Identification of the znuA-encoded periplasmic zinc transport protein of Haemophilus ducreyi

Expression of the cytolethal distending toxin in a geographically diverse collection of Haemophilus ducreyi clinical isolates

OBJECTIVE

To screen a collection of isolates of Haemophilus ducreyi for expression of the cytolethal distending toxin (CDT).

METHODS

45 clinical isolates of H ducreyi were screened for cytotoxic activity by examining the effect of culture supernatants on Hela cells. Expression was confirmed using immunoblotting with CDT specific monoclonal antibodies and the presence of the cdt genes determined by amplification of the cdt genes in a multiplex polymerase chain assay.

RESULTS

Of the 45 clinical isolates, six isolates from differing geographical origins did not demonstrate cytotoxic activity. Expression of CDT was also not detected in these six isolates using immunoblotting and the genes cdtA, cdtB, and cdtC were not amplified using PCR. The remaining isolates demonstrated cytotoxic activity, expressed the CDT proteins, and the presence of the cdt genes was confirmed.

CONCLUSIONS

CDT is considered a virulence factor of H ducreyi but was found to be absent in 13% of isolates from different geographical origins.

Identification of Actinobacillus pleuropneumoniae genes important for survival during infection in its natural host

Actinobacillus pleuropneumoniae is a strict respiratory tract pathogen of swine and is the causative agent of porcine pleuropneumonia. We have used signature-tagged mutagenesis (STM) to identify genes required for survival of the organism within the pig. A total of 2,064 signature-tagged Tn10 transposon mutants were assembled into pools of 48 each, and used to inoculate pigs by the endotracheal route. Out of 105 mutants that were consistently attenuated in vivo, only 11 mutants showed a >2-fold reduction in growth in vitro compared to the wild type, whereas 8 of 14 mutants tested showed significant levels of attenuation in pig as evidenced from competitive index experiments. Inverse PCR was used to generate DNA sequence of the chromosomal domains flanking each transposon insertion. Only one sibling pair of mutants was identified, but three apparent transposon insertion hot spots were found--an anticipated consequence of the use of a Tn10-based system. Transposon insertions were found within 55 different loci, and similarity (BLAST) searching identified possible analogues or homologues for all but four of these. Matches included proteins putatively involved in metabolism and transport of various nutrients or unknown substances, in stress responses, in gene regulation, and in the production of cell surface components. Ten of the sequences have homology with genes involved in lipopolysaccharide and capsule production. The results highlight the importance of genes involved in energy metabolism, nutrient uptake and stress responses for the survival of A. pleuropneumoniae in its natural host: the pig.

Zn(II) metabolism in prokaryotes

It is difficult to over-state the importance of Zn(II) in biology. It is a ubiquitous essential metal ion and plays a role in catalysis, protein structure and perhaps as a signal molecule, in organisms from all three kingdoms. Of necessity, organisms have evolved to optimise the intracellular availability of Zn(II) despite the extracellular milieu. To this end, prokaryotes contain a range of Zn(II) import, Zn(II) export and/or binding proteins, some of which utilise either ATP or the chemiosmotic potential to drive the movement of Zn(II) across the cytosolic membrane, together with proteins that facilitate the diffusion of this ion across either the outer or inner membranes of prokaryotes. This review seeks to give an overview of the systems currently classified as altering Zn(II) availability in prokaryotes.

Isolation and characterization of mini-Tn5Km2 insertion mutants of Brucella abortus deficient in internalization and intracellular growth in HeLa cells

Brucella spp. are facultative intracellular pathogens that have the ability to survive and multiply in professional and nonprofessional phagocytes and cause abortion in domestic animals and undulant fever in humans. The mechanism and factors of virulence are not fully understood. To identify genes related to internalization and multiplication in host cells, Brucella abortus was mutagenized by mini-Tn5Km2 transposon that carryied the kanamycin resistance gene, 4,400 mutants were screened, and HeLa cells were infected with each mutant. Twenty-three intracellular-growth-defective mutants were screened and were characterized for internalization and intracellular growth. From these results, we divided the mutants into the following three groups: class I, no internalization and intracellular growth within HeLa cells; class II, an internalization similar to that of the wild type but with no intracellular growth; and class III, internalization twice as high as the wild type but with no intracellular growth. Sequence analysis of DNA flanking the site of transposon showed various insertion sites of bacterial genes that are virulence-associated genes, including virB genes, an ion transporter system, and biosynthesis- and metabolism-associated genes. These internalization and intracellular-growth-defective mutants in HeLa cells also showed defective intracellular growth in macrophages. These results suggest that the virulence-associated genes isolated here contributed to the intracellular growth of both nonprofessional and professional phagocytes.

Mutations in the lspA1 and lspA2 genes of Haemophilus ducreyi affect the virulence of this pathogen in an animal model system

Haemophilus ducreyi 35000HP contains two genes, lspA1 and lspA2, whose predicted protein products have molecular weights of 456,000 and 543,000, respectively (C. K. Ward, S. R. Lumbley, J. L. Latimer, L. D. Cope, and E. J. Hansen, J. Bacteriol. 180:6013-6022, 1998). We have constructed three H. ducreyi 35000HP mutants containing antibiotic resistance cartridges in one or both of the lspA1 and lspA2 open reading frames. Western blot analysis using LspA1- and LspA2-specific monoclonal antibodies indicated that the wild-type parent strain 35000HP expressed LspA1 protein that was readily detectable in culture supernatant fluid together with a barely detectable amount of LspA2 protein. The lspA2 mutant 35000HP.2 expressed LspA1 protein that was detectable in culture supernatant fluid and no LspA2 protein. In contrast, the H. ducreyi lspA1 mutant 35000HP.1, which did not express the LspA1 protein, expressed a greater quantity of the LspA2 protein than did the wild-type parent strain. The lspA1 lspA2 double mutant 35000HP.12 expressed neither LspA1 nor LspA2. The three mutant strains adhered to human foreskin fibroblasts and to a human keratinocyte cell line in vitro at a level that was not significantly different from that of the wild-type strain 35000HP. Lack of expression of the LspA1 protein by both the lspA1 mutant and the lspA1 lspA2 double mutant was associated with an increased tendency to autoagglutinate. When evaluated in the temperature-dependent rabbit model for chancroid, the lspA1 lspA2 double mutant was substantially less virulent than the wild-type strain 35000HP. The results of these studies indicated that H. ducreyi requires both the LspA1 and LspA2 proteins to be fully virulent in this animal model for experimental chancroid.

The high-affinity zinc-uptake system znuACB is under control of the iron-uptake regulator (fur) gene in the animal pathogen Pasteurella multocida

The Pasteurella multocida znuACB genes encoding a high-affinity zinc-uptake system have been identified and cloned. In contrast to what happens in Escherichia coli, znuA is not physically linked to znuCB. Through lacZ transcriptional fusions it has been demonstrated that zinc negatively regulates both znuA and znuCB operons. Nevertheless, and contrary to that determined so far for all other znuACB bacterial systems known, P. multocida znuACB genes are not under control of the zur gene, which is absent in this bacterial species, but rather are under its iron-uptake regulator (fur) gene. Furthermore, construction of defective mutants has demonstrated that P. multocida znuA and znuCB transcriptional units are required for virulence of this organism in a mouse model.

ZntB is a novel Zn2+ transporter in Salmonella enterica serovar Typhimurium

A Zn2+ transport system encoded by the zntB locus of Salmonella enterica serovar Typhimurium has been identified. The protein encoded by this locus is homologous to the CorA family of Mg2+ transport proteins and is widely distributed among the eubacteria. Mutations at zntB confer an increased sensitivity to the cytotoxic effects of Zn2+ and Cd2+, a phenotype that suggests that the encoded protein mediates the efflux of both cations. A direct analysis of transport activity identified a capacity for Zn2+ efflux. These data identify ZntB as a zinc efflux pathway in the enteric bacteria and assign a new function to the CorA family of cation transporters.

Role of the high-affinity zinc uptake znuABC system in Salmonella enterica serovar typhimurium virulence

The Salmonella enterica serovar Typhimurium znuABC genes encoding a high-affinity zinc uptake system and its regulatory zur gene have been cloned. Salmonella serovar Typhimurium zur and znuC knockout mutants have been constructed by marker exchange. The 50% lethal dose of the znuC mutant increased when either orally or intraperitoneally inoculated in BALB/c mice, while virulence of the zur mutant decreased only when mice were intraperitoneally challenged.

Haemophilus ducreyi requires the flp gene cluster for microcolony formation in vitro

Haemophilus ducreyi, the etiologic agent of chancroid, has been shown to form microcolonies when cultured in the presence of human foreskin fibroblasts. We identified a 15-gene cluster in H. ducreyi that encoded predicted protein products with significant homology to those encoded by the tad (for tight adhesion) locus in Actinobacillus actinomycetemcomitans that is involved in the production of fimbriae by this periodontal pathogen. The first three open reading frames in this H. ducreyi gene cluster encoded predicted proteins with a high degree of identity to the Flp (fimbria-like protein) encoded by the first open reading frame of the tad locus; this 15-gene cluster in H. ducreyi was designated flp. RT-PCR analysis indicated that the H. ducreyi flp gene cluster was likely to be a polycistronic operon. Mutations within the flp gene cluster resulted in an inability to form microcolonies in the presence of human foreskin fibroblasts. In addition, the same mutants were defective in the ability to attach to both plastic and human foreskin fibroblasts in vitro. An H. ducreyi mutant with an inactivated tadA gene exhibited a small decrease in virulence in the temperature-dependent rabbit model for experimental chancroid, whereas another H. ducreyi mutant with inactivated flp-1 and flp-2 genes was as virulent as the wild-type parent strain. These results indicate that the flp gene cluster is essential for microcolony formation by H. ducreyi, whereas this phenotypic trait is not linked to the virulence potential of the pathogen, at least in this animal model of infection.

Characterization of Haemophilus ducreyi cdtA, cdtB, and cdtC mutants in in vitro and in vivo systems

Haemophilus ducreyi expresses a soluble cytolethal distending toxin (CDT) that is encoded by the cdtABC gene cluster and can be detected in culture supernatant fluid by its ability to kill HeLa cells. The cdtA, cdtB, and cdtC genes of H. ducreyi were cloned independently into plasmid vectors, and their encoded proteins expressed singly or in various combinations in an Escherichia coli background. All three gene products had to be expressed in order for E. coli-derived culture supernatant fluids to demonstrate cytotoxicity for HeLa cells. Isogenic H. ducreyi cdtA and cdtB mutants were constructed and used in combination with the wild-type parent strain and a previously described H. ducreyi cdtC mutant (M. K. Stevens, J. L. Latimer, S. R. Lumbley, C. K. Ward, L. D. Cope, T. Lagergard, and E. J. Hansen, Infect. Immun. 67:3900-3908, 1999) to determine the relative contributions of the CdtA, CdtB, and CdtC proteins to CDT activity. Expression of CdtA, CdtB, and CdtC appeared necessary for H. ducreyi-derived culture supernatant fluid to exhibit cytotoxicity for HeLa cells. Whole-cell sonicates and periplasmic extracts from the cdtB and cdtC mutants had no effect on HeLa cells, whereas these same fractions from a cdtA mutant had a very modest cytotoxic effect on these same human cells. CdtA appeared to be primarily associated with the H. ducreyi cell envelope, whereas both CdtB and CdtC were present primarily in the soluble fraction from sonicated cells. Both the cdtA mutant and the cdtB mutant were found to be fully virulent in the temperature-dependent rabbit model for experimental chancroid.

Identification and characterization of a high-affinity zinc uptake system in Neisseria gonorrhoeae

A search of the gonococcal genome database using the known zinc-binding protein (znuA) sequences from Escherichia coli and Haemophilus influenzae identified an open reading frame encoding a putative gonococcal ZnuA. The consensus amino acid sequence of this open reading frame possessed a characteristic 30-amino acid histidine-rich metal-binding motif (repetitive HDH sequence) containing 43% histidine and 37% aspartic acid and glutamic acid. Subsequently, two adjacent open reading frames with homology to E. coli and H. influenzae znuB and znuC were located upstream of znuA. When partially purified from sonicated cell-free supernatants by CM-Sepharose chromatography, the mature gonococcal ZnuA had an estimated molecular mass of 38 kDa by SDS-PAGE. The presence of a DNA sequence encoding a 19-amino acid signal peptide and the solubility of the mature ZnuA suggested that this protein was located in the periplasm. Inactivation of the Neisseria gonorrhoeae F62 znuA by insertional mutagenesis resulted in a mutant that had a growth rate lower than that of the wild-type parent strain and that required high concentrations of ZnCl2 (> or = 200 microM) for optimal growth. Using a chemically defined agar medium, the gonococcal ZnuA mutant grew only in the presence of Zn(2+), whereas Mg(2+), Ca(2+), Ni(2+), Fe(2+), Cu(2+), Mn(2+) and Cd(2+) had either no effect or were growth inhibitory.

Investigation of the interaction among the components of the cytolethal distending toxin of Haemophilus ducreyi

The cytolethal distending toxin (CDT) of Haemophilus ducreyi is encoded by the cdtABC genes, but the composition of active CDT is not known. Both immunoaffinity and metal affinity chromatographic methods were used to purify H. ducreyi CDT from recombinant Escherichia coli strains bearing wild-type or mutated H. ducreyi cdtABC genes. Both affinity-purified preparations contained CdtA, CdtB, and CdtC proteins. These purification efforts also revealed that the formation of a noncovalent CdtB-CdtC complex and production of a fully active CDT complex required the presence of a functional CdtA protein. When purified recombinant CdtB and CdtC proteins were mixed, only very slight CDT activity was detected. In contrast, when a bacterial cell extract containing CdtA was mixed with purified preparations of both CdtB and CdtC, full CDT activity was reconstituted in vitro. These results indicate that CdtA is essential for normal H. ducreyi CDT activity and that CdtA likely modifies or alters either CdtB or CdtC or both to form the active CDT complex.

A histidine-rich metal binding domain at the N terminus of Cu,Zn-superoxide dismutases from pathogenic bacteria: a novel strategy for metal chaperoning

A group of Cu,Zn-superoxide dismutases from pathogenic bacteria is characterized by histidine-rich N-terminal extensions that are in a highly exposed and mobile conformation. This feature allows these proteins to be readily purified in a single step by immobilized metal affinity chromatography. The Cu,Zn-superoxide dismutases from both Haemophilus ducreyi and Haemophilus parainfluenzae display anomalous absorption spectra in the visible region due to copper binding at the N-terminal region. Reconstitution experiments of copper-free enzymes demonstrate that, under conditions of limited copper availability, this metal ion is initially bound at the N-terminal region and subsequently transferred to an active site. Evidence is provided for intermolecular pathways of copper transfer from the N-terminal domain of an enzyme subunit to an active site located on a distinct dimeric molecule. Incubation with EDTA rapidly removes copper bound at the N terminus but is much less effective on the copper ion bound at the active site. This indicates that metal binding by the N-terminal histidines is kinetically favored, but the catalytic site binds copper with higher affinity. We suggest that the histidine-rich N-terminal region constitutes a metal binding domain involved in metal uptake under conditions of metal starvation in vivo. Particular biological importance for this domain is inferred by the observation that its presence enhances the protection offered by periplasmic Cu,Zn-superoxide dismutase toward phagocytic killing.

zur: a Zn(2+)-responsive regulatory element of Staphylococcus aureus

A putative operon encoding a probable zinc-responsive regulatory element (zur) and components of an ABC-type transporter (mreA mreB) have been characterized in Staphylococcus aureus. The zur gene was inactivated but apparently this did not alter Zn(2+) uptake. Expression of mreAB zur is at a low level under a range of ion conditions. To allow inducible expression of the operon, a construct was made placing it under the control of the IPTG-inducible P(spac) promoter. Using this approach, it was shown that zur is able to repress expression of the entire operon in a Zn(2+)-dependent manner, and that mreA and mreB are likely to be involved in high-affinity ion uptake. zur has no apparent role in pathogenicity in a lesion model of S. aureus infection.

Neisserial TonB-dependent outer-membrane proteins: detection, regulation and distribution of three putative candidates identified from the genome sequences

Computer searches were carried out of the gonococcal and meningococcal genome databases for previously unknown members of the TonB-dependent family (Tdf) of outer-membrane receptor proteins. Seven putative non-contiguous genes were found and three of these (identified in gonococcal strain FA1090) were chosen for further study. Consensus motif analysis of the peptide sequences was consistent with the three genes encoding TonB-dependent receptors. In view of the five previously characterized TonB-dependent proteins of pathogenic neisseriae, the putative genes were labelled tdfF, tdfG and tdfH. TdfF had homology with the siderophore receptors FpvA of Pseudomonas aeruginosa and FhuE of Escherichia coli, whereas TdfG and TdfH had homology with the haemophore receptor HasR of Serratia marcescens. The aim of this project was to characterize these proteins and determine their expression, regulation, distribution and surface exposure. Strain surveys of iron-stressed commensal and pathogenic neisseriae revealed that TdfF is unlikely to be expressed, TdfG is expressed by gonococci only and that TdfH is expressed by both meningococci and gonococci. Expression of TdfH was unaffected by iron availability. Susceptibility of TdfH to cleavage by proteases in live gonococci was consistent with surface exposure of this protein. TdfH may function as a TonB-dependent receptor for a non-iron nutrient source. Furthermore, TdfH is worthy of future investigation as a potential meningococcal vaccine candidate as it is a highly conserved, widely distributed and surface-exposed outer-membrane protein.

Effect of metals on Candida albicans growth in the presence of chemical chelators and human abscess fluid

Calprotectin is a calcium- and zinc-binding protein that is present in abscess fluid supernatants and appears to inhibit microbial growth through competition for zinc. In the present study, growth inhibition by chemical chelators was compared with that produced by human abscess fluid to determine whether other chelators, perhaps with different metal specificities, would have the same or different patterns of metal reversibility as abscess fluid. Zinc was found to be more potent than the other metals tested in reversing C. albicans growth inhibition by human abscess fluid and three chemical chelators, even though in some cases the stability constants of certain of these chelators were higher for other metals. For example, in the presence of the chelator diethylenetriaminopentaacetic acid, zinc stimulated Candida growth at a 10-fold lower concentration than did iron, even though this chelator has a stability constant for iron that is almost 10(10) higher than that for zinc. These results suggest that the zinc specificity of calprotectin's C. albicans growth inhibition can best be explained by the marked sensitivity of this organism to zinc deprivation rather than by selective binding of this metal by the protein.

A new family of high-affinity ABC manganese and zinc permeases

Phylogenetic analysis of 47 extracellular putative metal binding receptors (MBRs) belonging to the newly defined cluster suggests the existence of two subclusters. The question of substrate specificity of the corresponding ATP binding cassette (ABC) permeases is discussed, based on data collected from 19 of them concerning their regulation, metal requirement of permease mutants, metal uptake and metal binding. The proposal that the two subclusters correspond to paralogous metal permeases dedicated primarily to manganese and to zinc transport is made. The question of a direct role of MBRs as adhesins of Gram-positive bacteria is then discussed and the importance of metal permeases for cellular processes and host-bacteria interactions is reviewed.

Host factors that influence the behaviour of bacterial pathogens in vivo

Interest is increasing in how bacteria behave and produce virulence determinants within the infected host. There are three aspects of this process; observations on the bacteria themselves, recognition of host factors that affect them and investigation of metabolic interactions between the two. The first aspect is relatively easy to investigate and attracts much interest. The second and third are difficult to work on and hence understudied. The review aims to stimulate interest in them by indicating methods of investigation and describing some successful studies. After discussing host factors that determine growth in vivo consideration is given to factors that influence the production of the determinants of mucosal colonization, penetration, interference with host defence and damage to the host. The final section deals with the influence of host-derived cytidine 5'-monophospho-N-acetyl neuraminic acid and lactate on the pathogenicity of gonococci, meningococci and Haemophilus influenzae.

Challenge of investigating biologically relevant functions of virulence factors in bacterial pathogens

Recent innovations have increased enormously the opportunities for investigating the molecular basis of bacterial pathogenicity, including the availability of whole-genome sequences, techniques for identifying key virulence genes, and the use of microarrays and proteomics. These methods should provide powerful tools for analysing the patterns of gene expression and function required for investigating host-microbe interactions in vivo. But, the challenge is exacting. Pathogenicity is a complex phenotype and the reductionist approach does not adequately address the eclectic and variable outcomes of host-microbe interactions, including evolutionary dynamics and ecological factors. There are difficulties in distinguishing bacterial 'virulence' factors from the many determinants that are permissive for pathogenicity, for example those promoting general fitness. A further practical problem for some of the major bacterial pathogens is that there are no satisfactory animal models or experimental assays that adequately reflect the infection under investigation. In this review, we give a personal perspective on the challenge of characterizing how bacterial pathogens behave in vivo and discuss some of the methods that might be most relevant for understanding the molecular basis of the diseases for which they are responsible. Despite the powerful genomic, molecular, cellular and structural technologies available to us, we are still struggling to come to grips with the question of 'What is a pathogen?'

Evidence for the transport of zinc(II) ions via the pit inorganic phosphate transport system in Escherichia coli

A locus involved in zinc(II) uptake in Escherichia coli K-12 was identified through the generation of a zinc(II)-resistant mutant by transposon (Tn10dCam) mutagenesis. The mutation was located within the pitA gene, which encodes the low-affinity inorganic phosphate transport system (Pit). The pitA mutant accumulated reduced amounts of zinc(II) when exposed to 0.5-2.0 mM ZnSO(4) during growth in Luria-Bertani medium.

Chancroid: from clinical practice to basic science

Chancroid is a sexually transmitted disease caused by the bacterium Haemophilus ducreyi. It usually presents as a genital ulcer and may be associated with regional lymphadenopathy and bubo formation. H. ducreyi infection is predominantly seen in tropical resource-poor regions of the world where it is frequently the most common etiological cause of genital ulceration. Genital ulcer disease has been shown to be an extremely important co-factor in HIV transmission. With the advent of the AIDS epidemic, there has been increased research effort to elucidate those factors involved in the pathogenesis of chancroid. Several putative virulence factors have now been identified and isogenic H. ducreyi mutants constructed by mutagenesis of their encoding genes. This approach has facilitated investigations into the role each of these putative virulence factors may play in H. ducreyi pathogenesis through the use of in vitro and in vivo model systems. One major goal of current chancroid research is to identify antigens which are immunogenic and could form the basis of a vaccine against H. ducreyi infection. Such a vaccine, if shown to be effective in decreasing the prevalence of chancroid, could have the added benefit of slowing down the HIV incidence rates in those populations where chancroid is a major co-factor for HIV transmission.