations were genetically separable from the BRM6 mutation, and both insertions exerted strong polar effects on expression of the downstream gene defined by the BRM6 mutation, suggesting a polycistronic transcriptional organization of these alcaligin biosynthesis genes. Subcloning and complementation experiments localized the putative Bordetella promoter to a 0.7-kb BamHI-SphI subregion of the cloned genomic DNA fragment. Nucleotide sequencing, phenotypic analysis of mutants, and protein expression by the 4.5-kb DNA fragment in Escherichia coli suggested the presence of three alcaligin system genes, namely, alcA, alcB, and alcC. The deduced protein products of alcA, alcB, and alcC have significant primary amino acid sequence similarities with known microbial siderophore biosynthesis enzymes. Primer extension analysis mapped the transcriptional start site of the putative alcaligin biosynthesis operon containing alcABC to a promoter region overlapping a proposed Fur repressor-binding site and demonstrated iron regulation at the transcriptional level.

Purification, spectroscopic analysis and biological activity of the macrocyclic dihydroxamate siderophore alcaligin produced by Bordetella pertussis and Bordetella bronchiseptica

Hydroxamate siderophores of virulent Bordetella pertussis and Bordetella bronchiseptica strains were purified using a simple large-scale isolation procedure, and identified by various spectroscopic techniques as the macrocyclic dihydroxamate siderophore trivially known as alcaligin, 1,8(S),11,18(S)- tetrahydroxy-1,6,11,16-tetraazacycloeicosane-2,5,12,15-tetrone+ ++, which was previously isolated from the taxonomically-related bacterial species Alcaligenes denitrificans subsp. xylosoxydans. Alcaligin purified from iron-depleted cultures of B. pertussis and B. bronchiseptica exhibited specific growth-promoting activity under iron-restricted conditions for Bordetella indicator strains, and ere active in [55Fe]ferric alcaligin transport assays. Evidence suggests that several C2-symmetric conformations of alcaligin exist simultaneously in both methanolic and aqueous solution.

Siderophore-mediated iron acquisition mechanisms in Vibrio vulnificus biotype 2

Vibrio vulnificus biotype 2 is a primary pathogen for eels and, as has recently been suggested, an opportunistic pathogen for humans. In this study we have investigated the ability of V. vulnificus biotype 2 to obtain iron by siderophore-mediated mechanisms and evaluated the importance of free iron in vibriosis. The virulence degree for eels was dependent on iron availability from host fluids, as was revealed by a reduction in the 50% lethal dose for iron-overloaded eels. This biotype produced both phenolate- and hydroxamate-type siderophores of an unknown nature and two new outer membrane proteins of around 84 and 72 kDa in response to iron starvation. No alterations in lipopolysaccharide patterns were detected in response to iron stress. Finally, our data suggest that V. vulnificus biotype 2 uses the hydroxamate-type siderophore for removal of iron from transferrin rather than relying on a receptor for this iron-binding protein.

Identification of alcA, a Bordetella bronchiseptica gene necessary for alcaligin production

The alcA gene, essential for the production of the dihydroxamate siderophore, alcaligin, by Bordetella bronchiseptica, was cloned and sequenced. The alcA gene was identified on a 4.7-kb EcoRI genomic fragment adjacent to a Tn5lac transposon insertion that inactivated alcaligin production in strain MBORD846. Analysis of the alcA nucleotide sequence revealed a putative Fur-binding site, suggesting that expression of this gene is repressed by iron. The deduced amino-acid sequence of this open reading frame had significant homology with the Escherichia coli iucD gene product, an enzyme required for biosynthesis of the dihydroxamate siderophore aerobactin.

bvg Repression of alcaligin synthesis in Bordetella bronchiseptica is associated with phylogenetic lineage

Recent studies have shown that Bordetella bronchiseptica utilizes a siderophore-mediated transport system for acquisition of iron from the host iron-binding proteins lactoferrin and transferrin. We recently identified the B. bronchiseptica siderophore as alcaligin, which is also produced by B. pertussis. Alcaligin production by B. bronchiseptica is repressed by exogenous iron, a phenotype of other microbes that produce siderophores. In this study, we report that alcaligin production by B. bronchiseptica RB50 and GP1SN was repressed by the Bordetella global virulence regulator, bvg, in addition to being Fe repressed. Modulation of bvg locus expression with 50 mM MgSO4 or inactivation of bvg by deletion allowed strain RB50 to produce alcaligin. In modulated organisms, siderophore production remained Fe repressed. These observations contrasted with our previous data indicating that alcaligin production by B. bronchiseptica MBORD846 and B. pertussis was repressed by Fe but bvg independent. Despite bvg repression of alcaligin production, strain RB50 was still able to acquire Fe from purified alcaligin, suggesting that expression of the bacterial alcaligin receptor was not repressed by bvg. We tested 114 B. bronchiseptica strains and found that bvg repression of alcaligin production was strongly associated with Bordetella phylogenetic lineage and with host species from which the organisms were isolated.

Cloning and initial characterization of the Bordetella pertussis fur gene

In several Gram-negative pathogens the fur (ferric uptake regulator) gene product controls the expression of many genes involved in iron uptake and virulence. To facilitate the study of iron-regulated gene expression in Bordetella pertussis, we cloned the fur gene from this organism. The B. pertussis fur gene product was 54% identical to the Escherichia coli Fur and complemented two E. coli fur mutants. As with the E. coli fur gene, sequences upstream of the B. pertussis fur were homologous to the consensus Fur-binding site and to the consensus catabolite activator protein binding site.

Identification of alcaligin as the siderophore produced by Bordetella pertussis and B. bronchiseptica

The siderophores produced by iron-starved Bordetella pertussis and B. bronchiseptica were purified and were found to be identical. Using mass spectrometry and proton nuclear magnetic resonance, we determined that the siderophore produced by these organisms was identical to alcaligin, a siderophore produced by Alcaligenes denitrificans.

Results of passive and active immunization directed against ferric aerobactin in experimental enterobacterial infections in mice and chickens

Production of aerobactin has been reported to be a virulence factor in members of the family Enterobacteriaceae. To investigate the protection afforded by humoral immunity directed towards aerobactin in infectious diseases caused by aerobactin-producing strains, we tested the efficacy of mAbAERO1, a murine monoclonal antibody directed to ferric aerobactin, which, in vitro, was found to impair the growth of aerobactin-dependent strains of Enterobacteriaceae under iron-limited conditions. The mortality of mice experimentally infected with the aerobactin-producing strains Escherichia coli V2019 (LD50 = 3.5 x 10(5) CFU/mice) or Klebsiella pneumoniae Caroli (LD50 = 1.3 CFU/mice) was not reduced when 1 mg of mAbAERO1 was injected intravenously 1 h before or 1 h after bacterial challenge. Nor was mortality reduced after challenge with either E. coli V2019 or K. pneumoniae Caroli, even though the active immunization of mice with purified FeAero (ferric aerobactin) conjugated with thyroglobulin as followed by a rise in systemic anti-FeAero antibodies. Lastly, chicks born of hens immunized with FeAero showed evidence of antibody transmission towards FeAero, but were not protected when challenged with E. coli MT78, an aerobactin-producing strain highly virulent for chickens. Therefore, under the experimental conditions tested, humoral immunity against aerobactin appeared to play only a minor role in protection against infections caused by aerobactin-producing members of the family Enterobacteriaceae. However, other experimental models should be tested to confirm these observations.

Membrane protein expression by Actinobacillus actinomycetemcomitans in response to iron availability

Iron-limited growth conditions under anaerobiosis were established for Actinobacillus actinomycetemcomitans strains Y4, JP2, and 75 by use of the ferrous ion chelator 2,2'-dipyridyl. Growth inhibition was reversible with both ferrous and ferric iron sources. Sarcosyl-insoluble membrane fractions of iron-stressed anaerobic A. actinomycetemcomitans cultures revealed a similar iron-repressible protein of approximately 70 kDa in A. actinomycetemcomitans strains Y4, JP2, and 75. This 70-kDa protein was recognized by serum from localized juvenile periodontitis patients and a periodontally healthy subject. This suggested that the 70-kDa iron-repressible protein may be expressed in vivo. When A. actinomycetemcomitans was grown under aerobic conditions, the ferric iron chelator, ethylenediamine di(o-hydroxyphenylacetic acid) (EDDA) was utilized for growth limitation. EDDA inhibition was reversible in strain Y4 with ferrous and ferric iron sources. An iron-repressible protein of approximately 70 kDa was also noted in iron-stressed aerobic cultures. The 70-kDa protein may be involved in iron transport by A. actinomycetemcomitans. Preliminary experiments were performed to examine potential iron transport systems for A. actinomycetemcomitans. Production of the two most common chemical types of siderophore was not detected in A. actinomycetemcomitans culture supernatants. Iron-starved A. actinomycetemcomitans cells did not bind transferrin or lactoferrin in a dot blot assay.

Expression of the Haemophilus influenzae transferrin receptor is repressible by hemin but not elemental iron alone

The absolute requirement for elemental iron and the porphyrin nucleus for growth of Haemophilus influenzae led us to investigate the role of iron and hemin in regulation of expression of the H. influenzae transferrin receptor. H. influenzae type b strain H1689 was grown in brain heart infusion broth supplemented with beta-NAD and either 10 or 0.1 microgram of hemin ml-1. Transferrin-binding ability was determined with a dot blot assay using human transferrin-horseradish peroxidase conjugate. Cells grown in media with 0.1 microgram of hemin ml-1 bound transferrin, but organisms grown in media with 10 micrograms ml-1 did not. In hemin-restricted media, transferrin binding occurred despite addition of up to 10 mM ferric nitrate, ferric citrate, or ferric PPi, whereas addition of 10 micrograms of hemoglobin ml-1 repressed expression. The breadth of species distribution of this mode of regulation was determined with strains previously characterized by multilocus enzyme electrophoresis. When grown in hemin-restricted media, 24 of 28 type b strains and 52 of 57 serologically nontypeable strains exhibited transferrin binding, although none did so in hemin- and iron-sufficient media. Strain H1689 and serologically nontypeable strain HI1423 grown in heat-inactivated pooled normal human serum, human cerebrospinal fluid, or human breast milk exhibited transferrin binding. Growth in these fluids with 10 micrograms of added hemin ml-1 abolished transferrin binding, whereas addition of 10 mM ferric nitrate did not. These data suggest that the transferrin receptor of H. influenzae is regulated by levels of hemin but not elemental iron alone and that this property is widely distributed among several major cloned families in the species.

A siderophore production mutant of Bordetella bronchiseptica cannot use lactoferrin as an iron source

Bordetella bronchiseptica secreted a hydroxamate siderophore when grown in Fe-depleted medium. A Tn5lac insertion mutant of B. bronchiseptica, DBB22, did not produce this hydroxamate siderophore and was incapable of using lactoferrin as an Fe source. Our data suggest that B. bronchiseptica uses a siderophore for removal of Fe from lactoferrin and transferrin rather than relying upon a receptor for these host Fe-binding proteins.

Signal transduction and virulence regulation in human and animal pathogens

Pathogens have developed many strategies for survival in animals and humans which possess very effective defense mechanisms. Although there are many different ways, in which pathogenic bacteria solved the problem to overcome the host defense, some common features of virulence mechanisms can be detected even in phylogenetically very distant bacteria (Finlay and Falkow (1989) Microb. Rev. 6, 1375-1383). One important feature is that the regulation of expression of virulence factors and the exact timing of their expression is very important for many of the pathogenic bacteria, as most of them have to encounter different growth situations during an infection cycle, which require a fast adaptation to the new situation by the expression of different factors. This review gives an overview about the mechanisms used by pathogenic bacteria to accomplish the difficult task of regulation of their virulence potential in response to environmental changes. In addition, the relationship of these virulence regulatory systems with other signal transduction mechanisms not involved in pathogenicity is discussed.

Isolation and characterization of Bordetella bronchiseptica mutants deficient in siderophore activity

Iron acquisition by the gram-negative pathogens Bordetella bronchiseptica and Bordetella pertussis is thought to occur by hydroxamate siderophore-mediated transport as well as an apparently siderophore-independent process by which host transferrins bind to bacterial surface receptors. We constructed B. bronchiseptica mutants deficient in siderophore activity by insertional mutagenesis with miniTn5/lacZ1. The mutants could be placed into four distinct complementation groups, as determined from cross-feeding assays which demonstrated restored siderophore synthesis. Mutants deficient in siderophore activity were BRM1, BRM6, and BRM9, exhibiting approximately 36 to 41% of wild-type siderophore levels, and BRM3 and BRM8, which appeared to produce very little or no detectable siderophore. Mutant BRM4 was found to be a leucine auxotroph, while mutants BRM2 and BRM7 could synthesize siderophore only in low-iron medium which was supplemented with various amino acids. Evaluation of all transcriptional fusions revealed an apparent lack of iron-regulated lacZ expression. Genomic regions flanking the transposable element in the siderophore mutants were homologous with B. pertussis chromosomal DNA, while bioassays suggested siderophore cross-feeding between B. pertussis and B. bronchiseptica. These results indicate probable similarity between the siderophore biosynthetic and transport systems of the two species.