Use of electroporation to construct isogenic mutants of Haemophilus ducreyi

The Haemophilus ducreyi LspA1 protein inhibits phagocytosis by using a new mechanism involving activation of C-terminal Src kinase

Haemophilus ducreyi causes chancroid, a sexually transmitted infection. A primary means by which this pathogen causes disease involves eluding phagocytosis; however, the molecular basis for this escape mechanism has been poorly understood. Here, we report that the LspA virulence factors of H. ducreyi inhibit phagocytosis by stimulating the catalytic activity of C-terminal Src kinase (Csk), which itself inhibits Src family protein tyrosine kinases (SFKs) that promote phagocytosis. Inhibitory activity could be localized to a 37-kDa domain (designated YL2) of the 456-kDa LspA1 protein. The YL2 domain impaired ingestion of IgG-opsonized targets and decreased levels of active SFKs when expressed in mammalian cells. YL2 contains tyrosine residues in two EPIYG motifs that are phosphorylated in mammalian cells. These tyrosine residues were essential for YL2-based inhibition of phagocytosis. Csk was identified as the predominant mammalian protein interacting with YL2, and a dominant-negative Csk rescued phagocytosis in the presence of YL2. Purified Csk phosphorylated the tyrosines in the YL2 EPIYG motifs. Phosphorylated YL2 increased Csk catalytic activity, resulting in positive feedback, such that YL2 can be phosphorylated by the same kinase that it activates. Finally, we found that the Helicobacter pylori CagA protein also inhibited phagocytosis in a Csk-dependent manner, raising the possibility that this may be a general mechanism among diverse bacteria. Harnessing Csk to subvert the Fcγ receptor (FcγR)-mediated phagocytic pathway represents a new bacterial mechanism for circumventing a crucial component of the innate immune response and may potentially affect other SFK-involved cellular pathways.

Phagocytosis is a critical component of the immune system that enables pathogens to be contained and cleared. A number of bacterial pathogens have developed specific strategies to either physically evade phagocytosis or block the intracellular signaling required for phagocytic activity. Haemophilus ducreyi, a sexually transmitted pathogen, secretes a 4,153-amino-acid (aa) protein (LspA1) that effectively inhibits FcγR-mediated phagocytic activity. In this study, we show that a 294-aa domain within this bacterial protein binds to C-terminal Src kinase (Csk) and stimulates its catalytic activity, resulting in a significant attenuation of Src kinase activity and consequent inhibition of phagocytosis. The ability to inhibit phagocytosis via Csk is not unique to H. ducreyi, because we found that the Helicobacter pylori CagA protein also inhibits phagocytosis in a Csk-dependent manner. Harnessing Csk to subvert the FcγR-mediated phagocytic pathway represents a new bacterial effector mechanism for circumventing the innate immune response.

The Haemophilus ducreyi Fis protein is involved in controlling expression of the lspB-lspA2 operon and other virulence factors

Expression of the lspB-lspA2 operon encoding a virulence-related two-partner secretion system in Haemophilus ducreyi 35000HP is directly regulated by the CpxRA regulatory system (M. Labandeira-Rey, J. R. Mock, and E. J. Hansen, Infect. Immun. 77:3402-3411, 2009). In the present study, we show that this secretion system is also regulated by the small nucleoid-associated protein Fis. Inactivation of the H. ducreyi fis gene resulted in a reduction in expression of both the H. ducreyi LspB and LspA2 proteins. DNA microarray experiments showed that a H. ducreyi fis deletion mutant exhibited altered expression levels of genes encoding other important H. ducreyi virulence factors, including DsrA and Flp1, suggesting a possible global role for Fis in the control of virulence in this obligate human pathogen. While the H. ducreyi Fis protein has a high degree of sequence and structural similarity to the Fis proteins of other bacteria, its temporal pattern of expression was very different from that of enterobacterial Fis proteins. The use of a lacZ-based transcriptional reporter provided evidence which indicated that the H. ducreyi Fis homolog is a positive regulator of gyrB, a gene that is negatively regulated by Fis in enteric bacteria. Taken together, the Fis protein expression data and the observed regulatory effects of Fis in H. ducreyi suggest that this small DNA binding protein has a regulatory role in H. ducreyi which may differ in substantial ways from that of other Fis proteins.

Inactivation of the Haemophilus ducreyi luxS gene affects the virulence of this pathogen in human subjects

Haemophilus ducreyi 35000HP contains a homologue of the luxS gene, which encodes an enzyme that synthesizes autoinducer 2 (AI-2) in other gram-negative bacteria. H. ducreyi 35000HP produced AI-2 that functioned in a Vibrio harveyi-based reporter system. A H. ducreyi luxS mutant was constructed by insertional inactivation of the luxS gene and lost the ability to produce AI-2. Provision of the H. ducreyi luxS gene in trans partially restored AI-2 production by the mutant. The luxS mutant was compared with its parent for virulence in the human challenge model of experimental chancroid. The pustule-formation rate in 5 volunteers was 93.3% (95% confidence interval, 81.7%-99.9%) at 15 parent sites and 60.0% (95% confidence interval, 48.3%-71.7%) at 15 mutant sites (1-tailed P < .001). Thus, the luxS mutant was partially attenuated for virulence. This is the first report of AI-2 production contributing to the pathogenesis of a genital ulcer disease.

Regulation of expression of the Haemophilus ducreyi LspB and LspA2 proteins by CpxR

The LspA1, LspA2, and LspB proteins of Haemophilus ducreyi comprise a two-partner secretion system that has been shown to be necessary for H. ducreyi to inhibit phagocytosis by immune cells in vitro. Inactivation of lspA1 resulted in increased levels of LspA2, suggesting that these two proteins are differentially controlled (C. J. Ward et al., Infect. Immun. 71:2478-2486, 2003). Expression of LspA2 but not LspA1 was shown to be both growth phase dependent and affected by the presence of fetal calf serum (FCS) in the growth medium. In addition, neither LspA1 nor LspA2 could be detected in culture supernatant fluid in the absence of FCS. DNA microarray analysis revealed that 324 H. ducreyi genes were differentially regulated after growth in the presence of FCS. Among these, the CpxRA two-component sensory transduction system was downregulated by the presence of FCS. Inactivation of cpxR resulted in increased expression of both LspB and LspA2. Electrophoretic mobility shift assays showed that a recombinant H. ducreyi CpxR protein bound the promoter region of the lspB-lspA2 operon. The cpxR and cpxA genes were shown to be part of an operon containing two additional genes in H. ducreyi 35000HP. This is the first description of a two-component sensory transduction system regulating a proven virulence factor of H. ducreyi.

Construction of non-polar mutants in Haemophilus influenzae using FLP recombinase technology

Background

Nontypeable Haemophilus influenzae (NTHi) is a gram-negative bacterium that causes otitis media in children as well as other infections of the upper and lower respiratory tract in children and adults. We are employing genetic strategies to identify and characterize virulence determinants in NTHi. NTHi is naturally competent for transformation and thus construction of most mutants by common methodologies is relatively straightforward. However, new methodology was required in order to construct unmarked non-polar mutations in poorly expressed genes whose products are required for transformation. We have adapted the lambda red/FLP-recombinase-mediated strategy used in E. coli for use in NTHi.

Results

A cassette containing a spectinomycin resistance gene and an rpsL gene flanked by FRT sites was constructed. A PCR amplicon containing 50 base pairs of DNA homologous to the 5' and 3' ends of the gene to be disrupted and the cassette was generated, then recombineered into the target NTHi gene, cloned on a plasmid, using the lambda recombination proteins expressed in E. coli DY380. Thus, the gene of interest was replaced by the cassette. The construct was then transformed into a streptomycin resistant NTHi strain and mutants were selected on spectinomycin-containing growth media. A plasmid derived from pLS88 with a temperature sensitive replicon expressing the FLP recombinase gene under the control of the tet operator/repressor was constructed. This plasmid was electroporated into the NTHi mutant at the permissive temperature and FLP expression was induced using anhydrotetracycline. The recombinase recognizes the FRT sites and eliminates the antibiotic cassette by site-specific recombination, creating the unmarked non-polar mutation. The plasmid is cured by growth of cells at the restrictive temperature.

Conclusion

The products of the genes in the NTHi pilABCD operon are required for type IV pilus biogenesis and have a role in transformation. We demonstrated the utility of our methodology by the construction of a non-polar pilA mutation in NTHi strain 2019 and complementation of the mutation with a plasmid containing the pilA gene. Utilization of this approach allowed us to readily generate unmarked non-polar mutations in NTHi genes.

Unveiling electrotransformation of Moraxella catarrhalis as a process of natural transformation

The human respiratory tract pathogen Moraxella catarrhalis is a naturally competent microorganism. However, electrotransformation has long been used to introduce foreign DNA into this organism. This study demonstrated that electrotransformants obtained with linear or circular nonreplicating plasmid DNA originated exclusively from natural transformation processes taking place during the recovery phase after the application of current. Only replicating plasmid DNA could be introduced into M. catarrhalis by electrotransformation, in a type IV pilus-independent manner. Electrotransformation with homologous genomic DNA indicated that restriction of double-stranded DNA was independent of type III restriction-methylation systems. Nontransformability of M. catarrhalis by electrotransformation was observed using double- as well as single-stranded DNA. In addition, the study showed that natural competence is a very constant feature of M. catarrhalis.

Identification of genes involved in the expression of atypical lipooligosaccharide structures from a second class of Haemophilus ducreyi

Haemophilus ducreyi is a gram-negative bacterium that is the causative agent of chancroid. Strain 35000HP has been well characterized and is representative of the majority of H. ducreyi strains. Strain 35000HP produces a lipooligosaccharide (LOS) that contains D-glycero-D-manno-heptose in the main oligosaccharide chain extension; the lbgB gene has been shown to encode the DD-heptosyltransferase. The lbgB gene is found in a gene cluster together with the lbgA gene, which encodes for the galactosyltransferase I. These two genes are flanked by two housekeeping genes, rpmE and xthA, encoding the ribosomal protein L31 and the exonuclease III, respectively. Recently, a second group of H. ducreyi strains have been identified. Strain 33921, a representative of the class II strains, produces an LOS that lacks DD-heptose in the oligosaccharide portion of its LOS. To better understand the biosynthesis of the DD-heptose-deficient 33921 LOS, we cloned and sequenced the corresponding lbgAB genomic region from strain 33921. Similar to strain 35000HP, the 33921 genome contains xthA and rpmE. However, between these two genes we identified genes encoding two putative glycosyltransferases that were not highly homologous to the 35000HP lbgAB genes. In this study, we demonstrate that the product of one of these genes encodes a galactosyltransferase. In addition, dot blot hybridization determined that 3 of 35 strains tested had the atypical transferases present, as did 4 strains characterized as class II strains by other criterion. These data indicate that the lbgAB genes can serve as one indicator of the classification of H. ducreyi strains.

Identification of Francisella tularensis genes affected by iron limitation

Cells of an attenuated live vaccine strain (LVS) of F. tularensis grown under iron-restricted conditions were found to contain increased quantities of several proteins relative to cells of this same strain grown under iron-replete conditions. Mass spectrometric analysis identified two of these proteins as IglC and PdpB, both of which are encoded by genes located in a previously identified pathogenicity island in F. tularensis LVS. Regions with homology to the consensus Fur box sequence were located immediately in front of the iglC and pdpB open reading frames (ORFs), and in silico analysis of the F. tularensis Schu4 genome detected a number of predicted 5' untranslated regions that contained putative Fur boxes. The putative Fur box preceding Francisella iron-regulated gene A (figA) had the highest degree of identity with the consensus Fur box sequence. DNA microarray analysis showed that nearly 80 of the genes in the F. tularensis LVS genome were up- or down-regulated at least twofold under iron-restricted growth conditions. When tested for possible siderophore production by means of the Chrome Azurol S assay, a wild-type F. novicida strain produced a large reaction zone whereas its figA mutant produced very little reactivity in this assay. In addition, a cross-feeding experiment demonstrated that this siderophore-like activity produced by the wild-type F. novicida strain could enhance the ability of the F. novicida figA mutant to grow under iron-restricted conditions. This study provides the first identification of iron-regulated genes in F. tularensis LVS and evidence for the production of a siderophore-like molecule by F. novicida.

Expression of the LspA1 and LspA2 proteins by Haemophilus ducreyi is required for virulence in human volunteers

Haemophilus ducreyi colocalizes with polymorphonuclear leukocytes and macrophages and evades phagocytosis during experimental infection of human volunteers. H. ducreyi contains two genes, lspA1 and lspA2, which encode predicted proteins of 456 and 543 kDa, respectively. Compared to its wild-type parent, an lspA1 lspA2 double mutant does not inhibit phagocytosis by macrophage and myelocytic cell lines in vitro and is attenuated in an experimental rabbit model of chancroid. To test whether expression of LspA1 and LspA2 was necessary for virulence in humans, six volunteers were experimentally infected. Each volunteer was inoculated with three doses (ranging from 85 to 112 CFU) of the parent (35000HP) in one arm and three doses (ranging from 60 to 822 CFU) of the mutant (35000HP Omega 12) in the other arm. The papule formation rates were 88% (95% confidence interval [95% CI], 76.8 to 99.9%) at 18 parent sites and 72% (95% CI, 44.4 to 99.9%) at 18 mutant sites (P = 0.19). However, papules were significantly smaller at mutant sites (mean size, 24.8 mm(2)) than at parent sites (mean size, 39.1 mm(2)) 24 h after inoculation (P = 0.0002). The pustule formation rates were 44% (95% CI, 5.8 to 77.6%) at parent sites and 0% (95% CI, 0 to 39.4%) at mutant sites (P = 0.009). With the caveat that biosafety regulations preclude testing of a complemented mutant in human subjects, these results indicate that expression of LspA1 and LspA2 facilitates the ability of H. ducreyi to initiate disease and to progress to pustule formation in humans.

The LspB protein is involved in the secretion of the LspA1 and LspA2 proteins by Haemophilus ducreyi

The LspA1 and LspA2 proteins of Haemophilus ducreyi 35000 are two very large macromolecules that can be detected in concentrated culture supernatant fluid. Both of these proteins exhibit homology with the N-terminal region of the Bordetella pertussis filamentous hemagglutinin (FHA), which is involved in secretion of the latter macromolecule. The lspA2 open reading frame is flanked upstream by a gene, lspB, that encodes a predicted protein with homology to the B. pertussis FhaC outer membrane protein that is involved in secretion of FHA across the outer membrane. The H. ducreyi lspB gene encodes a protein with a predicted molecular mass of 66,573 Da. Reverse transcription-PCR analysis suggested that the lspB gene was transcribed together with the lspA2 gene on a single mRNA transcript. Polyclonal H. ducreyi LspB antiserum reacted with a 64-kDa antigen present in the Sarkosyl-insoluble cell envelope fraction of H. ducreyi 35000, which indicated that the LspB protein is likely an outer membrane protein. Concentrated culture supernatant fluids from H. ducreyi lspB and lspA1 lspB mutants did not contain detectable LspA1 and detectable LspA2, respectively. However, complementation of the lspB mutant with the wild-type lspB gene on a plasmid restored LspB protein expression and resulted in release of detectable amounts of the LspA1 protein into culture supernatant fluid. When evaluated in the temperature-dependent rabbit model of infection, the lspB mutant was attenuated in the ability to cause lesions and was never recovered in a viable form from lesions. These results indicated that the H. ducreyi LspB protein is involved in secretion of the LspA1 and LspA2 proteins across the outer membrane.

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.

Development of a rapid immunodiagnostic test for Haemophilus ducreyi

Haemophilus ducreyi is the etiologic agent of chancroid, a sexually transmitted disease that increases the rate of transmission of human immunodeficiency virus. Chancroid ulcerations are difficult to distinguish from those produced by syphilis and herpes. Diagnosis based solely on clinical grounds is inaccurate, and culture is insensitive. Highly sensitive PCR has largely superseded culture as the preferred method of laboratory diagnosis; however, neither culture nor PCR is feasible where chancroid is endemic. We developed a rapid (15-min) diagnostic test based on monoclonal antibodies (MAbs) to the hemoglobin receptor of H. ducreyi, HgbA. This outer membrane protein is conserved in all strains of H. ducreyi tested and is required for the establishment of experimental human infection. MAbs to HgbA were generated and tested for cross-reactivity against a panel of geographically diverse strains. Three MAbs were found to be unique and noncompetitive and bound to all strains of H. ducreyi tested. Using an immunochromatography format, we evaluated the sensitivity and specificity of the test using geographically diverse strains of H. ducreyi, other Haemophilus strains, and other bacteria known to superinfect genital ulcers. All H. ducreyi strains were positive, and all other bacteria were negative, resulting in a specificity of 100%. The minimum number of CFU of H. ducreyi detected was 2 x 10(6) CFU, and the minimum amount of purified HgbA protein detected was 8.5 ng. Although this level of sensitivity may not be sufficient to detect H. ducreyi in all clinical specimens, further work to increase the sensitivity could potentially make this a valuable bedside tool in areas where chancroid is endemic.

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.

Cloning, overexpression, purification, and immunobiology of an 85-kilodalton outer membrane protein from Haemophilus ducreyi

We have identified an 85-kDa outer membrane protein that is expressed by all tested strains of Haemophilus ducreyi. Studies of related proteins from other pathogenic bacteria, including Haemophilus influenzae, Pasteurella multocida, Neisseria gonorrhoeae, Neisseria meningitidis, and Shigella dysenteriae, suggested a role for these proteins in pathogenesis and immunity. In keeping with the first such described protein from Haemophilus influenzae type B, we termed the H. ducreyi protein D15. The gene encoding the H. ducreyi D15 protein was cloned and sequenced, and the deduced amino acid sequence was found to be most similar to sequences of the D15-related proteins from other Pasteurella spp. The arrangement of the flanking genes was similar to that of H. influenzae Rd and suggested that D15 was part of a multigene operon. Attempts to make a null mutation of the D15 gene were unsuccessful, paralleling results in other D15 gene studies. Overexpression of H. ducreyi D15 in Escherichia coli resulted in a source of recombinant D15 (rD15) from which it was readily purified. rD15 was immunogenic, and it was found that immunization of rabbits with an rD15 vaccine preparation conferred partial protection against a virulent challenge infection. Antisera to an N-terminal peptide recognized all tested strains of H. ducreyi.

Serum resistance in Haemophilus ducreyi requires outer membrane protein DsrA

Haemophilus ducreyi is resistant to killing by normal serum antibody and complement. We discovered an H. ducreyi outer membrane protein required for expression of serum resistance and termed it DsrA (for "ducreyi serum resistance A"). The dsrA locus was cloned, sequenced, and mutagenized. An isogenic mutant (FX517) of parent strain 35000 was constructed and characterized, and it was found to no longer express dsrA. FX517 was at least 10-fold more serum susceptible than 35000. DsrA was expressed by all strains of H. ducreyi tested, except three naturally occurring, avirulent, serum-sensitive strains. FX517 and the three naturally occurring dsrA-nonexpressing strains were complemented in trans with a plasmid expressing dsrA. All four strains were converted to a serum-resistant phenotype, including two that contained truncated lipooligosaccharide (LOS). Therefore, serum resistance in H. ducreyi does not require expression of full-length LOS but does require expression of dsrA. The dsrA locus from eight additional H. ducreyi strains was sequenced, and the deduced amino acid sequences were more than 85% identical. The major difference between the DsrA proteins was due to the presence of one, two, or three copies of the heptameric amino acid repeat NTHNINK. These repeats account for the variability in apparent molecular mass of the monomeric form of DsrA (28 to 35 kDa) observed in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Since DsrA is present in virulent strains, is highly conserved, and is required for serum resistance, we speculate that it may be a virulence factor and a potential vaccine candidate.

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

The znuA gene of Haemophilus ducreyi encodes a 32-kDa (mature) protein that has homology to both the ZnuA protein of Escherichia coli and the Pzp1 protein of H. influenzae; both of these latter proteins are members of a growing family of prokaryotic zinc transporters. Inactivation of the H. ducreyi 35000 znuA gene by insertional mutagenesis resulted in a mutant that grew more slowly than the wild-type parent strain in vitro unless ZnCl(2) was provided at a final concentration of 100 microM. Other cations tested did not restore growth of this H. ducreyi mutant to wild-type levels. The H. ducreyi ZnuA protein was localized to the periplasm, where it is believed to function as the binding component of a zinc transport system. Complementation of the znuA mutation with the wild-type H. ducreyi znuA gene provided in trans restored the ability of this H. ducreyi mutant to grow normally in the absence of exogenously added ZnCl2. The wild-type H. ducreyi znuA gene was also able to complement a H. influenzae pzp1 mutation. The H. ducreyi znuA isogenic mutant exhibited significantly decreased virulence (P = 0.0001) when tested in the temperature-dependent rabbit model for experimental chancroid. This decreased virulence was not observed when the znuA mutant was complemented with the wild-type H. ducreyi znuA gene provided in trans.

Target cell range of Haemophilus ducreyi hemolysin and its involvement in invasion of human epithelial cells

Haemophilus ducreyi, the causative agent of chancroid, produces a hemolysin, whose role in virulence is not well defined. To assess the possible role of hemolysin in pathogenesis, we evaluated its target cell range by using wild-type H. ducreyi 35000, nonhemolytic mutants with the hemolysin structural gene deleted, and isogenic strains expressing different amounts of hemolytic activity. The cytotoxicity of the various cell types was assessed by quantitating the release of lactate dehydrogenase into culture supernatants as a measure of cell lysis. In these experiments, human foreskin fibroblasts, human foreskin epithelial cells, and, to a lesser extent, HEp-2 cells were lysed by H. ducreyi hemolysin. Hemolysin also lysed human blood mononuclear cells and immune system cell lines including U937 macrophage-like cells, T lymphocytes, and B lymphocytes. In contrast, human polymorphonuclear leukocytes were not sensitive to hemolysin under the conditions tested. We also analyzed the effect of hemolysin on invasion of human epithelial cells and found that H. ducreyi strains expressing cloned hemolysin genes showed a 10-fold increase in invasion compared to the control strain. These data support the hypothesis that the H. ducreyi hemolysin is important in the pathogenesis of chancroid and may contribute to ulcer formation, invasion of epithelial cells, and evasion of the immune response.

Characterization of a Haemophilus ducreyi mutant deficient in expression of cytolethal distending toxin

Haemophilus ducreyi expresses a soluble cytolethal distending toxin (CDT) that kills HeLa, HEp-2, and other human epithelial cells in vitro. H. ducreyi CDT activity is encoded by a three-gene cluster (cdtABC), and antibody to the cdtC gene product can neutralize CDT activity in vitro (L. D. Cope, S. R. Lumbley, J. L. Latimer, J. Klesney-Tait, M. K. Stevens, L. S. Johnson, M. Purven, R. S. Munson, Jr., T. Lagergard, J. D. Radolf, and E. J. Hansen, Proc. Natl. Acad. Sci. USA 94:4056-4061, 1997). Culture supernatant fluid from a recombinant Escherichia coli strain containing the H. ducreyi cdtABC gene cluster readily killed both HeLa cells and HaCaT keratinocytes and had a modest inhibitory effect on the growth of human foreskin fibroblasts. Insertional inactivation of the cdtC gene in this recombinant E. coli strain eliminated the ability of this strain to kill HeLa cells and HaCaT keratinocytes. This mutated H. ducreyi cdtABC gene cluster was used to construct an isogenic H. ducreyi cdtC mutant. Monoclonal antibodies against the H. ducreyi CdtA, CdtB, and CdtC proteins were used to characterize protein expression by this cdtC mutant. Culture supernatant fluid from this H. ducreyi cdtC mutant did not detectably affect any of the human cells used in this study. The presence of the wild-type H. ducreyi cdtC gene in trans in this H. ducreyi mutant restored its ability to express a CDT that killed both HeLa cells and HaCaT keratinocytes. The isogenic H. ducreyi cdtC mutant was shown to be as virulent as its wild-type parent strain in the temperature-dependent rabbit model for experimental chancroid. Lack of expression of the H. ducreyi CdtC protein also did not affect the ability of this H. ducreyi mutant to survive in the skin of rabbits.

Prevalence of, antibody response to, and immunity induced by Haemophilus ducreyi hemolysin

Haemophilus ducreyi, the etiologic agent of chancroid, a genital ulcer disease, produces a cell-associated hemolysin whose role in virulence is not well defined. Hemolysin is encoded by two genes, hhdA and hhdB, which, based on their homology to Serratia marcescens shlA and shlB genes, are believed to encode the hemolysin structural protein and a protein required for secretion and modification of this protein, respectively. In this study, we determined the prevalence and expression of the hemolysin genes in 90 H. ducreyi isolates obtained from diverse geographic locations from 1952 to 1996 and found that all strains contained DNA homologous to the hhdB and hhdA genes. In addition, all strains expressed a hemolytic activity. We also determined that hemolysin is expressed in vivo and is immunogenic, as indicated by the induction of antibodies to hemolysin in both the primate and rabbit disease models as well as in human patients with naturally acquired chancroid. Wild-type strain 35000 and isogenic hemolysin-negative mutants showed no difference in lesion development in the temperature-dependent rabbit model. However, immunization of rabbits with the purified hemolysin protein reduced the recovery of wild-type H. ducreyi, but not hemolysin-negative mutants, from lesions. Our study indicates that hemolysin is a possible candidate for vaccine development due to its immunogenicity, expression in vitro and in vivo by most, if not all, strains, and the effect of immunization on reducing the recovery of viable H. ducreyi in experimental disease in rabbits.

Characterization of a WaaF (RfaF) homolog expressed by Haemophilus ducreyi

Haemophilus ducreyi lipooligosaccharide (LOS) is capable of inducing an inflammatory response in skin (A. A. Campagnari, L. M. Wild, G. Griffiths, R. J. Karalus, M. A. Wirth, and S. M. Spinola, Infect. Immun. 59:2601-2608, 1991) and likely contributes to the virulence of this sexually transmitted pathogen (B. A. Bauer, M. K. Stevens, and E. J. Hansen, Infect. Immun. 68:4290-4298, 1998). An open reading frame in H. ducreyi 35000 was found to encode a predicted protein that was 59% identical to the protein product of the rfaF (waaF) gene of Salmonella typhimurium. The H. ducreyi waaF gene was able to complement an S. typhimurium rfaF (waaF) mutant, a result which confirmed the identity of this gene. In contrast to the rfaF (waaF) gene of enteric bacteria, the H. ducreyi waaF gene was not located adjacent to other genes involved in lipopolysaccharide expression. Inactivation of the H. ducreyi waaF gene by insertion mutagenesis resulted in expression of a LOS that migrated much faster than wild-type LOS in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The LOS of this mutant also did not bind a monoclonal antibody directed against a cell surface-exposed epitope of wild-type H. ducreyi LOS. Testing of the wild-type H. ducreyi strain and its isogenic waaF mutant in the temperature-dependent rabbit model for dermal lesion production by H. ducreyi revealed that this waaF mutant was less virulent than the wild-type parent strain. Complementation of the H. ducreyi waaF mutant with the wild-type H. ducreyi waaF gene resulted in expression of both wild-type LOS and wild-type virulence by this mutant.

The aadA gene of plasmid R100 confers resistance to spectinomycin and streptomycin in Myxococcus xanthus

Plasmids with the aadA gene from plasmid R100, which confers resistance to the aminoglycosides spectinomycin and streptomycin in Escherchia coli, can be introduced into wild-type Myxococcus xanthus, strain DK1622, by electroporation. Recombinant M. xanthus strains with integrated plasmids carrying the aadA gene acquire resistance to high levels of these antibiotics. Selection for aadA in M. xanthus can be carried out independently of, or simultaneously with, selection for resistance to kanamycin. The kinds and frequencies of recombination events observed between integrative plasmids with aadA and the M. xanthus chromosome are similar to those observed after the transformation of yeast. Cleavage of integrative plasmid DNA at a site adjacent to a region of homology between the plasmid and the M. xanthus genome favors the targeted disruption of M. xanthus genes by allele replacement.

Involvement of the Haemophilus ducreyi gmhA gene product in lipooligosaccharide expression and virulence

The lipooligosaccharide (LOS) present in the outer membrane of Haemophilus ducreyi is likely a virulence factor for this sexually transmitted pathogen. An open reading frame in H. ducreyi 35000 was found to encode a predicted protein that had 87% identity with the protein product of the gmhA (isn) gene of Haemophilus influenzae. In H. influenzae type b, inactivation of the gmhA gene caused the synthesis of a significantly truncated LOS which possessed only lipid A and a single 2-keto-3-deoxyoctulosonic acid molecule (A. Preston, D. J. Maskell, A. Johnson, and E. R. Moxon, J. Bacteriol. 178:396-402, 1996). The H. ducreyi gmhA gene was able to complement a gmhA-deficient Escherichia coli strain, a result which confirmed the identity of this gene. When the gmhA gene of H. ducreyi was inactivated by insertion of a cat cartridge, the resultant H. ducreyi gmhA mutant, 35000.252, expressed a LOS that migrated much faster than wild-type LOS in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. When the wild-type H. ducreyi strain and its isogenic gmhA mutant were used in the temperature-dependent rabbit model for dermal lesion production by H. ducreyi, the gmhA mutant was found to be substantially less virulent than the wild-type parent strain. The H. ducreyi gmhA gene was amplified by PCR from the H. ducreyi chromosome and cloned into the pLS88 vector. When the H. ducreyi gmhA gene was present in trans in gmhA mutant 35000.252, expression of the gmhA gene product restored the virulence of this mutant to wild-type levels. These results indicate that the gmhA gene product of H. ducreyi is essential for the expression of wild-type LOS by this pathogen.

Cloning and characterization of tdhA, a locus encoding a TonB-dependent heme receptor from Haemophilus ducreyi

Haemophilus ducreyi is unable to synthesize heme and must acquire it from its only known host, humans. We cloned and sequenced a gene encoding an outer membrane receptor for heme. It was designated tdhA (for TonB-dependent heme receptor A) since it was related by sequence homology to the family of TonB-dependent receptors. TdhA was strikingly similar to open reading frame HI0113 from the genome of Haemophilus influenzae Rd and also shared homology with five other heme receptors, including HxuC, HemR, HmuR, ChuA, and ShuA, from gram-negative bacteria. An Escherichia coli hemA tonB mutant strongly expressing H. ducreyi tdhA grew on low levels of heme as a source of heme only when an intact H. ducreyi Ton system plasmid was present, formally demonstrating functional TonB dependence. tdhA was expressed poorly in vitro by H. ducreyi and only under conditions of heme limitation. A survey of H. ducreyi revealed that all tested strains but one synthesized small amounts of TdhA in vitro under heme-limiting conditions. Surprisingly, an isogenic mutant of tdhA as well as its parent, 35000, both required the same high levels of heme for growth (50 microgram/ml [77 microM] on agar medium). This result, together with previous findings, suggests that in vitro, the uptake of heme by H. ducreyi is mediated by a TonB- and TdhA-independent mechanism, possibly diffusion.

Facile construction of mutations in Haemophilus ducreyi using lacZ as a counter-selectable marker

Haemophilus ducreyi is a Gram-negative bacterium which is the causative agent of chancroid, an ulcerative sexually transmitted disease. In order to understand the pathogenesis of H. ducreyi disease, studies designed to identify potential virulence determinants and construct mutants deficient in the elaboration of these determinants have been undertaken in several laboratories. At the present time, construction of isogenic mutants is accomplished by electroporation of linearized DNA containing insertionally inactivated H. ducreyi genes followed by selection for the resistance marker encoded on the inactivated gene. In our experience, certain mutants are difficult to construct using this procedure. In the construction of strains containing lacZ as a reporter gene, we observed that the growth of lacZ expressing H. ducreyi was inhibited in the presence of X-gal. We have exploited this observation to develop a new strategy for the construction of isogenic H. ducreyi mutants.

Periplasmic copper-zinc superoxide dismutase protects Haemophilus ducreyi from exogenous superoxide

Haemophilus ducreyi causes chancroid, a sexually transmitted genital ulcer disease implicated in increased heterosexual transmission of HIV. As part of an effort to identify H. ducreyi gene products involved in virulence and pathogenesis, we created random TnphoA insertion mutations in an H. ducreyi 35000 library cloned in Escherichia coli. Inserts encoding exported or secreted PhoA fusion proteins were characterized by DNA sequencing. One such clone encoded a Cu-Zn superoxide dismutase (SOD) enzyme. The Cu-Zn SOD was periplasmic in H. ducreyi and accounted for most of the detectable SOD activity in whole-cell lysates of H. ducreyi grown in vitro. To investigate the function of the Cu-Zn SOD, we created a Cu-Zn SOD-deficient H. ducreyi strain by inserting a cat cassette into the sodC gene. The wild-type and Cu-Zn SOD null mutant strains were equally resistant to excess cytoplasmic superoxide induced by paraquat, demonstrating that the Cu-Zn SOD did not function in the detoxification of cytoplasmic superoxide. However, the Cu-Zn SOD null strain was significantly more susceptible to killing by extracellular superoxide than the wild type. This result suggests that the H. ducreyi Cu-Zn SOD may play a role in bacterial defence against oxidative killing by host immune cells during infection.

Role of the Haemophilus ducreyi Ton system in internalization of heme from hemoglobin

By cloning into Escherichia coli and construction of isogenic mutants of Haemophilus ducreyi, we showed that the hemoglobin receptor (HgbA) is TonB dependent. An E. coli hemA tonB mutant expressing H. ducreyi hgbA grew on low levels of hemoglobin as a source of heme only when an intact H. ducreyi Ton system plasmid was present. In contrast, growth on heme by the E. coli hemA tonB mutant expressing hgbA was observed only at high concentrations of heme, was TonB independent, and demonstrated that H. ducreyi HgbA was not sufficient to function as a typical TonB-dependent heme receptor in E. coli. Allelic replacement of the wild-type H. ducreyi exbB, exbD, and tonB loci with the exbB, exbD, and tonB deletion resulted in an H. ducreyi isogenic mutant unable to utilize hemoglobin but able to utilize hemin at the same levels as the parent strain to fulfill its heme requirement. This finding confirms the TonB dependence of HgbA-mediated hemoglobin utilization and suggests that uptake of hemin in H. ducreyi is TonB independent. Additionally, the H. ducreyi Ton system mutant synthesized increased amounts of HgbA and other heme-regulated outer membrane proteins, consistent with derepression of these proteins due to lower intracellular heme and/or iron concentrations in the mutant. Sequencing of the Ton system genes revealed that the arrangement of the genes was exbB exbD tonB. The proximity and structure of these genes suggested that they are transcribed as an operon. This arrangement, as well as the DNA and deduced amino acid sequences of these H. ducreyi genes, was most similar to those from other pasteurellae.

A protective epitope of Moraxella catarrhalis is encoded by two different genes

The high-molecular-weight UspA protein of Moraxella catarrhalis has been described as being both present on the surface of all M. catarrhalis disease isolates examined to date and a target for a monoclonal antibody (MAb 17C7) which enhanced pulmonary clearance of this organism in a mouse model system (M. E. Helminen et al., J. Infect. Dis. 170:867-872, 1994). A recombinant bacteriophage that formed plaques which bound MAb 17C7 was shown to contain a M. catarrhalis gene, designated uspA1, that encoded a protein with a calculated molecular weight of 88,271. Characterization of an isogenic uspA1 mutant revealed that elimination of expression of UspA1 did not eliminate the reactivity of M. catarrhalis with MAb 17C7. In addition, N-terminal amino acid analysis of internal peptides derived from native UspA protein and Southern blot analysis of M. catarrhalis chromosomal DNA suggested the existence of a second UspA-like protein. A combination of epitope mapping and ligation-based PCR methods identified a second M. catarrhalis gene, designated uspA2, which also encoded the MAb 17C7-reactive epitope. The UspA2 protein had a calculated molecular weight of 62,483. Both the isogenic uspA1 mutant and an isogenic uspA2 mutant possessed the ability to express a very-high-molecular-weight antigen that bound MAb 17C7. Southern blot analysis indicated that disease isolates of M. catarrhalis likely possess both uspA1 and uspA2 genes. Both UspA1 and UspA2 most closely resembled adhesins produced by other bacterial pathogens.

Alterations in levels of DnaK and GroEL result in diminished survival and adherence of stressed Haemophilus ducreyi

Haemophilus ducreyi is a hemin-requiring bacterium causing the genital ulcer disease chancroid. Previously we demonstrated that the heat shock protein GroEL was immunogenic and possibly highly expressed in a mammalian host. The present study was initiated to (i) determine the relative amounts of GroEL expressed by H. ducreyi during in vitro exposure to stresses and (ii) evaluate whether a high level of GroEL is directly or indirectly required for survival and adherence of stressed H. ducreyi. Using scanning densitometry of sodium dodecyl sulfate-polyacrylamide gel electrophoresis protein profiles, we found that H. ducreyi expressed high basal levels of GroEL, averaging fivefold greater than in Escherichia coli. These high GroEL levels increased up to twofold upon exposure of the organism to heat shock or high levels of hydrogen peroxide and during adherence to two human genital cell lines. Furthermore, when the gene for DnaK was present on a multicopy plasmid in H. ducreyi, a 1.8-fold increase in DnaK and a 2.3-fold reduction in GroEL were seen. These results suggest that DnaK serves as a negative modulator of H. ducreyi GroEL. Subsequently we found that H. ducreyi with lower GroEL had diminished ability to survive when challenged by heat and oxidative stresses. In addition, the long, parallel chains characteristic of virulent strains of H. ducreyi were absent when GroEL was lowered, so that fewer bacterial cells adhered to the human cells. These results suggest that the unusually high basal levels of GroEL are involved, either directly or indirectly, in the survival, chaining, and adherence of H. ducreyi in the presence of the combined stresses of the host environment.

Distribution, cloning, characterisation and mutagenesis of sodC, the gene encoding copper/zinc superoxide dismutase, a potential determinant of virulence, in Haemophilus ducreyi

The sodC gene encoding the periplasmic enzyme copper/zinc superoxide dismutase (CuZnSOD) has been cloned from Haemophilus ducreyi, the causative agent of the genital ulcer disease, chancroid. Examination of a collection of diverse strains indicates that it is present throughout the species. Cloned sodC has been expressed in E. coli and shown to encode active enzyme. Insertional mutagenesis was used to construct a non-functional version of the gene. This has been transferred into the chromosome of the parent H. ducreyi strain by electroporation and homologous recombination, in preparation for studies of the role of this enzyme in the interactive biology of the organism with its host, perhaps in protecting bacteria from superoxide radicals and their reactive progeny generated by neutrophils in the context of host defence.

The major outer membrane protein of Haemophilus ducreyi consists of two OmpA homologs

The major outer membrane protein (MOMP) of Haemophilus ducreyi is an OmpA homolog that migrates on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels as three species with apparent molecular weights ranging from 37,000 to 43,000. Monoclonal antibodies directed against this macromolecule were used to identify recombinant clones containing fragments of the gene encoding this protein. Nucleotide sequence analysis of these fragments confirmed that the MOMP encoded by the intact gene (momp) was a member of the OmpA family of outer membrane proteins. Construction of an isogenic H. ducreyi mutant unable to express the MOMP led to the discovery of a second outer membrane protein which migrated at the same rate on SDS-PAGE gels as the MOMP. N-terminal amino acid sequence analysis of this second protein revealed that its N terminus was nearly identical to that of the MOMP and also had homology with members of the OmpA family. Nucleotide sequence analysis of the region downstream from the momp gene revealed the presence of a partial open reading frame encoding a predicted OmpA-like protein. A modification of anchored PCR technology was used to obtain the nucleotide sequence of this downstream gene which was shown to encode a second OmpA homolog (OmpA2). The N-terminal amino acid sequence of OmpA2 was identical to that of the OmpA-like protein detected in the momp mutant. The H. ducreyi MOMP and OmpA2 proteins, which comigrated on SDS-PAGE gels and which were encoded by the tandem arranged momp and ompA2 genes, were 72% identical.

Identification of tandem genes involved in lipooligosaccharide expression by Haemophilus ducreyi

A transposon insertion mutant of Haemophilus ducreyi 35000 possessing a truncated lipooligosaccharide (LOS) failed to bind the LOS-specific monoclonal antibody 3E6 (M. K. Stevens, L. D. Cope, J. D. Radolf, and E. J. Hansen, Infect. Immun. 63:2976-2982, 1995). This transposon was found to have inserted into the first of two tandem genes and also caused a deletion of chromosomal DNA upstream of this gene. These two genes, designated lbgA and lbgB, encoded predicted proteins with molecular masses of 25,788 and 40,236 Da which showed homology with proteins which function in lipopolysaccharide biosynthetic in other gram-negative bacteria. The tandem arrangement of the lbgA and lbgB genes was found to be conserved among H. ducreyi strains. Isogenic LOS mutants, constructed by the insertion of a cat cartridge into either the lbgA or the lbgB gene, expressed an LOS phenotype indistinguishable from that of the original transposon-derived LOS mutant. The wild-type LOS phenotype could be restored by complementation with the appropriate wild-type allele. These two LOS mutants proved to be as virulent as the wild-type parent strain in an animal model. A double mutant with a deletion of the lbgA and lbgB genes yielded equivocal results when its virulence was tested in an animal model.

Lack of expression of the global regulator OxyR in Haemophilus influenzae has a profound effect on growth phenotype

A pBR322-based library of chromosomal DNA from the nontypeable Haemophilus influenzae TN106 was screened for the expression of transferrin-binding activity in Escherichia coli. A recombinant clone expressing transferrin-binding activity contained a 3.7-kb fragment of nontypeable H. influenzae DNA. Nucleotide sequence analysis of this insert revealed the presence of two complete open reading frames encoding proteins of approximately 26 and 34 kDa. Mini-Tn10kan transposon mutagenesis at different sites within the open reading frame encoding the 34-kDa protein resulted in the abolition of transferrin-binding activity in the recombinant E. coli clone. The deduced amino acid sequence of the 34-kDa protein had 70% identity with the OxyR protein of E. coli; this latter macromolecule is a member of the LysR family of transcriptional activators. When a mutated H. influenzae oxyR gene was introduced into the chromosome of the wild-type H. influenzae strain by allelic exchange, the resulting oxyR mutant still exhibited wild-type levels of transferrin-binding activity but was unable to grow on media containing the heme precursor protoporphyrin IX (PPIX) in place of heme. This mutant also exhibited reduced growth around disks impregnated with heme sources. Supplementation of the PPIX-based growth media with catalase or sodium pyruvate resulted in normal growth of the H. influenzae oxyR mutant. Provision of the wild-type H. influenzae oxyR gene in trans also permitted the growth of this mutant on a PPIX-based medium. Exogenously supplied catalase restored the growth of this mutant with heme sources to nearly wild-type levels. These results indicate that expression of a wild-type OxyR protein by H. influenzae is essential to allow this organism to protect itself against oxidative stresses in vitro.

The conserved 18,000-molecular-weight outer membrane protein of Haemophilus ducreyi has homology to PAL

Haemophilus ducreyi expresses an 18,000-molecular-weight outer membrane protein that contains a conserved surface-exposed epitope recognized by monoclonal antibody 3B9. Monoclonal antibody 3B9 cross-reacts with proteins of similar molecular weight found in many Haemophilus sp. strains, including P6, a candidate vaccine for Haemophilus influenzae. The gene encoding the 18,000-molecular-weight outer membrane protein was identified by screening a lambdagt11 genomic library with 3B9. The coding sequence of the gene was localized to a 471-bp open reading frame, designated pal (peptidoglycan-associated lipoprotein). Translation of pal predicted a mature polypeptide with a molecular weight of 15,000 that had extensive homology with P6 and Escherichia coli PAL. The predicted signal peptide had features characteristic of a prokaryotic lipoprotein, and processing of PAL was sensitive to globomycin in H. ducreyi. The sequences encoding mature H. ducreyi PAL were subcloned into the vector pRSET B and expressed as a polyhistidine-containing fusion protein that bound 3B9. In Western blot (immunoblot) analysis, serum samples obtained from healthy subjects and patients with chancroid or other genital ulcer diseases contained antibodies to purified PAL. Antibodies that bound to PAL were removed by absorption with a lysate of Haemophilus sp. antigens, suggesting that patients with chancroid do not develop an H. ducreyi-specific antibody response to PAL.

A hemoglobin-binding outer membrane protein is involved in virulence expression by Haemophilus ducreyi in an animal model

Haemophilus ducreyi exhibits a requirement for exogenously supplied heme for aerobic growth in vitro. Nine of ten wild-type isolates of H. ducreyi were shown to contain a readily detectable hemoglobin-binding activity. Spontaneous hemoglobin-binding-negative mutants of two of these wild-type isolates lost the ability to express an outer membrane protein with an apparent molecular mass of approximately 100 kDa. Similarly, the single wild-type isolate that lacked the ability to bind hemoglobin also appeared to lack expression of this same 100-kDa protein. A monoclonal antibody (5A9) to this 100-kDa protein was used to identify a recombinant clone which possessed an H. ducreyi chromosomal fragment containing the gene encoding the 100-kDa protein; this protein was designated hemoglobin utilization protein A (HupA). Nucleotide sequence analysis of the hupA gene revealed that the predicted protein, with a calculated molecular mass of 108 kDa, was similar to TonB-dependent outer membrane proteins of other bacteria. Increasing the concentration of heme in the growth medium resulted in decreased expression of the HupA protein. Mutant analysis was used to prove that the HupA protein was essential for the utilization by H. ducreyi of both hemoglobin and hemoglobin-haptoglobin as sources of heme in vitro. In addition, it was found that an isogenic hupA mutant was less virulent than the wild-type parent strain in the temperature-dependent rabbit model for dermal lesion production by H. ducreyi.

Fine tangled pili expressed by Haemophilus ducreyi are a novel class of pili

Haemophilus ducreyi synthesizes fine, tangled pili composed predominantly of a protein whose apparent molecular weight is 24,000 (24K). A hybridoma, 2D8, produced a monoclonal antibody (MAb) that bound to a 24K protein in H. ducreyi strains isolated from diverse geographic locations. A lambda gt11 H. ducreyi library was screened with MAb 2D8. A 3.5-kb chromosomal insert from one reactive plaque was amplified and ligated into the pCRII vector. The recombinant plasmid, designated pHD24, expressed a 24K protein in Escherichia coli INV alpha F that bound MAb 2D8. The coding sequence of the 24K gene was localized by exonuclease III digestion. The insert contained a 570-bp open reading frame, designated ftpA (fine, tangled pili). Translation of ftpA predicted a polypeptide with a molecular weight of 21.1K. The predicted N-terminal amino acid sequence of the polypeptide encoded by ftpA was identical to the N-terminal amino acid sequence of purified pilin and lacked a cleavable signal sequence. Primer extension analysis of ftpA confirmed the lack of a leader peptide. The predicted amino acid sequence lacked homology to known pilin sequences but shared homology with the sequences of E. coli Dps and Treponema pallidum antigen TpF1 or 4D, proteins which associate to form ordered rings. An isogenic pilin mutant, H. ducreyi 35000ftpA::mTn3(Cm), was constructed by shuttle mutagenesis and did not contain pili when examined by electron microscopy. We conclude that H. ducreyi synthesizes fine, tangled pili that are composed of a unique major subunit, which may be exported by a signal sequence independent mechanism.

Characterization of the hemolytic activity of Haemophilus ducreyi

H. ducreyi is the causative agent of chancroid, a genital ulcer disease most prevalent in developing countries. Chancroid enhances the heterosexual transmission of human immunodeficiency virus and is identified in focal outbreaks in the United States, but little is known about its pathogenesis. We studied the hemolysin produced by H. ducreyi because this molecule might be an important virulence factor in the pathogenesis of chancroid. Ten strains of H. ducreyi were tested on newly devised blood agar plates and were found to have hemolytic activity. We examined the hemolytic activity of H. ducreyi 35000 further and found that it was heat labile, cell associated, greatest at pH 7.0, and produced in logarithmic- but not stationary-phase cultures. Using transposons Tn916 and Tn1545-delta 3, we have isolated three classes of transposon mutants of strain 35000: those with no detectable hemolytic activity, those with reduced hemolytic activity, and those with enhanced hemolytic activity. Transposon insertions in the nonhemolytic mutants were located in a DNA sequence which hybridized to the Proteus mirabilis hemolysin gene. Analysis of clones containing overlapping sections of this region served to further localize the H. ducreyi hemolysin gene and allow its expression in Escherichia coli and complementation of the nonhemolytic defect in an H. ducreyi mutant. These experiments indicate that H. ducreyi 35000 produces a hemolysin that is related to the calcium-independent hemolysin produced by P. mirabilis. Further experiments are needed to define the similarity of the H. ducreyi hemolysin to other calcium-independent hemolysins and to determine its role in the pathogenesis of chancroid.

A system for generalized mutagenesis of Haemophilus ducreyi

The lack of a generalized mutagenesis system for Haemophilus ducreyi has hampered efforts to identify virulence factors expressed by this sexually transmitted pathogen. To address this issue, the transposable element Tn1545-delta 3, which encodes resistance to kanamycin, was evaluated for its ability to insert randomly into the H. ducreyi chromosome and produce stable, isogenic mutants. Electroporation of H. ducreyi with 1 microgram of plasmid pMS1 carrying Tn1545-delta 3 resulted in the production of 10(4) kanamycin-resistant transformants; Southern blot analysis of a number of these transformants indicated that insertion of the transposon into the chromosome occurred at a number of different sites. This pMS1-based transposon delivery system was used to produce an H. ducreyi mutant that expressed an altered lipooligosaccharide (LOS). Passage of this mutant in vitro in the presence or absence of kanamycin did not affect the stability of the transposon insertion. To confirm that the observed mutant phenotype was the result of the transposon insertion, a chromosomal fragment containing Tn1545-delta 3 was cloned from this H. ducreyi LOS mutant. Electroporation of the wild-type H. ducreyi strain with this DNA fragment yielded numerous kanamycin-resistant transformants, the majority of which had the same altered LOS phenotype as the original mutant. Southern blot analysis confirmed the occurrence of proper allelic exchange in the LOS-deficient transformants obtained in this backcross experiment. The ability of Tn1545-delta 3 to produce insertion mutations in H. ducreyi should facilitate genetic analysis of this pathogen.

Directed genomic integration in Actinobacillus actinomycetemcomitans: generation of defined leukotoxin-negative mutants

To develop targeted gene integration in the periodontal pathogen Actinobacillus actinomycetemcomitans, a ColE1-based, spectinomycin-resistant plasmid containing a segment of the leukotoxin gene was electroporated into strain JP2. In all of the stable spectinomycin-resistant transformants that arose, the plasmid had recombined into the genomic leukotoxin locus since ColE1-based vectors cannot replicate extrachromosomally in A. actinomycetemcomitans. Directed genomic integration was then used to construct a leukotoxin-negative strain by transforming the leukotoxin-producing strain JP2 with a ColE1-based plasmid containing an internal fragment of the leukotoxin gene. Cytotoxicity assays proved that these transformants had < 0.1% of the leukotoxin activity of the parental strain. These results demonstrate that integration-based approaches can be used for generating isogenic mutants in specific virulence genes in A. actinomycetemcomitans.

Chancroid and Haemophilus ducreyi: an update

Haemophilus ducreyi is a fastidious gram-negative bacillus that causes the sexually transmitted infection chancroid. Chancroid is a major genital ulcerative disease in Africa, Southeast Asia, the Caribbean, and Latin America and is of increasing concern in the United States. Genital ulcerative disease and chancroid in particular have been associated with facilitating the transmission of human immunodeficiency virus. The diagnosis of chancroid based on the clinical appearance of the genital lesion or on the isolation of H. ducreyi on selective medium is relatively insensitive. However, recent advances in nonculture diagnostic tests have enhanced our ability to diagnose chancroid. There has been renewed interest in understanding the pathogenesis of H. ducreyi. In vitro and in vivo models have been developed to help identify important virulence determinants. Through the use of biochemical and molecular techniques, macromolecular components that may be important in virulence have been identified.

rpsL+: a dominant selectable marker for gene replacement in mycobacteria

Molecular genetic manipulations in mycobacteria would benefit from procedures which efficiently select for double-crossover events by homologous recombination. Here we describe a vector-host system for gene replacement in mycobacteria, the utility of which was investigated using functional inactivation of the pyrF gene in Mycobacterium smegmatis as a model. This system is based on the expression of the wild-type rpsL gene coding for ribosomal protein S12 in a streptomycin-resistant host. Owing to the absence of a mycobacterial origin the plasmids are unable to replicate autonomously in mycobacteria. The first selection for maintenance of cloned sequences is conferred by the kanamycin-resistance gene. The second simultaneous selection by streptomycin is against maintenance of cloned sequences which contain the gene encoding the streptomycin-sensitive allele of the rpsL gene. By placing the gene for positive selection and that used for negative selection within and outside the target gene of interest, respectively, gene replacement is obtained. A one-step double selection procedure provides a means to distinguish strictly between gene replacement by double crossover versus homologous recombination by single crossover events. The system should have considerable potential for genera or species where single-crossover events or even illegitimate recombination are the predominant recombination mechanisms. It should also be of wide use for the construction of mutants without a selectable phenotype.

Characterization of the hgbA locus encoding a hemoglobin receptor from Haemophilus ducreyi

Haemophilus ducreyi can bind hemoglobin and use it as a source of heme, for which it has an obligate requirement. We previously identified and purified HgbA, a hemoglobin-binding outer membrane protein from H. ducreyi. In this report, we describe the molecular cloning, expression, DNA sequence, and mutagenesis of the structural gene for HgbA, hgbA. H. ducreyi and recombinant Escherichia coli expressing hgbA bound [125I]hemoglobin, establishing HgbA as a receptor. Insertions or deletions in the cloned hgbA gene abolished expression of HgbA and hemoglobin binding in E. coli. Mutagenesis of H. ducreyi by allelic exchange of insertions into hgbA abolished its ability to bind [125I]hemoglobin or utilize hemoglobin as a source of heme. The deduced protein sequence was similar to those of the TonB-dependent family of outer membrane receptors. The most similar member was HutA (heme receptor) from Vibrio cholerae. Tbp1 and Lbp1 (transferrin and lactoferrin receptors, respectively, from pathogenic Neisseria spp.) also showed very significant homology. Thus, by characterizing the hgbA locus, this work elucidates a potentially important role of HgbA in obtaining heme and/or iron from the host.

A gene cluster involved in the utilization of both free heme and heme:hemopexin by Haemophilus influenzae type b

The utilization of heme bound to the serum glycoprotein hemopexin by Haemophilus influenzae type b (Hib) strain DL42 requires the presence of the 100-kDa heme:hemopexin-binding protein encoded by the hxuA gene (M. S. Hanson, S. E. Pelzel, J. Latimer, U. Muller-Eberhard, and E. J. Hansen, Proc. Natl. Acad. Sci. USA 89:1973-1977, 1992). Nucleotide sequence analysis of a 5-kb region immediately upstream from the hxuA gene revealed the presence of two genes, designated hxuC and hxuB, which encoded outer membrane proteins. The 78-kDa HxuC protein had similarity to TonB-dependent outer membrane proteins of other organisms, whereas the 60-kDa HxuB molecule most closely resembled the ShlB protein of Serratia marcescens. A set of three isogenic Hib mutants with cat cartridges inserted individually into their hxuA, hxuB, and hxuC genes was constructed. None of these mutants could utilize heme:hemopexin. The hxuC mutant was also unable to utilize low levels of free heme, whereas both the hxuA and hxuB mutants could utilize free heme. When the wild-type hxuC gene was present in trans, the hxuC mutant regained its ability to utilize low levels of free heme but still could not utilize heme:hemopexin. The hxuA mutant could utilize heme:hemopexin when a functional hxuA gene from a nontypeable H. influenzae strain was present in trans. Complementation analysis using this cloned nontypeable H. influenzae hxuA gene also indicated that the HxuB protein likely functions in the release of soluble HxuA from the Hib cell. These studies indicate that at least two and possible three gene products are required for utilization of heme bound to hemopexin by Hib strain DL42.

Use of tissue culture and animal models to identify virulence-associated traits of Haemophilus ducreyi

To identify virulence-associated properties of Haemophilus ducreyi, 34 strains of this sexually transmitted pathogen were evaluated for in vitro phenotypic characteristics of potential relevance to chancroid pathogenesis and for their ability to produce lesions in the temperature-dependent animal model for chancroid. Of the 34 strains tested, all but three produced a cytopathic effect on human foreskin fibroblasts (HFF) and all but six strains formed large microcolonies on HFF monolayers. A subset of 12 selected strains underwent more extensive analyses and, when evaluated for both their cytadherence kinetics and growth in the presence of HFF monolayers, it was found that several of these strains had a very limited ability to attach to HFF cells. When the same 12 strains were tested in the temperature-dependent rabbit model, only the seven strains which were positive in all of these in vitro-based tests readily produced lesions. In contrast, the five strains that were noted to be deficient in one or more of the phenotypic characteristics scored in the in vitro systems did not produce lesions. This association between the traits measured in vitro and the ability to produce dermal lesions was significant (P = 0.0012). These results suggest that in vitro behavior may be used to predict the virulence potential of H. ducreyi strains. Moreover, the phenotypic characteristics described in this study are appropriate focal points for efforts to determine the molecular basis of the virulence of this pathogen.

Identification of a new locus involved in expression of Haemophilus influenzae type b lipooligosaccharide

Previous studies have shown that changes in the expression of the Haemophilus influenzae type b (Hib) lipooligosaccharide (LOS) epitope reactive with monoclonal antibody (MAb) 5G8 can be correlated with alterations in the virulence of some Hib strains. To identify the locus involved in expression of this particular LOS epitope, a genomic library was constructed in the plasmid shuttle vector pGJB103 from Hib strain DL42, which constitutively expressed LOS reactive with MAb 5G8. This library was used to transform a second Hib strain (DL180) that normally does not express this LOS epitope, and a recombinant clone was identified that bound MAb 5G8. Subcloning of different regions of the Hib DL42 DNA insert in this recombinant plasmid determined that a 1.9-kb EcoRI fragment, designated lex-2, was responsible for transforming Hib strain DL180 to reactivity with MAb 5G8. Nucleotide sequence analysis revealed the presence of two contiguous open reading frames (ORFs) in lex-2, the first of which contained 18 tandem repeats of the nucleotide tetramer GCAA near its 5' end. Sequence analysis of PCR-derived products from MAb 5G8-reactive and -nonreactive Hib DL180 colonies established that 18 GCAA repeats were associated with expression of the LOS epitope that bound MAb 5G8. Mutational analysis determined that a functional ORF 2 was essential for expression of the MAb 5G8-reactive LOS epitope. The nucleotide tetramer GCAA repeat present in ORF 1 was also detected in at least two different chromosomal regions in all Hib strains tested. The availability of the cloned lex-2 locus should facilitate future analysis of the complex regulatory mechanisms involved in expression of LOS epitopes by this pathogen.

Identification of a locus involved in the utilization of iron by Haemophilus influenzae

Haemophilus influenzae has an absolute requirement for heme for aerobic growth. This organism can satisfy this requirement by synthesizing heme from iron and protoporphyrin IX (PPIX). H. influenzae type b (Hib) strain DL42 was found to be unable to form single colonies when grown on a medium containing free iron and PPIX in place of heme. In contrast, the nontypeable H. influenzae (NTHI) strain TN106 grew readily on the same medium. A genomic library from NTHI strain TN106 was used to transform Hib strain DL42, and recombinants were selected on a medium containing iron and PPIX in place of heme. A recombinant plasmid with an 11.5-kb NTHI DNA insert was shown to confer on Hib strain DL42 the ability to grow on iron and PPIX. Nucleotide sequence analysis revealed that this NTHI DNA insert contained three genes, designated hitA, hitB, and hitC, which encoded products similar to the SfuABC proteins of Serratia marcescens, which have been shown to constitute a periplasmic binding protein-dependent iron transport system in this enteric organism. The NTHI HitA protein also was 69% identical to the ferric-binding protein of Neisseria gonorrhoeae. Inactivation of the cloned NTHI hitC gene by insertion of an antibiotic resistance cartridge eliminated the ability of the recombinant plasmid to complement the growth deficiency of Hib DL42. Construction of an isogenic NTHI TN106 mutant lacking a functional hitC gene revealed that this mutation prevented this strain from growing on a medium containing iron and PPIX in place of heme. This NTHI hitC mutant was also unable to utilize either iron bound to transferrin or iron chelates. These results suggest that the products encoded by the hitABC genes are essential for the utilization of iron by NTHI.

Molecular gene cloning and nucleotide sequencing and construction of an aroA mutant of Pasteurella haemolytica serotype A1

The aroA gene of Pasteurella haemolytica serotype A1 was cloned by complementation of the aroA mutation in Escherichia coli K-12 strain AB2829. The nucleotide sequence of a 2.2-kb fragment encoding aroA predicted an open reading frame product 434 amino acids long that shows homology to other bacterial AroA proteins. Several strategies to inactivate aroA were unsuccessful. Gene replacement was finally achieved by constructing a replacement plasmid with aroA inactivated by insertion of a P. haemolytica ampicillin resistance fragment into a unique NdeI site in aroA. A hybrid plasmid was constructed by joining the aroA replacement plasmid with a 4.2-kb P. haemolytica plasmid which encodes streptomycin resistance. Following PhaI methylation, the replacement plasmid was introduced by electroporation into P. haemolytica NADC-D60, a plasmidless strain of serotype 1A. Allelic exchange between the replacement plasmid and the chromosome of P. haemolytica gave rise to an ampicillin-resistant mutant which grew on chemically defined P. haemolytica medium supplemented with aromatic amino acids but failed to grow on the same medium lacking tryptophan. Southern blot analysis confirmed that aroA of the mutant was inactivated and that the mutant was without a plasmid.