Expression of peptidoglycan-associated lipoprotein is required for virulence in the human model of Haemophilus ducreyi infection

The peptidoglycan-associated lipoprotein gene mutant elicits robust immunological defense in mice against Salmonella enteritidis

Background

Salmonella enteritidis (S. enteritidis), a zoonotic pathogen with a broad host range, presents a substantial threat to global public health safety. Vaccination stands as an effective strategy for the prevention and control of S. enteritidis infection, highlighting an immediate clinical need for the creation of safe and efficient attenuated live vaccines.

Methods

In this study, a S. enteritidis peptidoglycan-associated lipoprotein (pal) gene deletion strain (Δpal), was constructed. To assess its virulence, we conducted experiments on biofilm formation capability, motility, as well as cell and mouse infection. Subsequently, we evaluated the immune-protective effect of Δpal.

Results

It was discovered that deletion of the pal gene reduced the biofilm formation capability and motility of S. enteritidis. Cell infection experiments revealed that the Δpal strain exhibited significantly decreased abilities in invasion, adhesion, and intracellular survival, with downregulation of virulence gene expression, including mgtC, invH, spvB, sipA, sipB, ssaV, csgA, and pipB. Mouse infection experiments showed that the LD50 of Δpal increased by 104 times, and its colonization ability in mouse tissue organs was significantly reduced. The results indicated that the pal gene severely affected the virulence of S. enteritidis. Further, immunogenicity evaluation of Δpal showed a significant enhancement in the lymphocyte transformation proliferation capability of immunized mice, producing high titers of specific IgG and IgA, suggesting that Δpal possesses good immunogenicity. Challenge protection tests demonstrated that the strain could provide 100% immune protection against wild-type strains in mice.

Discussion

This study proves that the pal gene influences the virulence of S. enteritidis, and Δpal could serve as a candidate strain for attenuated live vaccines, laying the foundation for the development of attenuated live vaccines against Salmonella.

Peptidoglycan-associated lipoprotein contributes to the virulence of Acinetobacter baumannii and serves as a vaccine candidate

The role of peptidoglycan-associated lipoprotein (Pal) in A. baumannii pathogenesis remains unclear. Here, we illustrated its role by constructing a pal deficient A. baumannii mutant and its complementary strain.Transcriptome analysis of the WT and pal mutant revealed a total of 596 differentially expressed genes. Gene Ontology analysis revealed that pal deficiency caused the downregulation of genes related to material transport and metabolic processes. The pal mutant showed a slower growth and was sensitive to detergent and serum killing compared to WT strain, whereas, the complemented pal mutant showed rescued phenotype. The pal mutant caused decreased mortality in mice pneumonia infection compared to WT strain, while the complemented pal mutant showed increased mortality. Mice immunized with recombinant Pal showed 40% protection against A. baumannii-mediated pneumonia. Collectively, these data indicate Pal is a virulence factor of A. baumannii and may serve as a potential target for preventive or therapeutic interventions.

Haemophilus ducreyi Infection Induces Oxidative Stress, Central Metabolic Changes, and a Mixed Pro- and Anti-inflammatory Environment in the Human Host

Few studies have investigated host-bacterial interactions at sites of infection in humans using transcriptomics and metabolomics. Haemophilus ducreyi causes cutaneous ulcers in children and the genital ulcer disease chancroid in adults. We developed a human challenge model in which healthy adult volunteers are infected with H. ducreyi on the upper arm until they develop pustules. Here, we characterized host-pathogen interactions in pustules using transcriptomics and metabolomics and examined interactions between the host transcriptome and metabolome using integrated omics. In a previous pilot study, we determined the human and H. ducreyi transcriptomes and the metabolome of pustule and wounded sites of 4 volunteers (B. Griesenauer, T. M. Tran, K. R. Fortney, D. M. Janowicz, et al., mBio 10:e01193-19, 2019, https://doi.org/10.1128/mBio.01193-19). While we could form provisional transcriptional networks between the host and H. ducreyi, the study was underpowered to integrate the metabolome with the host transcriptome. To better define and integrate the transcriptomes and metabolome, we used samples from both the pilot study (n = 4) and new volunteers (n = 8) to identify 5,495 human differentially expressed genes (DEGs), 123 H. ducreyi DEGs, 205 differentially abundant positive ions, and 198 differentially abundant negative ions. We identified 42 positively correlated and 29 negatively correlated human-H. ducreyi transcriptome clusters. In addition, we defined human transcriptome-metabolome networks consisting of 9 total clusters, which highlighted changes in fatty acid metabolism and mitigation of oxidative damage. Taken together, the data suggest a mixed pro- and anti-inflammatory environment and rewired central metabolism in the host that provides a hostile, nutrient-limited environment for H. ducreyi. IMPORTANCE Interactions between the host and bacteria at sites of infection in humans are poorly understood. We inoculated human volunteers on the upper arm with the skin pathogen H. ducreyi or a buffer control and biopsied the resulting infected and sham-inoculated sites. We performed dual transcriptome sequencing (RNA-seq) and metabolic analysis on the biopsy samples. Network analyses between the host and bacterial transcriptomes and the host transcriptome-metabolome network were used to identify molecules that may be important for the virulence of H. ducreyi in the human host. Our results suggest that the pustule is highly oxidative, contains both pro- and anti-inflammatory components, and causes metabolic shifts in the host, to which H. ducreyi adapts to survive. To our knowledge, this is the first study to integrate transcriptomic and metabolomic responses to a single bacterial pathogen in the human host.

Breaking down the cell wall: Still an attractive antibacterial strategy

Since the advent of penicillin, humans have known about and explored the phenomenon of bacterial inhibition via antibiotics. However, with changes in the global environment and the abuse of antibiotics, resistance mechanisms have been selected in bacteria, presenting huge threats and challenges to the global medical and health system. Thus, the study and development of new antimicrobials is of unprecedented urgency and difficulty. Bacteria surround themselves with a cell wall to maintain cell rigidity and protect against environmental insults. Humans have taken advantage of antibiotics to target the bacterial cell wall, yielding some of the most widely used antibiotics to date. The cell wall is essential for bacterial growth and virulence but is absent from humans, remaining a high-priority target for antibiotic screening throughout the antibiotic era. Here, we review the extensively studied targets, i.e., MurA, MurB, MurC, MurD, MurE, MurF, Alr, Ddl, MurI, MurG, lipid A, and BamA in the cell wall, starting from the very beginning to the latest developments to elucidate antimicrobial screening. Furthermore, recent advances, including MraY and MsbA in peptidoglycan and lipopolysaccharide, and tagO, LtaS, LspA, Lgt, Lnt, Tol-Pal, MntC, and OspA in teichoic acid and lipoprotein, have also been profoundly discussed. The review further highlights that the application of new methods such as macromolecular labeling, compound libraries construction, and structure-based drug design will inspire researchers to screen ideal antibiotics.

Roles of the Tol/Pal System in Bacterial Pathogenesis and Its Application to Antibacterial Therapy

The Tol/Pal system (also written as “The Tol-Pal system”) is a set of protein complexes produced by most Gram-negative bacteria. It comprises the inner membrane-associated and the outer membrane-anchored subunits composed of the TolA, TolQ, and TolR proteins and the TolB and Pal proteins, respectively. Although the Tol/Pal system was first defined as bacterial proteins involved in colicin uptake of Escherichia coli, its global roles have been characterized in several studies as mentioned in this article. Pathogenesis of many Gram-negative pathogens is sustained by the Tol/Pal system. It is also essential for cell growth and fitness in some pathogens. Therefore, the Tol/Pal system is proposed as a potential target for antimicrobial chemotherapy. Although the tol/pal mutants are low in virulence, they still have the ability to stimulate the immune system. The Pal protein is highly immunogenic and induces both adaptive and innate immune responses. Therefore, the tol/pal mutant strains and Pal proteins also have potential vaccine properties. For these reasons, the Tol/Pal system represents a promising research target in the development of antibacterial therapeutic strategies for refractory infections caused by multi-drug-resistant (MDR), Gram-negative pathogens. In this paper, we summarize studies on the Tol/Pal system associated with bacterial pathogenesis and vaccine development.

The multifarious roles of Tol-Pal in Gram-negative bacteria

In the 1960s several groups reported the isolation and preliminary genetic mapping of Escherichia coli strains tolerant towards the action of colicins. These pioneering studies kick-started two new fields in bacteriology; one centred on how bacteriocins like colicins exploit the Tol (or more commonly Tol-Pal) system to kill bacteria, the other on the physiological role of this cell envelope-spanning assembly. The following half century has seen significant advances in the first of these fields whereas the second has remained elusive, until recently. Here, we review work that begins to shed light on Tol-Pal function in Gram-negative bacteria. What emerges from these studies is that Tol-Pal is an energised system with fundamental, interlinked roles in cell division – coordinating the re-structuring of peptidoglycan at division sites and stabilising the connection between the outer membrane and underlying cell wall. This latter role is achieved by Tol-Pal exploiting the proton motive force to catalyse the accumulation of the outer membrane peptidoglycan associated lipoprotein Pal at division sites while simultaneously mobilising Pal molecules from around the cell. These studies begin to explain the diverse phenotypic outcomes of tol-pal mutations, point to other cell envelope roles Tol-Pal may have and raise many new questions.

Interactions of the Skin Pathogen Haemophilus ducreyi With the Human Host

The obligate human pathogen Haemophilus ducreyi causes both cutaneous ulcers in children and sexually transmitted genital ulcers (chancroid) in adults. Pathogenesis is dependent on avoiding phagocytosis and exploiting the suppurative granuloma-like niche, which contains a myriad of innate immune cells and memory T cells. Despite this immune infiltrate, long-lived immune protection does not develop against repeated H. ducreyi infections—even with the same strain. Most of what we know about infectious skin diseases comes from naturally occurring infections and/or animal models; however, for H. ducreyi, this information comes from an experimental model of infection in human volunteers that was developed nearly three decades ago. The model mirrors the progression of natural disease and serves as a valuable tool to determine the composition of the immune cell infiltrate early in disease and to identify host and bacterial factors that are required for the establishment of infection and disease progression. Most recently, holistic investigation of the experimentally infected skin microenvironment using multiple “omics” techniques has revealed that non-canonical bacterial virulence factors, such as genes involved in central metabolism, may be relevant to disease progression. Thus, the immune system not only defends the host against H. ducreyi, but also dictates the nutrient availability for the invading bacteria, which must adapt their gene expression to exploit the inflammatory metabolic niche. These findings have broadened our view of the host-pathogen interaction network from considering only classical, effector-based virulence paradigms to include adaptations to the metabolic environment. How both host and bacterial factors interact to determine infection outcome is a current focus in the field. Here, we review what we have learned from experimental H. ducreyi infection about host-pathogen interactions, make comparisons to what is known for other skin pathogens, and discuss how novel technologies will deepen our understanding of this infection.

The Peptidoglycan-associated lipoprotein Pal contributes to the virulence of Burkholderia mallei and provides protection against lethal aerosol challenge

BURKHOLDERIA MALLEI

is a highly pathogenic bacterium that causes the fatal zoonosis glanders. The organism specifies multiple membrane proteins, which represent prime targets for the development of countermeasures given their location at the host-pathogen interface. We investigated one of these proteins, Pal, and discovered that it is involved in the ability of B. mallei to resist complement-mediated killing and replicate inside host cells in vitro, is expressed in vivo and induces antibodies during the course of infection, and contributes to virulence in a mouse model of aerosol infection. A mutant in the pal gene of the B. mallei wild-type strain ATCC 23344 was found to be especially attenuated, as BALB/c mice challenged with the equivalent of 5,350 LD50 completely cleared infection. Based on these findings, we tested the hypothesis that a vaccine containing the Pal protein elicits protective immunity against aerosol challenge. To achieve this, the pal gene was cloned in the vaccine vector Parainfluenza Virus 5 (PIV5) and mice immunized with the virus were infected with a lethal dose of B. mallei. These experiments revealed that a single dose of PIV5 expressing Pal provided 80% survival over a period of 40 days post-challenge. In contrast, only 10% of mice vaccinated with a PIV5 control virus construct survived infection. Taken together, our data establish that the Peptidoglycan-associated lipoprotein Pal is a critical virulence determinant of B. mallei and effective target for developing a glanders vaccine.

Roles of the Tol-Pal system in the Type III secretion system and flagella-mediated virulence in enterohemorrhagic Escherichia coli

The Tol-Pal system is a protein complex that is highly conserved in many gram-negative bacteria. We show here that the Tol-Pal system is associated with the enteric pathogenesis of enterohemorrhagic E. coli (EHEC). Deletion of tolB, which is required for the Tol-Pal system decreased motility, secretion of the Type III secretion system proteins EspA/B, and the ability of bacteria to adhere to and to form attaching and effacing (A/E) lesions in host cells, but the expression level of LEE genes, including espA/B that encode Type III secretion system proteins were not affected. The Citrobacter rodentium, tolB mutant, that is traditionally used to estimate Type III secretion system associated virulence in mice did not cause lethality in mice while it induced anti-bacterial immunity. We also found that the pal mutant, which lacks activity of the Tol-Pal system, exhibited lower motility and EspA/B secretion than the wild-type parent. These combined results indicate that the Tol-Pal system contributes to the virulence of EHEC associated with the Type III secretion system and flagellar activity for infection at enteric sites. This finding provides evidence that the Tol-Pal system may be an effective target for the treatment of infectious diseases caused by pathogenic E. coli.

The Tol-Pal System of Uropathogenic Escherichia coli Is Responsible for Optimal Internalization Into and Aggregation Within Bladder Epithelial Cells, Colonization of the Urinary Tract of Mice, and Bacterial Motility

Urinary tracts infection (UTI) caused by uropathogenic Escherichia coli (UPEC) is a common infectious disease. With the shortage of new antimicrobial agents, the increase in UPEC resistance to commonly used drugs, such as fluoroquinolones and β-lactams including carbapenems is a critical issue. UPEC invades urinary tract cells, where it aggregates, and subsequently, forms biofilm-like multicellular colonies termed intracellular bacterial communities (IBCs). This process allows the bacteria to establish infections and so may be a good potential target for new drugs to treat infections. Here, we show that deletion of the tolB gene, encoding a protein of the Tol-Pal system that was originally characterized as a protein complex for colicin uptake and maintenance of the outer membrane, decreases the level of bacterial internalization into and aggregation within cultured bladder epithelial cells and also inhibits the colonization of mice urinary tracts. The tolB mutant also exhibited defective motility because of impaired flagellum syntheses. The fliC and motA mutants, which are non-motile strains, also exhibited lower levels of bacterial internalization and aggregation than their wild-type parent. Additional deletion of tolB in the fliC mutant did not further decrease these, suggesting that the attenuated virulence of the tolB mutant is a result of defective motility. The tolA, tolQ, tolR, and pal mutants that lack other members of the Tol-Pal system also exhibited lower levels of motility and aggregation within bladder epithelial cells compared to their wild-type parent. These combined results suggest another role of the Tol-Pal system, i.e., that it is responsible for optimal internalization, aggregation followed by IBC formation within urinary tract cells, and bacterial motility.

A Class I Haemophilus ducreyi Strain Containing a Class II hgbA Allele Is Partially Attenuated in Humans: Implications for HgbA Vaccine Efficacy Trials

Haemophilus ducreyi causes chancroid and is a major cause of cutaneous ulcers in children. Due to environmental reservoirs, both class I and class II H. ducreyi strains persist in cutaneous ulcer regions of endemicity following mass drug administration of azithromycin, suggesting the need for a vaccine. The hemoglobin receptor (HgbA) is a leading vaccine candidate, but its efficacy in animal models is class specific. Controlled human infection models can be used to evaluate vaccines, but only a class I strain (35000HP) has been characterized in this model. As a prelude to evaluating HgbA vaccines in the human model, we tested here whether a derivative of 35000HP containing a class II hgbA allele (FX548) is as virulent as 35000HP in humans. In eight volunteers infected at three sites with each strain, the papule formation rate was 95.8% for 35000HP versus 62.5% for FX548 (P = 0.021). Excluding doses of FX548 that were ≥2-fold higher than those of 35000HP, the pustule formation rate was 25% for 35000HP versus 11.7% for FX548 (P = 0.0053). By Western blot analysis, FX548 and 35000HP expressed equivalent amounts of HgbA in whole-cell lysates and outer membranes. The growth of FX548 and 35000HP was similar in media containing hemoglobin or hemin. By whole-genome sequencing and single-nucleotide polymorphism analysis, FX548 contained no mutations in open reading frames other than hgbA We conclude that by an unknown mechanism, FX548 is partially attenuated in humans and is not a suitable strain for HgbA vaccine efficacy trials in the model.

Determination of an Interaction Network between an Extracellular Bacterial Pathogen and the Human Host

Dual RNA sequencing (RNA-seq) offers the promise of determining an interactome at a transcriptional level between a bacterium and the host but has yet to be done on any bacterial infection in human tissue. We performed dual RNA-seq and metabolomics analyses on wounded and infected sites following experimental infection of the arm with H. ducreyi. Our results suggest that H. ducreyi survives in an abscess by utilizing l-ascorbate as an alternative carbon source, possibly taking advantage of host ascorbic acid recycling, and that H. ducreyi also adapts by upregulating genes involved in anaerobic metabolism and inorganic ion and nutrient transport. To our knowledge, this is the first description of an interaction network between a bacterium and the human host at a site of infection.

A major gap in understanding infectious diseases is the lack of information about molecular interaction networks between pathogens and the human host. Haemophilus ducreyi causes the genital ulcer disease chancroid in adults and is a leading cause of cutaneous ulcers in children in the tropics. We developed a model in which human volunteers are infected on the upper arm with H. ducreyi until they develop pustules. To define the H. ducreyi and human interactome, we determined bacterial and host transcriptomic and host metabolomic changes in pustules. We found that in vivoH. ducreyi transcripts were distinct from those in the inocula, as were host transcripts in pustule and wounded control sites. Many of the upregulated H. ducreyi genes were found to be involved in ascorbic acid and anaerobic metabolism and inorganic ion/nutrient transport. The top 20 significantly expressed human pathways showed that all were involved in immune responses. We generated a bipartite network for interactions between host and bacterial gene transcription; multiple positively correlated networks contained H. ducreyi genes involved in anaerobic metabolism and host genes involved with the immune response. Metabolomic studies showed that pustule and wounded samples had different metabolite compositions; the top ion pathway involved ascorbate and aldarate metabolism, which correlated with the H. ducreyi transcriptional response and upregulation of host genes involved in ascorbic acid recycling. These data show that an interactome exists between H. ducreyi and the human host and suggest that H. ducreyi exploits the metabolic niche created by the host immune response.

Peptidoglycan-associated lipoprotein of Aggregatibacter actinomycetemcomitans induces apoptosis and production of proinflammatory cytokines via TLR2 in murine macrophages RAW 264.7 in vitro

Peptidoglycan-associated lipoprotein (PAL) is a conserved pro-inflammatory outer membrane lipoprotein in Gram-negative bacteria. Compared to systemic pathogens, little is known about the virulence properties of PAL in Aggregatibacter actinomycetemcomitans (AaPAL). The aims of this study were to investigate the cytolethality of AaPAL and its ability to induce pro-inflammatory cytokine production in macrophages. Mouse macrophages were stimulated with AaPAL, and the production of IL-1β, IL-6, TNF-α, and MCP-1 was measured after 6, 24, and 48 h. To investigate which receptor AaPAL employs for its interaction with macrophages, anti-toll-like receptor (TLR)2 and anti-TLR4 antibodies were used to block respective TLRs on macrophages. Metabolic activity and apoptosis of the macrophages were investigated after stimulation with AaPAL. AaPAL induced the production of MCP-1, TNF-α, IL-6, and IL-1β from mouse macrophages in order of decreasing abundance. The pre-treatment of macrophages with an anti-TLR2 antibody significantly diminished cytokine production. Under AaPAL stimulation, the metabolic activity of macrophages decreased in a dose- and time-dependent manner. Furthermore, AaPAL induced apoptosis in 56% of macrophages after 48 h of incubation. Our data suggest that AaPAL can kill macrophages by apoptosis. The results also emphasize the role of AaPAL as a potent pro-inflammatory agent in A. actinomycetemcomitans-associated infections.

Involvement of tolQ and tolR genes in Edwardsiella ictaluri virulence

Edwardsiella ictaluri is a Gram-negative intracellular facultative pathogen causing enteric septicemia of channel catfish (ESC). The Tol system, consisting of four envelope proteins TolQ, TolR, TolA, and TolB, are required for colicin import and contributes to bacterial virulence in several pathogenic bacteria. However, the Tol system and its importance in E. ictaluri virulence have not been investigated. Here we present construction and evaluation of the E. ictaluri TolQ, TolR and TolQR mutants (EiΔtolQ, EiΔtolR, and EiΔtolQR). The Tol mutants were developed using in-frame gene deletion and their attenuation and vaccine efficacy were determined in catfish fingerlings. The EiΔtolQ, EiΔtolR, and EiΔtolQR mutants showed reduced virulence in catfish (28.93%, 19.70%, and 39.82% mortality, respectively) compared to wild type (46.91% mortality). Further, vaccination with these mutants protected catfish against subsequent wild-type infection. This study suggests that the Tol system contributes to E. ictaluri virulence in catfish.

Outer membrane protein P4 is not required for virulence in the human challenge model of Haemophilus ducreyi infection

Background

Bacterial lipoproteins often play important roles in pathogenesis and can stimulate protective immune responses. Such lipoproteins are viable vaccine candidates. Haemophilus ducreyi, which causes the sexually transmitted disease chancroid, expresses a number of lipoproteins during human infection. One such lipoprotein, OmpP4, is homologous to the outer membrane lipoprotein e (P4) of H. influenzae. In H. influenzae, e (P4) stimulates production of bactericidal and protective antibodies and contributes to pathogenesis by facilitating acquisition of the essential nutrients heme and nicotinamide adenine dinucleotide (NAD). Here, we tested the hypothesis that, like its homolog, H. ducreyi OmpP4 contributes to virulence and stimulates production of bactericidal antibodies.

Results

We determined that OmpP4 is broadly conserved among clinical isolates of H. ducreyi. We next constructed and characterized an isogenic ompP4 mutant, designated 35000HPompP4, in H. ducreyi strain 35000HP. To test whether OmpP4 was necessary for virulence in humans, eight healthy adults were experimentally infected. Each subject was inoculated with a fixed dose of 35000HP on one arm and three doses of 35000HPompP4 on the other arm. The overall parent and mutant pustule formation rates were 52.4% and 47.6%, respectively (P = 0.74). These results indicate that expression of OmpP4 in not necessary for H. ducreyi to initiate disease or progress to pustule formation in humans. Hyperimmune mouse serum raised against purified, recombinant OmpP4 did not promote bactericidal killing of 35000HP or phagocytosis by J774A.1 mouse macrophages in serum bactericidal and phagocytosis assays, respectively.

Conclusions

Our data suggest that, unlike e (P4), H. ducreyi OmpP4 is not a suitable vaccine candidate. OmpP4 may be dispensable for virulence because of redundant mechanisms in H. ducreyi for heme acquisition and NAD utilization.

Activation of CpxRA in Haemophilus ducreyi primarily inhibits the expression of its targets, including major virulence determinants

Haemophilus ducreyi causes chancroid, a genital ulcer disease that facilitates the transmission of human immunodeficiency virus type 1. In humans, H. ducreyi is surrounded by phagocytes and must adapt to a hostile environment to survive. To sense and respond to environmental cues, bacteria frequently use two-component signal transduction (2CST) systems. The only obvious 2CST system in H. ducreyi is CpxRA; CpxR is a response regulator, and CpxA is a sensor kinase. Previous studies by Hansen and coworkers showed that CpxR directly represses the expression of dsrA, the lspB-lspA2 operon, and the flp operon, which are required for virulence in humans. They further showed that CpxA functions predominantly as a phosphatase in vitro to maintain the expression of virulence determinants. Since a cpxA mutant is avirulent while a cpxR mutant is fully virulent in humans, CpxA also likely functions predominantly as a phosphatase in vivo. To better understand the role of H. ducreyi CpxRA in controlling virulence determinants, here we defined genes potentially regulated by CpxRA by using RNA-Seq. Activation of CpxR by deletion of cpxA repressed nearly 70% of its targets, including seven established virulence determinants. Inactivation of CpxR by deletion of cpxR differentially regulated few genes and increased the expression of one virulence determinant. We identified a CpxR binding motif that was enriched in downregulated but not upregulated targets. These data reinforce the hypothesis that CpxA phosphatase activity plays a critical role in controlling H. ducreyi virulence in vivo. Characterization of the downregulated genes may offer new insights into pathogenesis.

Permeases of the sap transporter are required for cathelicidin resistance and virulence of Haemophilus ducreyi in humans

BACKGROUND

Haemophilus ducreyi encounters several classes of antimicrobial peptides (APs) in vivo and utilizes the sensitive-to-antimicrobial-peptides (Sap) transporter as one mechanism of AP resistance. A mutant lacking the periplasmic solute-binding component, SapA, was somewhat more sensitive to the cathelicidin LL-37 than the parent strain and was partially attenuated for virulence. The partial attenuation led us to question whether the transporter is fully abrogated in the sapA mutant.

METHODS

We generated a nonpolar sapBC mutant, which lacks both inner membrane permeases of the Sap transporter, and tested the mutant for virulence in human volunteers. In vitro, we compared LL-37 resistance phenotypes of the sapBC and sapA mutants.

RESULTS

Unlike the sapA mutant, the sapBC mutant was fully attenuated for virulence in human volunteers. In vitro, the sapBC mutant exhibited significantly greater sensitivity than the sapA mutant to killing by LL-37. Similar to the sapA mutant, the sapBC mutant did not affect H. ducreyi's resistance to human defensins.

CONCLUSIONS

Compared with the sapA mutant, the sapBC mutant exhibited greater attenuation in vivo, which directly correlated with increased sensitivity to LL-37 in vitro. These results strongly suggest that the SapBC channel retains activity when SapA is removed.

A novel protein refolding protocol for the solubilization and purification of recombinant peptidoglycan-associated lipoprotein from Xylella fastidiosa overexpressed in Escherichia coli

Xylella fastidiosa is a Gram-negative xylem-limited plant pathogenic bacterium responsible for several economically important crop diseases. Here, we present a novel and efficient protein refolding protocol for the solubilization and purification of recombinant X. fastidiosa peptidoglycan-associated lipoprotein (XfPal). Pal is an outer membrane protein that plays important roles in maintaining the integrity of the cell envelope and in bacterial pathogenicity. Because Pal has a highly hydrophobic N-terminal domain, the heterologous expression studies necessary for structural and functional protein characterization are laborious once the recombinant protein is present in inclusion bodies. Our protocol based on the denaturation of the XfPal-enriched inclusion bodies with 8M urea followed by buffer-exchange steps via dialysis proved effective for the solubilization and subsequent purification of XfPal, allowing us to obtain a large amount of relatively pure and folded protein. In addition, XfPal was biochemically and functionally characterized. The method for purification reported herein is valuable for further research on the three-dimensional structure and function of Pal and other outer membrane proteins and can contribute to a better understanding of the role of these proteins in bacterial pathogenicity, especially with regard to the plant pathogen X. fastidiosa.

Expression of the Flp proteins by Haemophilus ducreyi is necessary for virulence in human volunteers

Background

Haemophilus ducreyi, the causative agent of the sexually transmitted disease chancroid, contains a flp (fimbria like protein) operon that encodes proteins predicted to contribute to adherence and pathogenesis. H. ducreyi mutants that lack expression of Flp1 and Flp2 or TadA, which has homology to NTPases of type IV secretion systems, have decreased abilities to attach to and form microcolonies on human foreskin fibroblasts (HFF). A tadA mutant is attenuated in its ability to cause disease in human volunteers and in the temperature dependent rabbit model, but a flp1flp2 mutant is virulent in rabbits. Whether a flp deletion mutant would cause disease in humans is not clear.

Results

We constructed 35000HPΔflp1-3, a deletion mutant that lacks expression of all three Flp proteins but has an intact tad secretion system. 35000HPΔflp1-3 was impaired in its ability to form microcolonies and to attach to HFF in vitro when compared to its parent (35000HP). Complementation of the mutant with flp1-3 in trans restored the parental phenotype. To test whether expression of Flp1-3 was necessary for virulence in humans, ten healthy adult volunteers were experimentally infected with a fixed dose of 35000HP (ranging from 54 to 67 CFU) on one arm and three doses of 35000HPΔflp1-3 (ranging from 63 to 961 CFU) on the other arm. The overall papule formation rate for the parent was 80% (95% confidence interval, CI, 55.2%-99.9%) and for the mutant was 70.0% (95% CI, 50.5%-89.5%) (P = 0.52). Mutant papules were significantly smaller (mean, 11.2 mm²) than were parent papules (21.8 mm²) 24 h after inoculation (P = 0.018). The overall pustule formation rates were 46.7% (95% CI 23.7-69.7%) at 30 parent sites and 6.7% (95% CI, 0.1-19.1%) at 30 mutant sites (P = 0.001).

Conclusion

These data suggest that production and secretion of the Flp proteins contributes to microcolony formation and attachment to HFF cells in vitro. Expression of flp1-3 is also necessary for H. ducreyi to initiate disease and progress to pustule formation in humans. Future studies will focus on how Flp proteins contribute to microcolony formation and attachment in vivo.

A Haemophilus ducreyi CpxR deletion mutant is virulent in human volunteers

Haemophilus ducreyi 35000HP contains a homolog of the CpxRA 2-component signal transduction system, which controls the cell envelope stress response system in other gram-negative bacteria and regulates some important H. ducreyi virulence factors. A H. ducreyi cpxR mutant was compared with its parent for virulence in the human challenge model of experimental chancroid. The pustule formation rate in 5 volunteers was 33% (95% confidence interval [CI], 1.3%-65.3%) at 15 parent sites and 40% (95% CI, 18.1%-61.9%) at 15 mutant sites (P = .35). Thus, the cpxR mutant was not attenuated for virulence. Inactivation of the H. ducreyi cpxR gene did not reduce the ability of this mutant to express certain proven virulence factors, including the DsrA serum resistance protein and the LspA2 protein, which inhibits phagocytosis. These results expand our understanding of the involvement of the CpxRA system in regulating virulence expression in H. ducreyi.

A unique Coxiella burnetii lipoprotein involved in metal binding (LimB)

Coxiella burnetii is the bacterial agent of Q fever in humans. Here, we describe a unique, ~7.2 kDa, surface-exposed lipoprotein involved in metal binding which we have termed LimB. LimB was initially identified as a potential metal-binding protein on far-Western (FW) blots containing whole-cell lysate proteins when probed with nickel-coated horseradish peroxidase (Ni-HRP) and developed with a chemiluminescent HRP substrate. The corresponding identity of LimB as CBU1224a was established by matrix-assisted laser desorption ionization-tandem time-of-flight mass spectrometry. blast analyses with CBU1224a showed no significant similarity to sequences outside strains of C. burnetii. Additional in silico analyses revealed a putative 20 residue signal sequence with the carboxyl end demarcated by a potential lipobox (LSGC) whose Cys residue is predicted to serve as the N-terminal, lipidated Cys of mature LimB. The second residue of mature LimB is predicted to be Ala, an uncharged envelope localization residue. These features suggest that CBU1224a is synthesized as a prolipoprotein which is subsequently lipidated, secreted and anchored in the outer membrane. Mature LimB is predicted to contain 45 aa, of which there are 10 His and 5 Cys; both amino acids are frequently involved in binding transition metal cations. Recombinant LimB (rLimB) was generated and its Ni-HRP-binding activity demonstrated on FW blots. Ni-HRP binding by rLimB was inhibited by >95 % on FW blots done in the presence of EDTA, imidazole, Ni(2+) or Zn(2+), and roughly halved in the presence of Co(2+) or Fe(3+). The limB gene was maximally expressed at 3-7 days post-infection in Coxiella-infected Vero cells, coinciding with exponential phase growth. Two isoforms of LimB were detected on FW and Western blots, including a smaller (~7.2 kDa) species that was the predominant form in small cell variants and a larger isoform (~8.7 kDa) in large cell variants. LimB is Sarkosyl-insoluble, like many omps. The predicted surface location of LimB was verified by immunoelectron and immunofluorescence microscopy using anti-rLimB antibodies. Overall, the results suggest that LimB is a unique Coxiella lipoprotein that serves as a surface receptor for divalent metal cations and may play a role in acquiring at least one of these metals during intracellular growth.

Lipoprotein biosynthesis as a target for anti-Wolbachia treatment of filarial nematodes

BACKGROUND

Lymphatic filariasis and onchocerciasis are debilitating diseases caused by filarial nematodes. Disease pathogenesis is induced by inflammatory responses following the death of the parasite. Wolbachia endosymbionts of filariae are potent inducers of innate and adaptive inflammation and bacterial lipoproteins have been identified as the ligands that bind toll-like receptors (TLR) 2 and TLR6. Lipoproteins are important structural and functional components of bacteria and therefore enzymes involved in Wolbachia lipoprotein biosynthesis are potential chemotherapeutic targets.

RESULTS

Globomycin, a signal peptidase II (LspA) inhibitor, has activity against Gram-negative bacteria and a putative lspA gene has been identified from the Wolbachia genome of Brugia malayi (wBm). The amino acids required for function are strictly conserved and functionality was verified by complementation tests in a temperature-sensitive Escherichia coli lspA mutant. Also, transformation of wild type E. coli with Wolbachia lspA conferred significant globomycin resistance. A cell-based screen has been developed utilizing a Wolbachia-containing Aedes albopictus cell line to assay novel compounds active against Wolbachia. Globomycin was screened using this assay, which resulted in a dose-dependent reduction in Wolbachia load. Furthermore, globomycin was also effective in reducing the motility and viability of adult B. malayi in vitro.

CONCLUSIONS

These studies validate lipoprotein biosynthesis as a target in an organism for which no genetic tools are available. Further studies to evaluate drugs targeting this pathway are underway as part of the A-WOL drug discovery and development program.

Characterization of the CpxRA regulon in Haemophilus ducreyi

The Haemophilus ducreyi 35000HP genome encodes a homolog of the CpxRA two-component cell envelope stress response system originally characterized in Escherichia coli. CpxR, the cytoplasmic response regulator, was shown previously to be involved in repression of the expression of the lspB-lspA2 operon (M. Labandeira-Rey, J. R. Mock, and E. J. Hansen, Infect. Immun. 77:3402-3411, 2009). In the present study, the H. ducreyi CpxR and CpxA proteins were shown to closely resemble those of other well-studied bacterial species. A cpxA deletion mutant and a CpxR-overexpressing strain were used to explore the extent of the CpxRA regulon. DNA microarray and real-time reverse transcriptase (RT) PCR analyses indicated several potential regulatory targets for the H. ducreyi CpxRA two-component regulatory system. Electrophoretic mobility shift assays (EMSAs) were used to prove that H. ducreyi CpxR interacted with the promoter regions of genes encoding both known and putative virulence factors of H. ducreyi, including the lspB-lspA2 operon, the flp operon, and dsrA. Interestingly, the use of EMSAs also indicated that H. ducreyi CpxR did not bind to the promoter regions of several genes predicted to encode factors involved in the cell envelope stress response. Taken together, these data suggest that the CpxRA system in H. ducreyi, in contrast to that in E. coli, may be involved primarily in controlling expression of genes not involved in the cell envelope stress response.

Activation of the CpxRA system by deletion of cpxA impairs the ability of Haemophilus ducreyi to infect humans

Haemophilus ducreyi must adapt to the environment of the human host to establish and maintain infection in the skin. Bacteria generally utilize stress response systems, such as the CpxRA two-component system, to adapt to hostile environments. CpxRA is the only obvious two-component system contained in the H. ducreyi genome and negatively regulates the lspB-lspA2 operon, which encodes proteins that enable the organism to resist phagocytosis. We constructed an unmarked, in-frame H. ducreyi cpxA deletion mutant, 35000HPDeltacpxA. In human inoculation experiments, 35000HPDeltacpxA formed papules at a rate and size that were significantly less than its parent and was unable to form pustules compared to the parent. CpxA usually has kinase and phosphatase activities for CpxR, and the deletion of CpxA leads to the accumulation of activated CpxR due to the loss of phosphatase activity and the ability of CpxR to accept phosphate groups from other donors. Using a reporter construct, the lspB-lspA2 promoter was downregulated in 35000HPDeltacpxA, confirming that CpxR was activated. Deletion of cpxA downregulated DsrA, the major determinant of serum resistance in the organism, causing the mutant to become serum susceptible. Complementation in trans restored parental phenotypes. 35000HPDeltacpxA is the first H. ducreyi mutant that is impaired in its ability to form both papules and pustules in humans. Since a major function of CpxRA is to control the flow of protein traffic across the periplasm, uncontrolled activation of this system likely causes dysregulated expression of multiple virulence determinants and cripples the ability of the organism to adapt to the host.

Role played by CD4+FOXP3+ regulatory T Cells in suppression of host responses to Haemophilus ducreyi during experimental infection of human volunteers

Haemophilus ducreyi causes chancroid, a genital ulcer disease. Among human volunteers, the majority of experimentally infected individuals fail to clear the infection and form pustules. Here, we investigated the role played by CD4(+)FOXP3(+) regulatory T (T(reg)) cells in the formation of pustules. In pustules, there was a significant enrichment of CD4(+)FOXP3(+) T cells, compared with that in peripheral blood. The majority of lesional FOXP3(+) T cells were CD4(+), CD25(+), CD127(lo/-), and CTLA-4(+). FOXP3(+) T cells were found throughout pustules but were most abundant at their base. Significantly fewer lesional CD4(+)FOXP3(+) T cells expressed interferon gamma, compared with lesional CD4(+)FOXP3(-) effector T cells. Depletion of CD4(+)CD25(+) T cells from the peripheral blood of infected and uninfected volunteers significantly enhanced proliferation of H. ducreyi-reactive CD4(+) T cells. Our results indicate that the population of CD4(+)CD25(+)CD127(lo/-)FOXP3(+) T(reg) cells are expanded at H. ducreyi-infected sites and that these cells may play a role in suppressing the host immune response to the bacterium.

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.

A comprehensive proteome map of the Haemophilus parasuis serovar 5

Haemophilus parasuis is the causative agent of Glässer's disease of pigs, a disease associated with fibrinous polyserositis, polyarthritis and meningitis. Systematic reference maps of outer membrane, intracellular and extracellular proteome fractions of the clinical isolate H. parasuis SH0165 were examined by 2-DE coupled with MALDI-TOF MS. A total of 539 proteins spots were successfully identified, corresponding to 317 different proteins that were classified into functional categories. The majority of these proteins were linked to housekeeping functions in amino acid transport and metabolism, secondary metabolites biosynthesis, transport and catabolism and post-translational modification, protein turnover and chaperones. A significant number of outer membrane proteins were identified, such as Wza, Omp2, Omp5, D15 and PalA, which were supposed to play important roles in basic physiology of H. parasuis. In addition, several virulence-associated proteins involved in type I (TolC), type III (DsbA and DsbC) and type V (Autotransporter adhesins) secretion systems, and solute-binding proteins participating in iron-uptake systems were also identified in the present study.

Experimental infection of human volunteers with Haemophilus ducreyi: fifteen years of clinical data and experience

Haemophilus ducreyi causes chancroid, which facilitates transmission of human immunodeficiency virus type 1. To better understand the biology of H. ducreyi, we developed a human inoculation model. In the present article, we describe clinical outcomes for 267 volunteers who were infected with H. ducreyi. There was a relationship between papule formation and estimated delivered dose. The outcome (either pustule formation or resolution) of infected sites for a given subject was not independent; the most important determinants of pustule formation were sex and host effects. When 41 subjects were infected a second time, their outcomes segregated toward their initial outcome, confirming the host effect. Subjects with pustules developed local symptoms that required withdrawal from the study after a mean of 8.6 days. There were 191 volunteers who had tissue biopsy performed, 173 of whom were available for follow-up analysis; 28 (16.2%) of these developed hypertrophic scars, but the model was otherwise safe. Mutant-parent trials confirmed key features in H. ducreyi pathogenesis, and the model has provided an opportunity to study differential human susceptibility to a bacterial infection.

Peptidoglycan-associated lipoprotein (Pal) of Gram-negative bacteria: function, structure, role in pathogenesis and potential application in immunoprophylaxis

The protein Pal (peptidoglycan-associated lipoprotein) is anchored in the outer membrane (OM) of Gram-negative bacteria and interacts with Tol proteins. Tol-Pal proteins form two complexes: the first is composed of three inner membrane Tol proteins (TolA, TolQ and TolR); the second consists of the TolB and Pal proteins linked to the cell's OM. These complexes interact with one another forming a multiprotein membrane-spanning system. It has recently been demonstrated that Pal is essential for bacterial survival and pathogenesis, although its role in virulence has not been clearly defined. This review summarizes the available data concerning the structure and function of Pal and its role in pathogenesis.

A fibrinogen-binding lipoprotein contributes to the virulence of Haemophilus ducreyi in humans

A gene expression study of Haemophilus ducreyi identified the hypothetical lipoprotein HD0192, renamed here "fibrinogen binder A" (FgbA), as being preferentially expressed in vivo. To test the role played by fgbA in virulence, an isogenic fgbA mutant (35000HPfgbA) was constructed using H. ducreyi 35000HP, and 6 volunteers were experimentally infected with 35000HP or 35000HPfgbA. The overall pustule-formation rate was 61.1% at parent sites and 22.2% at mutant sites (P = .019). Papules were significantly smaller at mutant sites than at parent sites (13.3 vs. 37.9 mm(2); P = .002) 24 h after inoculation. Thus, fgbA contributed significantly to the virulence of H. ducreyi in humans. In vitro experiments demonstrated that fgbA encodes a fibrinogen-binding protein; no other fibrinogen-binding proteins were identified in 35000HP. fgbA was conserved among clinical isolates of both class I and II H. ducreyi strains, supporting the finding that fgbA is important for H. ducreyi infection.

Deletion of tolA in Salmonella Typhimurium generates an attenuated strain with vaccine potential

The Gram-negative Tol-Pal system of envelope proteins plays a key role in maintaining outer membrane integrity and contributes to the virulence of several pathogens. We have investigated the role of one of these proteins, TolA, in the biology of Salmonella enterica serovar Typhimurium. Deletion of tolA rendered strain SL1344 more susceptible to killing by bile and human serum. In addition the mutant had impaired membrane integrity and displayed alterations in LPS production. The tolA mutant was highly attenuated in mouse infections via the oral and intravenous routes. Importantly, each phenotype displayed by the mutant was complemented by provision of tolA in trans. The tolA gene therefore contributes to virulence, membrane integrity, LPS production and bile and serum resistance in S. enterica serovar Typhimurium SL1344. Finally, immunization with the tolA mutant provided significant protection against subsequent challenge with wild-type SL1344. The Tol-Pal system is therefore a potential target in the development of novel attenuated live vaccines against Salmonella and other Gram-negative pathogens.

The enterobacterial common antigen-like gene cluster of Haemophilus ducreyi contributes to virulence in humans

Haemophilus ducreyi 35000HP contains a cluster of homologues of genes required for the synthesis of enterobacterial common antigen (ECA), suggesting that H. ducreyi may express a putative ECA-like glycoconjugate. WecA initiates the synthesis of ECA by transferring N-acetylglucosamine to undecaprenyl-P, to form lipid I. A wecA mutant (35000HPwecA) was constructed, and 5 volunteers were inoculated at 3 sites with fixed doses of 35000HP on one arm and at 3 sites with varying doses of 35000HPwecA on the other arm. 35000HPwecA caused pustules to form at 3 sites inoculated with a dose 2.5-fold higher than that of 35000HP. However, at sites inoculated with similar doses of 35000HP and 35000HPwecA, pustules developed at 46.7% (95% confidence interval [CI], 23.3%-70.0%) of 15 parent-strain sites and at 8.3% (95% CI, 0.01%-23.6%) of 12 mutant-strain sites (P = .013). Thus, the expression of wecA contributes to the ability of H. ducreyi to cause pustules in humans.

Transposon mutagenesis reveals differential pathogenesis of Ralstonia solanacearum on tomato and Arabidopsis

Ralstonia solanacearum causes a deadly wilting disease on a wide range of crops. To elucidate pathogenesis of this bacterium in different host plants, we set out to identify R. solanacearum genes involved in pathogenesis by screening random transposon insertion mutants of a highly virulent strain, Pss190, on tomato and Arabidopsis thaliana. Mutants exhibiting various decreased virulence levels on these two hosts were identified. Sequence analysis showed that most, but not all, of the identified pathogenesis genes are conserved among distinct R. solanacearum strains. A few of the disrupted loci were not reported previously as being involved in R. solanacearum pathogenesis. Notably, a group of mutants exhibited differential pathogenesis on tomato and Arabidopsis. These results were confirmed by characterizing allelic mutants in one other R. solanacearum strain of the same phylotype. The significantly decreased mutants' colonization in Arabidopsis was found to be correlated with differential pathogenesis on these two plants. Differential requirement of virulence genes suggests adaptation of this bacterium in different host environments. Together, this study reveals commonalities and differences of R. solanacearum pathogenesis on single solanaceous and nonsolanaceous hosts, and provides important new insights into interactions between R. solanacearum and different host plants.

Haemophilus ducreyi LspA proteins are tyrosine phosphorylated by macrophage-encoded protein tyrosine kinases

The LspA proteins (LspA1 and LspA2) of Haemophilus ducreyi are necessary for this pathogen to inhibit the phagocytic activity of macrophage cell lines, an event that can be correlated with a reduction in the level of active Src family protein tyrosine kinases (PTKs) in these eukaryotic cells. During studies investigating this inhibitory mechanism, it was discovered that the LspA proteins themselves were tyrosine phosphorylated after wild-type H. ducreyi cells were incubated with macrophages. LspA proteins in cell-free concentrated H. ducreyi culture supernatant fluid could also be tyrosine phosphorylated by macrophages. This ability to tyrosine phosphorylate the LspA proteins was not limited to immune cell lineages but could be accomplished by both HeLa and COS-7 cells. Kinase inhibitor studies with macrophages demonstrated that the Src family PTKs were required for this tyrosine phosphorylation activity. In silico methods and site-directed mutagenesis were used to identify EPIYG and EPVYA motifs in LspA1 that contained tyrosines that were targets for phosphorylation. A total of four tyrosines could be phosphorylated in LspA1, with LspA2 containing eight predicted tyrosine phosphorylation motifs. Purified LspA1 fusion proteins containing either the EPIYG or EPVYA motifs were shown to be phosphorylated by purified Src PTK in vitro. Macrophage lysates could also tyrosine phosphorylate the LspA proteins and an LspA1 fusion protein via a mechanism that was dependent on the presence of both divalent cations and ATP. Several motifs known to interact with or otherwise affect eukaryotic kinases were identified in the LspA proteins.

Evaluation of the repertoire of the TonB-dependent receptors of Haemophilus ducreyi for their role in virulence in humans

Haemophilus ducreyi contains 3 TonB-dependent receptors: the hemoglobin receptor HgbA, which is required for virulence in humans; the heme receptor TdhA; and an uncharacterized conserved hypothetical protein TdX (HD0646). A double tdX/tdhA mutant (FX527) was constructed on the background of a human-passaged variant of strain 35000 (35000HP). Six volunteers were infected with 35000HP at 3 sites on one arm and with FX527 at 3 sites on the other. The pustule formation rate was 55.6% (95% confidence interval [CI], 35.7%-75.4%) at 18 parent-strain sites and 44.4% (95% CI, 15.0%-73.9%) at 18 mutant-strain sites (P = .51). Similar amounts of 35000HP and FX527 were recovered from pustules in semiquantitative culture. Thus, TdX and TdhA are not necessary for virulence, whereas HgbA is both necessary and sufficient for virulence in humans. The data suggest that hemoglobin is the sole source of heme/iron used by H. ducreyi in vivo and has implications for the potential of HgbA as a vaccine.

Identification of Haemophilus ducreyi genes expressed during human infection

To identify Haemophilus ducreyi transcripts that are expressed during human infection, we used selective capture of transcribed sequences (SCOTS) with RNA isolated from pustules obtained from three volunteers infected with H. ducreyi, and with RNA isolated from broth-grown bacteria used to infect volunteers. With SCOTS, competitive hybridization of tissue-derived and broth-derived sequences identifies genes that may be preferentially expressed in vivo. Among the three tissue specimens, we identified 531 genes expressed in vivo. Southern blot analysis of 60 genes from each tissue showed that 87 % of the identified genes hybridized better with cDNA derived from tissue specimens than with cDNA derived from broth-grown bacteria. RT-PCR on nine additional pustules confirmed in vivo expression of 10 of 11 selected genes in other volunteers. Of the 531 genes, 139 were identified in at least two volunteers. These 139 genes fell into several functional categories, including biosynthesis and metabolism, regulation, and cellular processes, such as transcription, translation, cell division, DNA replication and repair, and transport. Detection of genes involved in anaerobic and aerobic respiration indicated that H. ducreyi likely encounters both microenvironments within the pustule. Other genes detected suggest an increase in DNA damage and stress in vivo. Genes involved in virulence in other bacterial pathogens and 32 genes encoding hypothetical proteins were identified, and may represent novel virulence factors. We identified three genes, lspA1, lspA2 and tadA, known to be required for virulence in humans. This is the first study to broadly define transcripts expressed by H. ducreyi in humans.

Vesicle-independent extracellular release of a proinflammatory outer membrane lipoprotein in free-soluble form

Background

Aggregatibacter actinomycetemcomitans is an oral bacterium associated with aggressively progressing periodontitis. Extracellular release of bacterial outer membrane proteins has been suggested to mainly occur via outer membrane vesicles. This study investigated the presence and conservation of peptidoglycan-associated lipoprotein (AaPAL) among A. actinomycetemcomitans strains, the immunostimulatory effect of AaPAL, and whether live cells release this structural outer membrane lipoprotein in free-soluble form independent of vesicles.

Results

The pal locus and its gene product were confirmed in clinical A. actinomycetemcomitans strains by PCR-restriction fragment length polymorphism and immunoblotting. Culturing under different growth conditions revealed no apparent requirement for the AaPAL expression. Inactivation of pal in a wild-type strain (D7S) and in its spontaneous laboratory variant (D7SS) resulted in pleiotropic cellular effects. In a cell culture insert model (filter pore size 0.02 μm), AaPAL was detected from filtrates when strains D7S and D7SS were incubated in serum or broth in the inserts. Electron microscopy showed that A. actinomycetemcomitans vesicles (0.05–0.2 μm) were larger than the filter pores and that there were no vesicles in the filtrates. The filtrates were immunoblot negative for a cytoplasmic marker, cyclic AMP (cAMP) receptor protein. An ex vivo model indicated cytokine production from human whole blood stimulated by AaPAL.

Conclusion

Free-soluble AaPAL can be extracellularly released in a process independent of vesicles.

Analysis of whole cell lysate from the intercellular bacterium Coxiella burnetii using two gel-based protein separation techniques

Coxiella burnetii, the causative agent of Q fever, is an obligate intracellular gamma-proteobacterium, which replicates within large phagolysosome-like compartments formed in the host cell. The global protein profile of intracellular C. burnetii strain Nine Mile phase II was analyzed by two gel-based approaches coupled to MALDI-TOF MS. Colloidal Coomassie brilliant blue-stained 2-DE gels at the pH range 3-10 resolved over 600 protein spots and 125 spots in doubled-SDS-PAGE gels. Mass spectra obtained for each trypsin-digested protein-spot were compared to the C. burnetii genome database, and a total number of 185 different C. burnetii proteins were identified by both techniques. 2-DE in combination with MALDI-TOF MS, as a high-throughput method, allowed the identification of 172 proteins. On the other hand, the application of doubled-SDS-PAGE allowed the identification of 38 proteins, with some of them being very alkaline and membrane proteins not identified in the 2-DE approach. Most identified proteins were predicted to be involved in metabolism and biosynthesis. Several identified proteins are speculated to have a distinct and vital role in the pathogenesis and survival of C. burnetii within the harsh phagolysosomal environment.

Experimental infection with Haemophilus ducreyi in persons who are infected with HIV does not cause local or augment systemic viral replication

We infected 11 HIV-seropositive volunteers whose CD4(+) cell counts were >350 cells/ microL (7 of whom were receiving antiretrovirals) with Haemophilus ducreyi. The papule and pustule formation rates were similar to those observed in HIV-seronegative historical control subjects. No subject experienced a sustained change in CD4(+) cell count or HIV RNA level. The cellular infiltrate in biopsy samples obtained from the HIV-seropositive and HIV-seronegative subjects did not differ with respect to the percentage of leukocytes, neutrophils, macrophages, or T cells. The CD4(+):CD8(+) cell ratio in biopsy samples from the HIV-seropositive subjects was 1:3, the inverse of the ratio seen in the HIV-seronegative subjects (P<.0001). Although CD4(+) cells proliferated in lesions, in situ hybridization and reverse-transcription polymerase chain reaction for HIV RNA was negative. We conclude that experimental infection in HIV-seropositive persons is clinically similar to infection in HIV-seronegative persons and does not cause local or augment systemic viral replication. Thus, prompt treatment of chancroid may abrogate increases in viral replication associated with natural disease.

Complete genome sequence of Haemophilus somnus (Histophilus somni) strain 129Pt and comparison to Haemophilus ducreyi 35000HP and Haemophilus influenzae Rd

Haemophilus somnus can be either a commensal of bovine mucosal surfaces or an opportunistic pathogen. Pathogenic strains of H. somnus are a significant cause of systemic disease in cattle. We report the genome sequence of H. somnus 129Pt, a nonpathogenic commensal preputial isolate, and the results of a genome-wide comparative analysis of H. somnus 129Pt, Haemophilus influenzae Rd, and Haemophilus ducreyi 35000HP. We found unique genes in H. somnus 129Pt involved in lipooligosaccharide biosynthesis, carbohydrate uptake and metabolism, cation transport, amino acid metabolism, ubiquinone and menaquinone biosynthesis, cell surface adhesion, biosynthesis of cofactors, energy metabolism, and electron transport. There were also many genes in common among the three organisms. Our comparative analyses of H. somnus 129Pt, H. influenzae Rd, and H. ducreyi 35000HP revealed similarities and differences in the numbers and compositions of genes involved in metabolism, host colonization, and persistence. These results lay a foundation for research on the host specificities and niche preferences of these organisms. Future comparisons between H. somnus 129Pt and virulent strains will aid in the development of protective strategies and vaccines to protect cattle against H. somnus disease.

Construction of a mutant and characterization of the role of the vaccine antigen P6 in outer membrane integrity of nontypeable Haemophilus influenzae

Outer membrane protein P6 is the subject of investigation as a vaccine antigen to prevent infections caused by nontypeable Haemophilus influenzae, which causes otitis media in children and respiratory tract infections in adults with chronic lung disease. P6 induces protective immune responses in animal models and is the target of potentially protective immune responses in humans. P6 is a 16-kDa lipoprotein that shares homology with the peptidoglycan-associated lipoproteins of gram-negative bacteria and is highly conserved among strains of H. influenzae. To characterize the function of P6, an isogenic mutant was constructed by replacing the P6 gene with a chloramphenicol resistance cassette. The P6 mutant showed altered colony morphology and slower growth in vitro than that of the parent strain. By electron microscopy, the P6 mutant cells demonstrated increased size, variability in size, vesicle formation, and fragility compared to the parent cells. The P6 mutant showed hypersensitivity to selected antibiotics with different mechanisms of action, indicating increased accessibility of the agents to their targets. The P6 mutant was more sensitive to complement-mediated killing by normal human serum. Complementation of the mutation in trans completely or partially restored the phenotypes. We concluded that P6 plays a structural role in maintaining the integrity of the outer membrane by anchoring the outer membrane to the cell wall. The observation that the absence of expression of P6 is detrimental to the cell is a highly desirable feature for a vaccine antigen, supporting further investigation of P6 as a vaccine candidate for H. influenzae.

Immunoproteomics of Actinobacillus actinomycetemcomitans outer-membrane proteins reveal a highly immunoreactive peptidoglycan-associated lipoprotein

In a search for novel bioactive cell surface structures of periodontal pathogens, it was found that sera from two patients with Actinobacillus actinomycetemcomitans-associated infections reacted strongly at 17 kDa on immunoblots of A. actinomycetemcomitans outer-membrane protein (OMP) preparations. The 17 kDa antigen was also recognized by anti-CsgA (Escherichia coli curli major subunit) antibody. The 17 kDa A. actinomycetemcomitans protein was identified as peptidoglycan-associated lipoprotein (PAL; AaPAL) by two-dimensional immunoblotting and subsequent sequence analysis by mass spectrometry and bioinformatics tools. AaPAL was an OMP and a lipoprotein, and it had an OmpA-like domain. In a group of middle-aged subjects (n = 26), serum reactivity to AaPAL was associated with the presence of periodontitis but not with the oral detection of A. actinomycetemcomitans. Both human sera and rabbit antisera against three different types of antigens, the gel-purified AaPAL, A. actinomycetemcomitans whole-cell antigens, and CsgA, recognized putative PALs of oral haemophili in addition to AaPAL. The results demonstrated that the novel AaPAL is a conserved bacterial lipoprotein. It is expressed in vivo and is strongly immunoreactive. The antigenic cross-reactivity found between AaPAL and oral haemophili may enhance local and systemic immuno-inflammatory reactions in periodontitis.

Expression of Haemophilus ducreyi collagen binding outer membrane protein NcaA is required for virulence in swine and human challenge models of chancroid

Haemophilus ducreyi, the etiologic agent of the sexually transmitted genital ulcer disease chancroid, has been shown to associate with dermal collagen fibers within infected skin lesions. Here we describe NcaA, a previously uncharacterized outer membrane protein that is important for H. ducreyi collagen binding and host colonization. An H. ducreyi strain lacking the ncaA gene was impaired in adherence to type I collagen but not fibronectin (plasma or cellular form) or heparin. The mutation had no effect on serum resistance or binding to HaCaT keratinocytes or human foreskin fibroblasts in vitro. Escherichia coli expressing H. ducreyi NcaA bound to type I collagen, demonstrating that NcaA is sufficient to confer collagen attachment. The importance of NcaA in H. ducreyi pathogenesis was assessed using both swine and human experimental models of chancroid. In the swine model, 20% of lesions from sites inoculated with the ncaA mutant were culture positive for H. ducreyi 7 days after inoculation, compared to 73% of wild-type-inoculated sites. The average number of CFU recovered from mutant-inoculated lesions was also significantly reduced compared to that recovered from wild-type-inoculated sites at both 2 and 7 days after inoculation. In the human challenge model, 8 of 30 sites inoculated with wild-type H. ducreyi progressed to the pustular stage, compared to 0 of 30 sites inoculated with the ncaA mutant. Together these results demonstrate that the collagen binding protein NcaA is required for H. ducreyi infection.

A DltA mutant of Haemophilus ducreyi Is partially attenuated in its ability to cause pustules in human volunteers

Haemophilus ducreyi produces two outer membrane proteins, called DltA (H. ducreyi lectin A) and DsrA (H. ducreyi serum resistance A), that contribute to the ability of the organism to evade complement-mediated serum killing. In contrast to their isogenic parent strain, 35000HP, the DsrA mutant FX517 exhibits 0% survival in 50% normal human serum and the DltA mutant FX533 exhibits 23% survival. Compared to 35000HP, FX517 does not cause pustule formation in human volunteers. To test whether DltA was required for virulence in humans, seven volunteers were experimentally infected with 35000HP and FX533. Four subjects were inoculated with fixed doses of 35000HP (101 CFU or 130 CFU) at three sites on one arm and escalating doses of FX533 (range, 46 CFU to 915 CFU) at three sites on the other arm. Pustules only developed at mutant-injected sites at doses nearly twofold higher than that of the parent, suggesting that FX533 was partially attenuated. Three subjects were inoculated with similar doses of the parent (67 CFU) and mutant (104 CFU) at three sites. Pustules formed at five of nine parent sites and one of nine mutant sites. Overall, the papule and pustule formation rates for 35000HP and FX533 were similar for the trial. However, for the five subjects who received similar doses of the parent and mutant, pustules developed at 7 of 15 sites (46.7%; 95% confidence interval [CI], 16.9% to 76.5%) inoculated with the parent and at 1 of 15 (6.7%; 95% CI, 0.1% to 18.4%) sites inoculated with the mutant (P = 0.043). We concluded that the DltA mutant was attenuated in its ability to cause disease at doses similar to that of the parent.

Expression of OmpP2A and OmpP2B is not required for pustule formation by Haemophilus ducreyi in human volunteers

Haemophilus ducreyi express two porin proteins, termed OmpP2A and OmpP2B. To test whether expression of OmpP2A and OmpP2B was necessary for virulence in humans, eight volunteers were experimentally infected with the parent (35000HP) in one arm and a double OmpP2A OmpP2B mutant (35000HP::P2AB) in the other arm. The pustule formation rates were 58.3% (95% CI, 33.2-83.5%) for the parent and 41.7% (95% CI, 19.3-64.0%) for the mutant (P=0.25). Biopsy of 35000HP and 35000HP::P2AB-infected sites yielded similar amounts of bacteria in quantitative culture. These results indicate that expression of OmpP2A and OmpP2B is not necessary to initiate disease or to progress to pustule formation in humans.

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.

Tol-Pal proteins are critical cell envelope components of Erwinia chrysanthemi affecting cell morphology and virulence

The tol-pal genes are necessary for maintaining the outer-membrane integrity of Gram-negative bacteria. These genes were first described in Escherichia coli, and more recently in several other species. They are involved in the pathogenesis of E. coli, Haemophilus ducreyi, Vibrio cholerae and Salmonella enterica. The role of the tol-pal genes in bacterial pathogenesis was investigated in the phytopathogenic enterobacterium Erwinia chrysanthemi, assuming that this organism might be a good model for such a study. The whole Er. chrysanthemi tol-pal region was characterized. Tol-Pal proteins, except TolA, showed high identity scores with their E. coli homologues. Er. chrysanthemi mutants were constructed by introducing a uidA-kan cassette in the ybgC, tolQ, tolA, tolB, pal and ybgF genes. All the mutants were hypersensitive to bile salts. Mutations in tolQ, tolA, tolB and pal were deleterious for the bacteria, which required high concentrations of sugars or osmoprotectants for their viability. Consistent with this observation, they were greatly impaired in their cell morphology and division, which was evidenced by observations of cell filaments, spherical forms, membrane blebbing and mislocalized bacterial septa. Moreover, tol-pal mutants showed a reduced virulence in a potato tuber model and on chicory leaves. This could be explained by a combination of impaired phenotypes in the tol-pal mutants, such as reduced growth and motility and a decreased production of pectate lyases, the major virulence factor of Er. chrysanthemi.

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.

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.