Characterization of the hemolytic activity of Haemophilus ducreyi

Pathological analysis and antimicrobial susceptibility of Chryseobacterium balustinum RTFCP 298 isolated from diseased rainbow trout, Oncorhynchus mykiss

In this study, six isolates of Chryseobacterium balustinum were characterized from diseased rainbow trout fingerlings. The virulence characteristics, pathogenicity, and antimicrobial susceptibility pattern of these isolates were investigated. The bacterium showed positive results for catalase, cytochrome oxidase, and aesculin hydrolysis, while negative results were obtained for DNase, gelatinase, methyl red, Voges-Proskauer's reaction, Simon citrate, Hydrogen sulphide, and starch hydrolysis. Amino acid metabolism analysis revealed the inability to metabolize arginine, lysine, and ornithine decarboxylase. Molecular characterization (16S rRNA) and phylogenetic analysis revealed the test isolates as C. balustinum, closely related to strain WLT (99.85% similarity) and C. balustinum P-27 (99.77%). Virulence assay indicated haemolytic activity and biofilm formation by the test bacterium. The challenge test confirmed moderate pathogenicity in rainbow trout and established Koch's postulates. The clinical manifestations of infection included fin erosion, eye and body surface haemorrhage, exophthalmia, and organ liquefaction. The minimum inhibitory concentrations of various antimicrobials ranged from 1 to > 256 µg mL-1. The novel synthetic antimicrobial peptides exhibited MICs of 8 to > 256 µg mL-1, suggesting a potential control method. These findings suggest that C. balustinum is an opportunistic pathogen with moderate pathogenicity in rainbow trout. Further research on the host-pathogen relationship is necessary to understand virulence characteristics and pathogenicity in aquaculture.

Efficient Secretion of Murine IL-2 From an Attenuated Strain of Clostridium sporogenes, a Novel Delivery Vehicle for Cancer Immunotherapy

Despite a history dating back to the 1800s, using Clostridium bacteria to treat cancer has not advanced beyond the observation that they can colonise and partially destroy solid tumours. Progress has been hampered by their inability to eradicate the viable portion of tumours, and an instinctive anxiety around injecting patients with a bacterium whose close relatives cause tetanus and botulism. However, recent advances in techniques to genetically engineer Clostridium species gives cause to revisit this concept. This paper illustrates these developments through the attenuation of C. sporogenes to enhance its clinical safety, and through the expression and secretion of an immunotherapeutic. An 8.6 kb sequence, corresponding to a haemolysin operon, was deleted from the genome and replaced with a short non-coding sequence. The resultant phenotype of this strain, named C. sporogenes-NT, showed a reduction of haemolysis to levels similar to the probiotic strain, C. butyricum M588. Comparison to the parental strain showed no change in growth or sporulation. Following injection of tumour-bearing mice with purified spores of the attenuated strain, high levels of germination were detected in all tumours. Very low levels of spores and vegetative cells were detected in the spleen and lymph nodes. The new strain was transformed with four different murine IL-2-expressing plasmids, differentiated by promoter and signal peptide sequences. Biologically active mIL-2, recovered from the extracellular fraction of bacterial cultures, was shown to stimulate proliferation of T cells. With this investigation we propose a new, safer candidate for intratumoral delivery of cancer immunotherapeutics.

Difficult identification of Haemophilus influenzae, a typical cause of upper respiratory tract infections, in the microbiological diagnostic routine

Haemophilus influenzae is a key pathogen of upper respiratory tract infections. Its reliable discrimination from nonpathogenic Haemophilus spp. is necessary because merely colonizing bacteria are frequent at primarily unsterile sites. Due to close phylogenetic relationship, it is not easy to discriminate H. influenzae from the colonizer Haemophilus haemolyticus. The frequency of H. haemolyticus isolations depends on factors like sampling site, patient condition, and geographic region. Biochemical discrimination has been shown to be nonreliable. Multiplex PCR including marker genes like sodC, fucK, and hpd or sequencing of the 16S rRNA gene, the P6 gene, or multilocus-sequence-typing is more promising. For the diagnostic routine, such techniques are too expensive and laborious. If available, matrix-assisted laser-desorption-ionization time-of-flight mass spectrometry is a routine-compatible option and should be used in the first line. However, the used database should contain well-defined reference spectra, and the spectral difference between H. influenzae and H. haemolyticus is small. Fluorescence in-situ hybridization is an option for less well-equipped laboratories, but the available protocol will not lead to conclusive results in all instances. It can be used as a second line approach. Occasional ambiguous results have to be resolved by alternative molecular methods like 16S rRNA gene sequencing.

Difficulties in species identification within the genus Haemophilus - A pilot study addressing a significant problem for routine diagnostics

Diagnostic misidentifications of commensalic Haemophilus haemolyticus as pathogenic Haemophilus influenzae are frequent. This pilot study evaluates whether isolations of H. haemolyticus are frequent enough in Germany to cause a relevant diagnostic problem, considering the fact that even H. influenzae is a mere colonizer in about 30% of isolations. In microbiological laboratories of two hospitals located in Northern and Southern Germany, the distribution of Haemophilus spp. was analyzed during a six-month-period. Site of infection, sex, and age of the patients was taken into consideration. A total of 77 Haemophilus spp. isolates was acquired and discriminated on species level, comprising: 48 H. influenzae, 25 Haemophilus parainfluenzae, 3 H. haemolyticus, and 1 Haemophilus parahaemolyticus. The proportion of H. haemolyticus was calculated to range between 1.2% and 16.2 % within the 95% confidence limits. Commensalic Haemophilus spp. were isolated from oropharynx-associated sites only. H. influenzae, in contrast, was detected in clinically relevant materials like lower respiratory materials and conjunctiva swabs. Altogether, there was a low proportion of clinical H. haemolyticus isolates. Accordingly, the problem of unnecessary antibiotic therapies due to misidentifications of H. haemolyticus as H. influenzae is quantitatively negligible compared with the risk of confusing H. influenzae colonizations with infections.

Rapid discrimination of Haemophilus influenzae, H. parainfluenzae, and H. haemolyticus by fluorescence in situ hybridization (FISH) and two matrix-assisted laser-desorption-ionization time-of-flight mass spectrometry (MALDI-TOF-MS) platforms

Background

Due to considerable differences in pathogenicity, Haemophilus influenzae, H. parainfluenzae and H. haemolyticus have to be reliably discriminated in routine diagnostics. Retrospective analyses suggest frequent misidentifications of commensal H. haemolyticus as H. influenzae. In a multi-center approach, we assessed the suitability of fluorescence in situ hybridization (FISH) and matrix-assisted laser-desorption-ionization time-of-flight mass-spectrometry (MALDI-TOF-MS) for the identification of H. influenzae, H. parainfluenzae and H. haemolyticus to species level.

Methodology

A strain collection of 84 Haemophilus spp. comprising 50 H. influenzae, 25 H. parainfluenzae, 7 H. haemolyticus, and 2 H. parahaemolyticus including 77 clinical isolates was analyzed by FISH with newly designed DNA probes, and two different MALDI-TOF-MS systems (Bruker, Shimadzu) with and without prior formic acid extraction.

Principal Findings

Among the 84 Haemophilus strains analyzed, FISH led to 71 correct results (85%), 13 uninterpretable results (15%), and no misidentifications. Shimadzu MALDI-TOF-MS resulted in 59 correct identifications (70%), 19 uninterpretable results (23%), and 6 misidentifications (7%), using colony material applied directly. Bruker MALDI-TOF-MS with prior formic acid extraction led to 74 correct results (88%), 4 uninterpretable results (5%) and 6 misidentifications (7%). The Bruker MALDI-TOF-MS misidentifications could be resolved by the addition of a suitable H. haemolyticus reference spectrum to the system's database. In conclusion, no analyzed diagnostic procedure was free of errors. Diagnostic results have to be interpreted carefully and alternative tests should be applied in case of ambiguous test results on isolates from seriously ill patients.

Autotransporter and Two-Partner Secretion: Delivery of Large-Size Virulence Factors by Gram-Negative Bacterial Pathogens

A number of protein secretion mechanisms have been identified in gram-negative pathogens. Many of these secretion systems are dependent upon the Sec translocase for protein export from the cytoplasm into the periplasm and then utilize other mechanisms for transport from the periplasm through the outer membrane. In this article, we review secretion similarities between autotransporter and two-partner secretion systems, and we report similarities between the autotransporter secretion mechanism with that of intimin/invasins. Considering that many secreted proteins are virulence factors, a better understanding of their secretion mechanisms will aid in the development of disease treatments and new bacterial vaccines.

A role for Haemophilus ducreyi Cu,ZnSOD in resistance to heme toxicity

The Cu,Zn superoxide dismutase (Cu,ZnSOD) from Haemophilus ducreyi is the only enzyme of this class which binds a heme molecule at its dimer interface. To explore the role of the enzyme in this heme-obligate bacterium, a sodC mutant was created by insertional inactivation. No difference in growth rate was observed during heme limitation. In contrast, under heme rich conditions growth of the sodC mutant was impaired compared to the wild type strain. This growth defect was abolished by supplementation of exogenous catalase. Genetic complementation of the sodC mutant in trans demonstrated that the enzymatic property or the heme-binding activity of the protein could repair the growth defect of the sodC mutant. These results indicate that Cu,ZnSOD protects Haemophilus ducreyi from heme toxicity.

Immunization with the Haemophilus ducreyi hemoglobin receptor HgbA protects against infection in the swine model of chancroid

The etiologic agent of chancroid is Haemophilus ducreyi. To fulfill its obligate requirement for heme, H. ducreyi uses two TonB-dependent receptors: the hemoglobin receptor (HgbA) and a receptor for free heme (TdhA). Expression of HgbA is necessary for H. ducreyi to survive and initiate disease in a human model of chancroid. In this study, we used a swine model of H. ducreyi infection to demonstrate that an experimental HgbA vaccine efficiently prevents chancroid, as determined by several parameters. Histological sections of immunized animals lacked typical microscopic features of chancroid. All inoculated sites from mock-immunized pigs yielded viable H. ducreyi cells, whereas no viable H. ducreyi cells were recovered from inoculated sites of HgbA-immunized pigs. Antibodies from sera of HgbA-immunized animals bound to and initiated antibody-dependent bactericidal activity against homologous H. ducreyi strain 35000HP and heterologous strain CIP542 ATCC; however, an isogenic hgbA mutant of 35000HP was not killed, proving specificity. Anti-HgbA immunoglobulin G blocked hemoglobin binding to the HgbA receptor, suggesting a novel mechanism of protection through the limitation of heme/iron acquisition by H. ducreyi. Such a vaccine strategy might be applied to other bacterial pathogens with strict heme/iron requirements. Taken together, these data suggest continuing the development of an HgbA subunit vaccine to prevent chancroid.

Haemophilus ducreyi targets Src family protein tyrosine kinases to inhibit phagocytic signaling

Haemophilus ducreyi, the etiologic agent of the sexually transmitted disease chancroid, has been shown to inhibit phagocytosis of both itself and secondary targets in vitro. Immunodepletion of LspA proteins from H. ducreyi culture supernatant fluid abolished this inhibitory effect, indicating that the LspA proteins are necessary for the inhibition of phagocytosis by H. ducreyi. Fluorescence microscopy revealed that macrophages incubated with wild-type H. ducreyi, but not with a lspA1 lspA2 mutant, were unable to complete development of the phagocytic cup around immunoglobulin G-opsonized targets. Examination of the phosphotyrosine protein profiles of these two sets of macrophages showed that those incubated with wild-type H. ducreyi had greatly reduced phosphorylation levels of proteins in the 50-to-60-kDa range. Subsequent experiments revealed reductions in the catalytic activities of both Lyn and Hck, two members of the Src family of protein tyrosine kinases that are known to be involved in the proximal signaling steps of Fcgamma receptor-mediated phagocytosis. Additional experiments confirmed reductions in the levels of both active Lyn and active Hck in three different immune cell lines, but not in HeLa cells, exposed to wild-type H. ducreyi. This is the first example of a bacterial pathogen that suppresses Src family protein tyrosine kinase activity to subvert phagocytic signaling in hostcells.

Survival of enterococci and Tn916-like conjugative transposons in soil

The persistence of Enterococcus faecalis, fecal enterococci from swine waste, and Tn916-like elements was determined following inoculation into autoclaved and native soil microcosms. When cells of E. faecalis CG110 (Tn916) were inoculated into native microcosms, enterococcal viability in the soil decreased approximately 5 orders of magnitude (4.8 x 10(5) CFU/g soil to < 10 CFU/g) after 5 weeks. In autoclaved microcosms, the viability of E. faecalis decreased by only 20% in 5 weeks. In contrast, the content of Tn916, based on PCR of DNA extracts from soil microcosms, decreased by about 20% in both native and autoclaved microcosms. Similar results were obtained when the source of fecal enterococci and Tn916-like elements was swine waste. Because the concentration of Tn916-independent E. faecalis DNA (the D-alanine D-alanine ligase gene), based on PCR, decreased to nearly undetectable levels (at least 3 orders of magnitude) after 5 weeks in the native microcosms, the evidence suggests Tn916 stability in the soil results from en masse transfer of the transposon to the normal soil microflora and not survival of E. faecalis DNA in the soil system. Results from denaturing gradient gel electrophoresis suggest that multiple forms of Tn916 occur in swine waste, but only forms most like Tn916 exhibit stability in the soil.

Haemophilus ducreyi Outer membrane determinants, including DsrA, define two clonal populations

The Haemophilus ducreyi outer membrane component DsrA (for ducreyi serum resistance A) is necessary for complete resistance to normal human serum (NHS). When DsrA expression in 19 temporally and geographically diverse clinical isolates of H. ducreyi was examined by Western blotting, 5 of the strains expressed a different immunotype of the DsrA protein (DsrA(II)) than the well-characterized prototypical strain 35000HP (DsrA(I)). The predicted DsrA proteins expressed by the DsrA(II) strains were 100% identical to each other but only 48% identical to that of strain 35000HP. In addition to the DsrA(II) protein, class II strains also expressed variant forms of other outer membrane proteins (OMPs) including NcaA (necessary for collagen adhesion A), DltA (ducreyi lectin A), Hlp (H. ducreyi lipoprotein), major OMP, and/or OmpA2 (for OMP A2) and synthesized a distinct, faster-migrating lipooligosaccharide. Based on these data, strains expressing DsrA(I) were termed class I, and those expressing DsrA(II) were termed class II. Expression of dsrA(II) from strain CIP 542 ATCC in the class I dsrA(I) mutant FX517 (35000HP background), which does not express a DsrA protein, rendered this strain resistant to 50% NHS. This demonstrates that DsrA(II) protein is also critical to serum resistance. Taken together, these results indicate that there are two clonal populations of H. ducreyi. The implications of two classes of H. ducreyi strains differing in important antigenic outer membrane components are discussed.

Differential expression of porins OmpP2A and OmpP2B of Haemophilus ducreyi

Haemophilus ducreyi is a strict human pathogen and the causative agent of the sexually transmitted disease chancroid. The genome of the human-passaged strain of H. ducreyi (35000HP) contains two homologous genes whose protein products have estimated molecular masses of 46 and 43 kDa. A comparative analysis of the deduced amino acid sequences revealed that these proteins share 27 to 33% identity to the outer membrane protein P2 (OmpP2), a major porin of Haemophilus influenzae. Therefore, these proteins have been designated OmpP2A and OmpP2B, respectively. The detection of ompP2A and ompP2B transcripts by reverse transcriptase PCR indicated that these genes were independently transcribed in H. ducreyi 35000HP. Western blot analysis of outer membrane proteins isolated from a geographically diverse collection of H. ducreyi clinical isolates revealed that OmpP2A and OmpP2B were differentially expressed among these strains. Although PCR analysis suggested that ompP2A and ompP2B were conserved among the strains tested, the differential expression observed was due to nucleotide additions and partial gene deletions. Purified OmpP2A and OmpP2B were isolated under nondenaturing conditions, and subsequent analysis demonstrated that these two proteins exhibited porin activity. OmpP2A and OmpP2B are the first porins described for H. ducreyi.

Activation of Serratia marcescens hemolysin through a conformational change

For Serratia marcescens, secreted hemolysin/cytotoxin is not only secreted but also activated by an outer membrane protein. Excluding posttranslational processing by mass spectrometry, the conformation of active and inactive ShlA derivatives strongly differed in electrophoretic mobilities, gel permeation chromatography, sensitivity to trypsin, circular dichroism, and intrinsic fluorescence. We concluded that ShlB interacts with ShlA during secretion and imposes a conformational change in ShlA to form the active hemolysin.

Specificity of antibodies directed against the cytolethal distending toxin of Haemophilus ducreyi in patients with chancroid

Antibodies specific for the cytolethal-distending toxin of Haemophilus ducreyi (HdCDT) complex and for the CdtA, CdtB, and CdtC components were measured by ELISA in the sera of 50 patients with culture and/or PCR proven chancroid, 42 patients with periodontitis, 50 blood donors from Tanzania, 50 blood donors from Sweden. In addition, the biological activity e.g. neutralization capacity of the sera were tested. Our results demonstrate that majority of chancroid patients and healthy individuals had detectable levels of serum antibodies to HdCDT complex and to separate toxin components. However, high levels (> or =100 units) of antibodies to HdCDT complex were significantly more prevalent in the sera of patients with both chancroid and periodontitis than in the sera of the corresponding controls (P=0.001 and P=0.04, respectively). In the sera of the 50 patients with chancroid, antibodies to CdtA, CdtB, and CdtC were detected in 50, 35, and 34 individuals, respectively. Antibodies to CdtC, being less frequently detected than the antibodies to other components, show a good correlation with the neutralizing capacity of sera. High levels of neutralizing antibodies (> or =160) were detected in only 22 and 2% of the patients with chancroid and periodontitis, respectively. The data suggest that the low levels of anti-HdCDT antibodies, which include neutralizing antibodies, may contribute to limited protection in chancroid and since anti-HdCDT antibodies, may be detected in healthy individuals and in patients with certain disease conditions (e.g. periodontitis), they may not be specific markers for chancroid infection.

Signature Tagged Mutagenesis of Haemophilus influenzae identifies genes required for in vivo survival

The pathogenic bacterium Haemophilus influenzae causes meningitis, epiglottitis, pneumonia, otitis media and other infections. To further understand the genetic basis of invasive disease and to inform about the bacterium's requirements in an in vivo environment, we analysed a library of 1632 insertional Tn1545 -Delta3 transposon mutants for their capacity to cause systemic infection in an animal model. We identified 25 genes that are potentially essential for H. influenzae invasive disease, and are candidates for further exploratory research. Seven of the genes encode hypothetical proteins, the function of six of which could be tentatively assigned on the basis of functional motifs and low homology to other bacterial genes. Eleven genes encode central metabolic enzymes or transporters; eight encode proteins that interact with DNA or modify other proteins; and four encode enzymes involved in the elaboration of classical virulence determinants. Two genes have no known function. Independent mutagenesis of six of the 25 genes and determination of the competitive index confirmed that these genes are important or essential to the organism in an in vivo environment. This genome-wide analysis has identified metabolic and other genes required during invasive disease, and the findings may lead to new interventions to prevent and treat H. influenzae infections.

CpsK of Streptococcus agalactiae exhibits alpha2,3-sialyltransferase activity in Haemophilus ducreyi

Streptococcus agalactiae (GBS) is a major cause of serious newborn bacterial infections. Crucial to GBS evasion of host immunity is the production of a capsular polysaccharide (CPS) decorated with sialic acid, which inactivates the alternative complement pathway. The CPS operons of serotypes Ia and III GBS have been described, but the CPS sialyltransferase gene was not identified. We identified cpsK, an open reading frame in the CPS operon of most serotypes, which was homologous to the lipooligosaccharide (LOS) sialyltransferase gene, lst, of Haemophilus ducreyi. To determine if cpsK might encode a sialyltransferase, we complemented a H. ducreyi lst mutant with cpsK. CpsK was expressed in H. ducreyi and LOS was isolated and analysed for sialic acid content by SDS-PAGE and high-performance liquid chromatography (HPLC). Sialo-LOS was seen in the wild-type, cpsK- or lst-complemented mutant strains, but not in the mutant without cpsK. Addition of Neu5Ac to the LOS was confirmed by mass spectroscopy. Lectin binding studies detected terminal Neu5Ac(alpha 2-->3)Gal(beta 1- on LOS produced by the wild-type, cpsK or lst-complemented mutant strain LOS, compared with the mutant alone. Our data characterize the first sialyltransferase gene from a Gram- positive bacterium and provide compelling evidence that its product catalyses the alpha2,3 addition of Neu5Ac to H. ducreyi LOS and therefore the terminal side-chain of GBS CPS. Phylogenetic studies further indicated that lst and cpsK are related but distinct from sialyltransferases of most other bacteria and, along with their similar codon usage bias and G + C content, suggests acquisition by lateral transfer from an ancestral low G + C organism.

In vitro and in vivo interactions of Haemophilus ducreyi with host phagocytes

We investigated the phagocytosis of Haemophilus ducreyi both in vitro and in vivo. Human granulocyte and monocyte phagocytosis of opsonized and nonopsonized, fluorescence-labeled H. ducreyi was assessed by flow cytometry. Both Escherichia coli and noncapsulated H. influenzae were included as controls. The maximal percentage of granulocytes taken up by H. ducreyi was 35% after 90 min. In contrast, 95% of H. influenzae bacteria were phagocytosed by granulocytes after 30 min. These results indicated that H. ducreyi phagocytosis was slow and inefficient. Bacterial opsonization by using specific antibodies increased the percentage of granulocytes phagocytosing H. ducreyi from 24 to 49%. The nonphagocytosed bacteria were completely resistant to phagocytosis even when reexposed to granulocytes, indicating that the H. ducreyi culture comprised a mixture of phenotypes. The intracellular survival of H. ducreyi in granulocytes, in monocytes/macrophages, and in a monocyte cell line (THP-1) was quantified after application of gentamicin treatment to kill extracellular bacteria. H. ducreyi survival within phagocytes was poor; approximately 11 and <0.1% of the added bacteria survived intracellularly after 2 and 20 h of incubation, respectively, while no intracellular H. influenzae bacteria were recovered after 2 h of incubation with phagocytes. The role of phagocytes in the development of skin lesions due to H. ducreyi was also studied in vivo. Mice that were depleted of granulocytes and/or monocytes and SCID mice, which lacked T and B cells, were injected intradermally with approximately 10(6) CFU of H. ducreyi. Within 4 days of inoculation, the granulocyte-depleted mice developed lesions that persisted throughout the experimental period. This result reinforces the importance of granulocytes in the early innate defense against H. ducreyi infection. In conclusion, H. ducreyi is insufficiently phagocytosed to achieve complete eradication of the bacteria. Indeed, H. ducreyi has the ability to survive intracellularly for short periods within phagocytic cells in vitro. Since granulocytes play a major role in the innate defense against H. ducreyi infection in vivo, bacterial resistance to phagocytosis probably plays a crucial role in the pathogenesis of chancroid.

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.

Haemophilus ducreyi inhibits phagocytosis by U-937 cells, a human macrophage-like cell line

Haemophilus ducreyi is a gram-negative obligate human pathogen that causes the genital ulcer disease chancroid. Chancroid lesions are deep necrotic ulcers with an immune cell infiltrate that includes macrophages. Despite the presence of these phagocytic cells, chancroid ulcers can persist for months and live H. ducreyi can be isolated from these lesions. To analyze the interaction of H. ducreyi with macrophages, we investigated the ability of H. ducreyi strain 35000 to adhere to, invade, and survive within U-937 cells, a human macrophage-like cell line. We found that although H. ducreyi strain 35000 adhered efficiently to U-937 cells, few bacteria were internalized, suggesting that H. ducreyi avoids phagocytosis by human macrophages. The few bacteria that were phagocytosed in these experiments were rapidly killed. We also found that H. ducreyi inhibits the phagocytosis of a secondary target (opsonized sheep red blood cells). Antiphagocytic activity was found in logarithmic, stationary-phase, and plate-grown cultures and was associated with whole, live bacteria but not with heat-killed cultures, sonicates, or culture supernatants. Phagocytosis was significantly inhibited after a 15-min exposure to H. ducreyi, and a multiplicity of infection of approximately 1 CFU per macrophage was sufficient to cause a significant reduction in phagocytosis by U-937 cells. Finally, all of nine H. ducreyi strains tested were antiphagocytic, suggesting that this is a common virulence mechanism for this organism. This finding suggests a mechanism by which H. ducreyi avoids killing and clearance by macrophages in chancroid lesions and inguinal lymph nodes.

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.

Expression of cytolethal distending toxin and hemolysin is not required for pustule formation by Haemophilus ducreyi in human volunteers

Haemophilus ducreyi makes cytolethal distending toxin (CDT) and hemolysin. In a previous human challenge trial, an isogenic hemolysin-deficient mutant caused pustules with a rate similar to that of its parent. To test whether CDT was required for pustule formation, six human subjects were inoculated with a CDT mutant and parent at multiple sites. The pustule formation rates were similar at both parent and mutant sites. A CDT and hemolysin double mutant was constructed and tested in five additional subjects. The pustule formation rates were similar for the parent and double mutant. These results indicate that neither the expression of CDT, nor that of hemolysin, nor both are required for pustule formation by H. ducreyi in humans.

Isolation of Tn916-like conjugal elements from swine lot effluent

Isolates of Enterococcus faecalis obtained from a swine farrowing house outflow were examined for genetic elements similar to Tn916. Of the enterococci isolated, 71% were resistant to tetracycline. Among the tetracycline-resistant enterococci isolated from the outflow samples, approximately 34% were able to transfer the tetracycline resistance phenotype to Bacillus thuringiensis in cross-genus matings. The frequencies of transfer for 10 random isolates were comparable to those for transfer of Tn916 from E. faecalis to B. thuringiensis. In addition, these elements were shown to mobilize plasmid pC194 between Bacillus species, as did Tn916. Southern blot and polymerase chain reaction (PCR) analysis showed these elements share extensive structural homology with Tn916. The selected conjugal elements were capable of transfer to a Bacillus recipient in a soil environment. When the swine waste was introduced into the soil, the tetracycline resistant fecal enterococci levels rose from essentially undetectable levels to approximately 4 x 10(4) and remained at this level for 4 weeks. After six months, including one winter, levels had decreased to 5 x 10(3).

Cloning and characterization of the lipooligosaccharide galactosyltransferase II gene of Haemophilus ducreyi

Haemophilus ducreyi is the etiologic agent of chancroid, a genital ulcer disease. The lipooligosaccharide (LOS) is considered to be a major virulence determinant and has been implicated in the adherence of H. ducreyi to keratinocytes. Strain A77, an isolate from the Paris collection, is serum sensitive, poorly adherent to fibroblasts, and deficient in microcolony formation. Structural analysis indicates that the LOS of strain A77 lacks the galactose residue found in the N-acetyllactosamine portion of the strain 35000HP LOS as well as the sialic acid substitution. From an H. ducreyi 35000HP genomic DNA library, a clone complementing the defect in A77 was identified by immunologic screening with monoclonal antibody (MAb) 3F11, a MAb which recognizes the N-acetyllactosamine portion of strain 35000HP LOS. The clone contained a 4-kb insert that was sequenced. One open reading frame which encodes a protein with a molecular weight of 33,400 was identified. This protein has homology to glycosyltransferases of Haemophilus influenzae, Haemophilus somnus, Neisseria species, and Pasteurella haemolytica. The putative H. ducreyi glycosyltransferase gene was insertionally inactivated, and an isogenic mutant of strain 35000HP was constructed. The most complex LOS glycoform produced by the mutant has a mobility on sodium dodecyl sulfate-polyacrylamide gel identical to that of the LOS of strain A77 and lacks the 3F11-binding epitope. Structural studies confirm that the most complex glycoform of the LOS isolated from the mutant lacks the galactose residue found in the N-acetyllactosamine portion of the strain 35000HP LOS. Although previously published data suggested that the serum-sensitive phenotype of A77 was due to the LOS mutation, we observed that the complemented A77 strain retained its serum-sensitive phenotype and that the galactosyltransferase mutant retained its serum-resistant phenotype. Thus, the serum sensitivity of strain A77 cannot be attributed to the galactosyltransferase mutation in strain A77.

Stable shuttle vectors for Neisseria gonorrhoeae, Haemophilus spp. and other bacteria based on a single origin of replication

An origin of replication (ori) was obtained from a naturally occurring beta-lactamase-producing plasmid isolated from Neisseria gonorrhoeae and used to construct shuttle vectors capable of replicating in N. gonorrhoeae, Haemophilus ducreyi, Haemophilus influenzae and Escherichia coli. Using the gonococcal proAB genes, we complemented proline-requiring N. gonorrhoeae F62 and E. coli HB101 in trans. The first demonstration of the expression of the green fluorescent protein (GFP) in either N. gonorrhoeae or H. ducreyi was shown using this vector, indicating that GFP may be a useful tool in the analysis of these organisms. This is the first report of a gonococcal vector based on a broad host range, genetically defined ori, and should facilitate the molecular analysis of gonococcal and Haemophilus genes.

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.

Cytolethal distending toxin of Haemophilus ducreyi induces apoptotic death of Jurkat T cells

The immune response to Haemophilus ducreyi is mediated in part by T cells infiltrating the site of infection. In this study, we show that H. ducreyi antigen preparations inhibited the proliferation of peripheral blood mononuclear cells and primary human T-cell lines. H. ducreyi also inhibited Jurkat T-cell proliferation and induced apoptosis of Jurkat T cells, confirmed through the detection of DNA degradation and membrane unpacking. The cytotoxic product(s) was present in cell-free culture supernatant and whole-cell preparations of H. ducreyi and was heat labile. H. ducreyi produces two known heat-labile toxins, a hemolysin and a cytolethal distending toxin (CDT). Whole cells and supernatants prepared from a hemolysin-deficient mutant had the same inhibitory and apoptotic effects on Jurkat T cells as did its isogenic parent. Preparations made from an H. ducreyi cdtC mutant were less toxic and induced less apoptosis than the parent. The toxic activity of the cdtC mutant was restored by complementation in trans. CdtC-neutralizing antibodies also inhibited H. ducreyi-induced toxicity and apoptosis. The data suggest that CDT may interfere with T-cell responses to H. ducreyi by induction of apoptosis.

Examination of early interactions between Haemophilus ducreyi and host cells by using cocultured HaCaT keratinocytes and foreskin fibroblasts

Haemophilus ducreyi is the etiologic agent of chancroid, a sexually transmitted genital ulcer disease. Keratinocytes are likely the first cell type encountered by H. ducreyi upon infection of human skin; thus, the interaction between H. ducreyi and keratinocytes is probably important for the ability of H. ducreyi to establish infection. We have used the HaCaT keratinocyte cell line grown in monolayers and in cocultures with HS27 fibroblasts to investigate H. ducreyi interactions with keratinocytes and the host-cell response to H. ducreyi infection. Using quantitative adherence and gentamicin protection assays, we determined that approximately 13% of H. ducreyi adhered to HaCaT cell monolayers, while only a small proportion (0.0052%) was intracellular. By transmission electron microscopy, we observed numerous H. ducreyi organisms adherent to but rarely within HaCaT cells cocultured with fibroblasts. Both live H. ducreyi and purified H. ducreyi lipooligosaccharide (LOS) induced significant interleukin 8 (IL-8) expression from HaCaT cell-HS27 cell cocultures. However, the level of IL-8 expression in response to LOS alone was not as pronounced. H. ducreyi LOS was a more potent inducer of IL-8 from cocultures than Escherichia coli lipopolysaccharide (LPS) at the same concentration, suggesting a unique effect of H. ducreyi LOS on cocultures. Neither live H. ducreyi nor purified H. ducreyi LOS or E. coli LPS induced tumor necrosis factor alpha expression from cocultures. H. ducreyi induced drastically different cytokine profiles from cocultures than from HS27 or HaCaT cells cultured separately. IL-8 expression by skin cells in response to H. ducreyi infection in vivo may be responsible for the massive influx of polymorphonuclear leukocytes and other inflammatory cells to the site of infection. This influx of inflammatory cells may be partly responsible for the tissue destruction characteristic of chancroid.

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.

The haemolysin-secreting ShlB protein of the outer membrane of Serratia marcescens: determination of surface-exposed residues and formation of ion-permeable pores by ShlB mutants in artificial lipid bilayer membranes

The ShlB protein in the outer membrane of Serratia marcescens is the only protein known to be involved in secretion of the ShlA protein across the outer membrane. At the same time, ShlB converts ShlA into a haemolytic and a cytolytic toxin. Surface-exposed residues of ShlB were determined by reaction of an M2 monoclonal antibody with the M2 epitope DYKDDDDK inserted at 25 sites along the entire ShlB polypeptide. The antibody bound to the M2 epitope at 17 sites in intact cells, which indicated surface exposure of the epitope, and to 23 sites in isolated outer membranes. Two insertion mutants contained no ShlB(M2) protein in the outer membrane. The ShlB derivatives activated and/or secreted ShlA. To gain insights into the secretion mechanism, we studied whether highly purified ShlB and ShlB deletion derivatives formed pores in artificial lipid bilayer membranes. Wild-type ShlB formed channels with very low single channel conductance that rarely assumed an open channel configuration. In contrast, open channels with a considerably higher single channel conductance were observed with the deletion mutants ShlB(Delta65-186), ShlB(Delta87-153), and ShlB(Delta126-200). ShlB(Delta126-200) frequently formed permanently open channels, whereas the conductance caused by ShlB(Delta65-186) and ShlB(Delta87-153) did not assume a stationary value, but fluctuated rapidly between open and closed configurations. The results demonstrate the orientation of large portions of ShlB in the outer membrane and suggest that ShlB may function as a specialized pore through which ShlA is secreted.

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 cytolethal distending toxin from the chancroid bacterium Haemophilus ducreyi induces cell-cycle arrest in the G2 phase

The potent cytolethal distending toxin produced by Haemophilus ducreyi is a putative virulence factor in the pathogenesis of chancroid. We studied its action on eukaryotic cells, with the long-term goal of understanding the pathophysiology of the disease. Intoxication of cultured human epithelial-like cells, human keratinocytes, and hamster fibroblasts was irreversible, and appeared as a gradual distention of three- to fivefold the size of control cells. Organized actin assemblies appeared concomitantly with cell enlargement, promoted by a mechanism that probably does not involve small GTPases of the Rho protein family. Intoxicated cells did not proliferate. Similar to cells treated with other cytolethal distending toxins, these cells accumulated in the G2 phase of the cell cycle, demonstrating an increased level of the tyrosine phosphorylated (inactive) form of the cyclin-dependent kinase p34(cdc2). DNA synthesis was not affected until several hours after this increase, suggesting that the toxin acts directly on some kinase/phosphatase in the signaling network controlling the p34(cdc2) activity. We propose that this toxin has an important role both in the generation of chancroid ulcers and in their slow healing. The toxin may also be an interesting new tool for molecular studies of the eukaryotic cell- cycle machinery.

Haemophilus ducreyi secretes a filamentous hemagglutinin-like protein

We have identified two extremely large open reading frames (ORFs) in Haemophilus ducreyi 35000, lspA1 and lspA2, each of which encodes a predicted protein product whose N-terminal half is approximately 43% similar to the N-terminal half of Bordetella pertussis filamentous hemagglutinin (FhaB). To the best of our knowledge, lspA1 (12,500 nucleotides [nt]) and lspA2 (14,800 nt) are among the largest prokaryotic ORFs identified to date. The predicted proteins, LspA1 and LspA2, are 86% identical overall to each other and also have limited amino acid sequence similarity at their N termini to other secreted bacterial proteins, including certain hemolysins. Southern blot analysis indicated that lspA1 and lspA2 sequences were present in 15 other geographically diverse H. ducreyi strains. Reverse transcriptase PCR analysis of total RNA isolated from H. ducreyi 35000 grown in liquid medium, grown on solid agar medium, and isolated from lesions of H. ducreyi-infected rabbits indicated that lspA1 and lspA2 were transcribed both in vitro and in vivo. A 260-kDa protein present in culture supernatant from eight virulent H. ducreyi strains reacted with both polyclonal serum from rabbits infected with H. ducreyi 35000 and a monoclonal antibody predicted to bind both LspA1 and LspA2. This 260-kDa protein in H. ducreyi 35000 culture supernatant was shown to be the protein product of the lspA1 ORF based on its reactivity with a monoclonal antibody specific for LspA1. Four H. ducreyi strains, previously shown to be avirulent in the temperature-dependent rabbit model for chancroid, did not produce either LspA1 or LspA2 in vitro. This finding raised the possibility that LspA1, LspA2, or both may be involved in the ability of H. ducreyi to cause lesions in this animal model.

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.

Haemophilus ducreyi infection causes basal keratinocyte cytotoxicity and elicits a unique cytokine induction pattern in an In vitro human skin model

Haemophilus ducreyi is the etiologic agent of the sexually transmitted genital ulcer disease chancroid. Predominantly a cutaneous pathogen, H. ducreyi is present in chancroid ulcers that are characterized by extensive neutrophil accumulation in intraepidermal lesions accompanied by a mononuclear infiltrate in the dermis. We used an in vitro human skin model composed of foreskin fibroblasts and keratinocytes to examine host skin cell interactions with H. ducreyi 35000. Bacteria replicated and persisted in artificial skin for at least 14 days. We observed H. ducreyi inside suprabasal keratinocytes using transmission electron microscopy. Although no bacteria were seen in the basal keratinocyte region, these cells were disrupted in infected cocultures. H. ducreyi infection stimulated increased secretion of interleukin-6 (IL-6) and IL-8 by skin cells. Conversely, tumor necrosis factor alpha and IL-1alpha levels were not elevated. IL-8 produced in response to H. ducreyi infection may be involved in recruiting polymorphonuclear leukocytes and other inflammatory cells, thereby contributing to the tissue necrosis and ulcer formation characteristic of chancroid.

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.

Characterization of a transposon Tn916-generated mutant of Haemophilus ducreyi 35000 defective in lipooligosaccharide biosynthesis

To define the role of the surface lipooligosaccharide (LOS) of Haemophilus ducreyi in the pathogenesis of chancroid, Tn916 mutants of H. ducreyi 35000 defective in expression of the murine monoclonal antibody (MAb) 3F11 epitope on H. ducreyi LOS were identified by immunologic screening. One mutant, designated 1381, has an LOS which lacks the MAb 3F11 epitope and migrates with an increased mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The gene disrupted by the Tn916 element in strain 1381 was identified by cloning the sequences flanking the Tn916 element. The sequences were then used to probe a lambda DASHII genomic library. In strain 1381, Tn916 interrupts a gene which encodes an open reading frame (ORF) with an Mr of 40,246. This ORF has homology to the product of the rfaK gene of Escherichia coli. The major LOS glycoform produced by strain 1381 was analyzed by using a combination of mass spectrometry, linkage and composition analysis, and 1H nuclear magnetic resonance spectroscopy. The major LOS species was found to terminate in a single glucose attached to the heptose (L-glycero-D-manno-heptose, or Hep) trisaccharide core. In the wild-type strain 35000, glucose serves as the acceptor for the addition of the D-glycero-D-manno-heptose (or DDHep), which extends to form the mature branch of the H. ducreyi LOS. This mature oligosaccharide is in turn partially capped by the addition of sialic acid (NeuAc), i.e., NeuAc2 alpha-->3Gal beta1-->4GlcNAc beta1-->3Gal beta1-->4DDHep alpha1-->6Glc beta1 (W. Melaugh et al., Biochemistry 33:13070-13078, 1994). Since this LOS terminates prior to the addition of the branch DD-heptose, this gene is likely to encode the D-glycero-D-manno-heptosyltransferase. Strain 1381 exhibits a significant reduction in adherence to and invasion of primary human keratinocytes. This defect was complemented by the cloned heptosyltransferase gene, indicating that the terminal portion of the LOS oligosaccharide plays an important role in adherence to human keratinocytes.

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 functional complementation between Bordetella pertussis filamentous hemagglutinin and Proteus mirabilis HpmA hemolysin secretion machineries

The gram-negative bacterium Bordetella pertussis has adapted specific secretion machineries for each of its major secretory proteins. In particular, the highly efficient secretion of filamentous hemagglutinin (FHA) is mediated by the accessory protein FhaC. FhaC belongs to a family of outer membrane proteins which are involved in the secretion of large adhesins or in the activation and secretion of Ca2+-independent hemolysins by several gram-negative bacteria. FHA shares with these hemolysins a 115-residue-long amino-proximal region essential for its secretion. To compare the secretory pathways of these hemolysins and FHA, we attempted functional transcomplementation between FhaC and the Proteus mirabilis hemolysin accessory protein HpmB. HpmB could not promote the secretion of FHA derivatives. Likewise, FhaC proved to be unable to mediate secretion and activation of HpmA, the cognate secretory partner of HpmB. In contrast, ShlB, the accessory protein of the closely related Serratia marcescens hemolysin, was able to activate and secrete HpmA. Two invariant asparagine residues lying in the region of homology shared by secretory proteins and shown to be essential for the secretion and activation of the hemolysins were replaced in FHA by site-directed mutagenesis. Replacements of these residues indicated that both are involved in, but only the first one is crucial to, FHA secretion. This slight discrepancy together with the lack of functional complementation demonstrates major differences between the hemolysins and FHA secretion machineries.

Cloning and sequencing of the gene encoding the Cu,Zn-superoxide dismutase of Haemophilus ducreyi

The sodC gene of Haemophilus ducreyi was cloned and sequenced. The deduced amino acid sequence of this protein exhibited 71.6% identity and 81.8% similarity to the H. influenzae and H. parainfluenzae copper (Cu), zinc (Zn)-superoxide dismutase (SOD) enzymes. This gene was localized to a 2.2-kb H. ducreyi chromosomal DNA insert in plasmid pHdSOD. SOD activity was expressed in cell-free extracts of Escherichia coli containing the recombinant plasmid pHdSOD and was localized to the periplasmic space. The Cu,Zn-SOD produced by the H. ducreyi sodC gene did not complement the aerobic growth defect of an E. coli SOD-deficient mutant.

Haemophilus ducreyi hemolysin acts as a contact cytotoxin and damages human foreskin fibroblasts in cell culture

Haemophilus ducreyi, which causes the sexually transmitted disease chancroid, produces several factors that damage human cells. We used isogenic mutants of H. ducreyi 35000 to demonstrate that the hemolytic activity and the cytotoxic effect of H. ducreyi on human foreskin fibroblasts are due to the same toxin.