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

The regulation of bacterial two-partner secretion systems

Two-partner secretion (TPS) systems, also known as Type Vb secretion systems, allow the translocation of effector proteins across the outer membrane of Gram-negative bacteria. By secreting different classes of effectors, including cytolysins and adhesins, TPS systems play important roles in bacterial pathogenesis and host interactions. Here, we review the current knowledge on TPS systems regulation and highlight specific and common regulatory mechanisms across TPS functional classes. We discuss in detail the specific regulatory networks identified in various bacterial species and emphasize the importance of understanding the context-dependent regulation of TPS systems. Several regulatory cues reflecting host environment during infection, such as temperature and iron availability, are common determinants of expression for TPS systems, even across relatively distant species. These common regulatory pathways often affect TPS systems across subfamilies with different effector functions, representing conserved global infection-related regulatory mechanisms.

Comparative Genomic Analysis of the DUF34 Protein Family Suggests Role as a Metal Ion Chaperone or Insertase

Members of the DUF34 (domain of unknown function 34) family, also known as the NIF3 protein superfamily, are ubiquitous across superkingdoms. Proteins of this family have been widely annotated as "GTP cyclohydrolase I type 2" through electronic propagation based on one study. Here, the annotation status of this protein family was examined through a comprehensive literature review and integrative bioinformatic analyses that revealed varied pleiotropic associations and phenotypes. This analysis combined with functional complementation studies strongly challenges the current annotation and suggests that DUF34 family members may serve as metal ion insertases, chaperones, or metallocofactor maturases. This general molecular function could explain how DUF34 subgroups participate in highly diversified pathways such as cell differentiation, metal ion homeostasis, pathogen virulence, redox, and universal stress responses.

Maintaining Integrity Under Stress: Envelope Stress Response Regulation of Pathogenesis in Gram-Negative Bacteria

The Gram-negative bacterial envelope is an essential interface between the intracellular and harsh extracellular environment. Envelope stress responses (ESRs) are crucial to the maintenance of this barrier and function to detect and respond to perturbations in the envelope, caused by environmental stresses. Pathogenic bacteria are exposed to an array of challenging and stressful conditions during their lifecycle and, in particular, during infection of a host. As such, maintenance of envelope homeostasis is essential to their ability to successfully cause infection. This review will discuss our current understanding of the σ^E- and Cpx-regulated ESRs, with a specific focus on their role in the virulence of a number of model pathogens.

Contribution of the Cpx envelope stress system to metabolism and virulence regulation in Salmonella enterica serovar Typhimurium

The Cpx-envelope stress system regulates the expression of virulence factors in many Gram-negative pathogens. In Salmonella enterica serovar Typhimurium deletion of the sensor kinase CpxA but not of the response regulator CpxR results in the down regulation of the key regulator for invasion, HilA encoded by the Salmonella pathogenicity island 1 (SPI-1). Here, we provide evidence that cpxA deletion interferes with dephosphorylation of CpxR resulting in increased levels of active CpxR and consequently in misregulation of target genes. 14 potential operons were identified to be under direct control of CpxR. These include the virulence determinants ecotin, the omptin PgtE, and the SPI-2 regulator SsrB. The Tat-system and the PocR regulator that together promote anaerobic respiration of tetrathionate on 1,2-propanediol are also under direct CpxR control. Notably, 1,2-propanediol represses hilA expression. Thus, our work demonstrates for the first time the involvement of the Cpx system in a complex network mediating metabolism and virulence function.

Two-Partner Secretion: Combining Efficiency and Simplicity in the Secretion of Large Proteins for Bacteria-Host and Bacteria-Bacteria Interactions

Initially identified in pathogenic Gram-negative bacteria, the two-partner secretion (TPS) pathway, also known as Type Vb secretion, mediates the translocation across the outer membrane of large effector proteins involved in interactions between these pathogens and their hosts. More recently, distinct TPS systems have been shown to secrete toxic effector domains that participate in inter-bacterial competition or cooperation. The effects of these systems are based on kin vs. non-kin molecular recognition mediated by specific immunity proteins. With these new toxin-antitoxin systems, the range of TPS effector functions has thus been extended from cytolysis, adhesion, and iron acquisition, to genome maintenance, inter-bacterial killing and inter-bacterial signaling. Basically, a TPS system is made up of two proteins, the secreted TpsA effector protein and its TpsB partner transporter, with possible additional factors such as immunity proteins for protection against cognate toxic effectors. Structural studies have indicated that TpsA proteins mainly form elongated β helices that may be followed by specific functional domains. TpsB proteins belong to the Omp85 superfamily. Open questions remain on the mechanism of protein secretion in the absence of ATP or an electrochemical gradient across the outer membrane. The remarkable dynamics of the TpsB transporters and the progressive folding of their TpsA partners at the bacterial surface in the course of translocation are thought to be key elements driving the secretion process.

Validation of Reference Genes for Real-Time Quantitative PCR (qPCR) Analysis of Avibacterium paragallinarum

Real-time quantitative reverse transcription PCR (qRT-PCR) offers a robust method for measurement of gene expression levels. Selection of reliable reference gene(s) for gene expression study is conducive to reduce variations derived from different amounts of RNA and cDNA, the efficiency of the reverse transcriptase or polymerase enzymes. Until now reference genes identified for other members of the family Pasteurellaceae have not been validated for Avibacterium paragallinarum. The aim of this study was to validate nine reference genes of serovars A, B, and C strains of A. paragallinarum in different growth phase by qRT-PCR. Three of the most widely used statistical algorithms, geNorm, NormFinder and ΔCT method were used to evaluate the expression stability of reference genes. Data analyzed by overall rankings showed that in exponential and stationary phase of serovar A, the most stable reference genes were gyrA and atpD respectively; in exponential and stationary phase of serovar B, the most stable reference genes were atpD and recN respectively; in exponential and stationary phase of serovar C, the most stable reference genes were rpoB and recN respectively. This study provides recommendations for stable endogenous control genes for use in further studies involving measurement of gene expression levels.

The CpxRA two-component system contributes to Legionella pneumophila virulence

The bacterium Legionella pneumophila is capable of intracellular replication within freshwater protozoa as well as human macrophages, the latter of which results in the serious pneumonia Legionnaires' disease. A primary factor involved in these host cell interactions is the Dot/Icm Type IV secretion system responsible for translocating effector proteins needed to establish and maintain the bacterial replicative niche. Several regulatory factors have been identified to control the expression of the Dot/Icm system and effectors, one of which is the CpxRA two-component system, suggesting essentiality for virulence. In this study, we generated cpxR, cpxA and cpxRA in-frame null mutant strains to further delineate the role of the CpxRA system in bacterial survival and virulence. We found that cpxR is essential for intracellular replication within Acanthamoeba castellanii, but not in U937-derived macrophages. Transcriptome analysis revealed that CpxRA regulates a large number of virulence-associated proteins including Dot/Icm effectors as well as Type II secreted substrates. Furthermore, the cpxR and cpxRA mutant strains were more sodium resistant than the parental strain Lp02, and cpxRA expression reaches maximal levels during postexponential phase. Taken together, our findings suggest the CpxRA system is a key contributor to L. pneumophila virulence in protozoa via virulence factor regulation.

Role of the Gram-Negative Envelope Stress Response in the Presence of Antimicrobial Agents

Bacterial survival necessitates endurance of many types of antimicrobial compound. Many Gram-negative envelope stress responses, which must contend with an outer membrane and a dense periplasm containing the cell wall, have been associated with the status of protein folding, membrane homeostasis, and physiological functions such as efflux and the proton motive force (PMF). In this review, we discuss evidence that indicates an emerging role for Gram-negative envelope stress responses in enduring exposure to diverse antimicrobial substances, focusing on recent studies of the γ-proteobacterial Cpx envelope stress response.

Validation of reference genes for quantitative real-time PCR (qPCR) analysis of Actinobacillus suis

Background

Quantitative real-time PCR is a valuable tool for evaluating bacterial gene expression. However, in order to make best use of this method, endogenous reference genes for expression data normalisation must first be identified by carefully validating the stability of expression under experimental conditions. Therefore, the objective of this study was to validate eight reference genes of the opportunistic swine pathogen, Actinobacillus suis, grown in aerobic cultures with (Epinephrine) or without (Aerobic) epinephrine in the growth medium and in anoxic static cultures (Anoxic), and sampled during exponential and stationary phases.

Results

Using the RefFinder tool, expression data were analysed to determine whether comprehensive stability rankings of selected reference genes varied with experimental design. When comparing Aerobic and Epinephrine cultures by growth phase, pyk and rpoB were both among the most stably expressed genes, but when analysing both growth phases together, only pyk remained in the top three rankings. When comparing Aerobic and Anoxic samples, proS ranked among the most stable genes in exponential and stationary phase data sets as well as in combined rankings. When analysing the Aerobic, Epinephrine, and Anoxic samples together, only gyrA ranked consistently among the top three most stably expressed genes during exponential and stationary growth as well as in combined rankings; the rho gene ranked as least stably expressed gene in this data set.

Conclusions

Reference gene stability should be carefully assessed with the design of the experiment in mind. In this study, even the commonly used reference gene 16S rRNA demonstrated large variability in stability depending on the conditions studied and how the data were analysed. As previously suggested, the best approach may be to use a geometric mean of multiple genes to normalise qPCR results. As researchers continue to validate reference genes for various organisms in multiple growth conditions and sampling time points, it may be possible to make informed predictions as to which genes may be most suitable to validate for a given experimental design, but in the meantime, the reference genes used to normalise qPCR data should be selected with caution.

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

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

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

Haemophilus ducreyi Hfq contributes to virulence gene regulation as cells enter stationary phase

To adapt to stresses encountered in stationary phase, Gram-negative bacteria utilize the alternative sigma factor RpoS. However, some species lack RpoS; thus, it is unclear how stationary-phase adaptation is regulated in these organisms. Here we defined the growth-phase-dependent transcriptomes of Haemophilus ducreyi, which lacks an RpoS homolog. Compared to mid-log-phase organisms, cells harvested from the stationary phase upregulated genes encoding several virulence determinants and a homolog of hfq. Insertional inactivation of hfq altered the expression of ~16% of the H. ducreyi genes. Importantly, there were a significant overlap and an inverse correlation in the transcript levels of genes differentially expressed in the hfq inactivation mutant relative to its parent and the genes differentially expressed in stationary phase relative to mid-log phase in the parent. Inactivation of hfq downregulated genes in the flp-tad and lspB-lspA2 operons, which encode several virulence determinants. To comply with FDA guidelines for human inoculation experiments, an unmarked hfq deletion mutant was constructed and was fully attenuated for virulence in humans. Inactivation or deletion of hfq downregulated Flp1 and impaired the ability of H. ducreyi to form microcolonies, downregulated DsrA and rendered H. ducreyi serum susceptible, and downregulated LspB and LspA2, which allow H. ducreyi to resist phagocytosis. We propose that, in the absence of an RpoS homolog, Hfq serves as a major contributor of H. ducreyi stationary-phase and virulence gene regulation. The contribution of Hfq to stationary-phase gene regulation may have broad implications for other organisms that lack an RpoS homolog.

Pathogenic bacteria encounter a wide range of stresses in their hosts, including nutrient limitation; the ability to sense and respond to such stresses is crucial for bacterial pathogens to successfully establish an infection. Gram-negative bacteria frequently utilize the alternative sigma factor RpoS to adapt to stresses and stationary phase. However, homologs of RpoS are absent in some bacterial pathogens, including Haemophilus ducreyi, which causes chancroid and facilitates the acquisition and transmission of HIV-1. Here, we provide evidence that, in the absence of an RpoS homolog, Hfq serves as a major contributor of stationary-phase gene regulation and that Hfq is required for H. ducreyi to infect humans. To our knowledge, this is the first study describing Hfq as a major contributor of stationary-phase gene regulation in bacteria and the requirement of Hfq for the virulence of a bacterial pathogen in humans.

Everything old is new again: an update on current research on the Cpx envelope stress response

The Cpx envelope stress response (ESR) has been linked to proteins that are integrated into and secreted across the inner membrane for several decades. Initial studies of the cpx locus linked it to alterations in the protein content of both the inner and outer membrane, together with changes in proton motive driven transport and conjugation. Since the mid 1990s, the predominant view of the Cpx envelope stress response has been that it serves to detect and respond to secreted, misfolded proteins in the periplasm. Recent studies in Escherichia coli and other Gram negative organisms highlight a role for the Cpx ESR in specifically responding to perturbations that occur at the inner membrane (IM). It is clear that Cpx adaptation involves a broad suite of changes that encompass many functions in addition to protein folding. Interestingly, recent studies have refocused attention on Cpx-regulated phenotypes that were initially published over 30years ago, including antibiotic resistance and transport across the IM. In this review I will focus on the insights and models that have arisen from recent studies and that may help explain some of the originally published Cpx phenotypes. Although the molecular nature of the inducing signal for the Cpx ESR remains enigmatic, recently solved structures of signaling proteins are yielding testable models concerning the molecular mechanisms behind signaling. The identification of connections between the Cpx ESR and other stress responses in the cell reveals a complex web of interactions that involves Cpx-regulated expression of other regulators as well as small proteins and sRNAs. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

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

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

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.

On the evolution of the sexually transmitted bacteria Haemophilus ducreyi and Klebsiella granulomatis

Haemophilus ducreyi and Klebsiella (Calymmatobacterium) granulomatis are sexually transmitted bacteria that cause characteristic, persisting ulceration on external genitals called chancroid and granuloma inguinale, respectively. Those ulcers are endemic in developing countries or exist, as does granuloma inguinale, only in some geographic "hot spots."H. ducreyi is placed in the genus Haemophilus (family Pasteurellacae); however, this phylogenetic position is not obvious. The multiple ways in which the bacterium may be adapted to its econiche through specialized nutrient acquisitions; defenses against the immune system; and virulence factors that increase attachment, fitness, and persistence within genital tissue are discussed below. The analysis of K. granulomatis phylogeny demonstrated a high degree of identity with other Klebsiella species, and the name K. granulomatis comb. nov. was proposed. Because of the difficulty in growing this bacterium on artificial media, its characteristics have not been sufficiently defined. More studies are needed to understand bacterial genetics related to the pathogenesis and evolution of K. granulomatis.

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.

Deletion of mtrC in Haemophilus ducreyi increases sensitivity to human antimicrobial peptides and activates the CpxRA regulon

Haemophilus ducreyi resists killing by antimicrobial peptides encountered during human infection, including cathelicidin LL-37, α-defensins, and β-defensins. In this study, we examined the role of the proton motive force-dependent multiple transferable resistance (MTR) transporter in antimicrobial peptide resistance in H. ducreyi. We found a proton motive force-dependent effect on H. ducreyi's resistance to LL-37 and β-defensin HBD-3, but not α-defensin HNP-2. Deletion of the membrane fusion protein MtrC rendered H. ducreyi more sensitive to LL-37 and human β-defensins but had relatively little effect on α-defensin resistance. The mtrC mutant 35000HPmtrC exhibited phenotypic changes in outer membrane protein profiles, colony morphology, and serum sensitivity, which were restored to wild type by trans-complementation with mtrC. Similar phenotypes were reported in a cpxA mutant; activation of the two-component CpxRA regulator was confirmed by showing transcriptional effects on CpxRA-regulated genes in 35000HPmtrC. A cpxR mutant had wild-type levels of antimicrobial peptide resistance; a cpxA mutation had little effect on defensin resistance but led to increased sensitivity to LL-37. 35000HPmtrC was more sensitive than the cpxA mutant to LL-37, indicating that MTR contributed to LL-37 resistance independent of the CpxRA regulon. The CpxRA regulon did not affect proton motive force-dependent antimicrobial peptide resistance; however, 35000HPmtrC had lost proton motive force-dependent peptide resistance, suggesting that the MTR transporter promotes proton motive force-dependent resistance to LL-37 and human β-defensins. This is the first report of a β-defensin resistance mechanism in H. ducreyi and shows that LL-37 resistance in H. ducreyi is multifactorial.

Identification of gene products involved in the oxidative stress response of Moraxella catarrhalis

Moraxella catarrhalis is subjected to oxidative stress from both internal and environmental sources. A previous study (C. D. Pericone, K. Overweg, P. W. Hermans, and J. N. Weiser, Infect. Immun. 68:3990-3997, 2000) indicated that a wild-type strain of M. catarrhalis was very resistant to killing by exogenous hydrogen peroxide (H₂O₂). The gene encoding OxyR, a LysR family transcriptional regulator, was identified and inactivated in M. catarrhalis strain O35E, resulting in an increase in sensitivity to killing by H₂O₂ in disk diffusion assays and a concomitant aerobic serial dilution effect. Genes encoding a predicted catalase (KatA) and an alkyl hydroperoxidase (AhpCF) showed dose-dependent upregulation in wild-type cells exposed to H₂O₂. DNA microarray and real-time reverse transcription-PCR (RT-PCR) analyses identified M. catarrhalis genes whose expression was affected by oxidative stress in an OxyR-dependent manner. Testing of M. catarrhalis O35E katA and ahpC mutants for their abilities to scavenge exogenous H₂O₂ showed that the KatA catalase was responsible for most of this activity in the wild-type parent strain. The introduction of the same mutations into M. catarrhalis strain ETSU-4 showed that the growth of a ETSU-4 katA mutant was markedly inhibited by the addition of 50 mM H₂O₂ but that this mutant could still form a biofilm equivalent to that produced by its wild-type parent strain.

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.

Host-pathogen interplay of Haemophilus ducreyi

PURPOSE OF REVIEW

Haemophilus ducreyi, the causative agent of the sexually transmitted infection chancroid, is primarily a pathogen of human skin. During infection, H. ducreyi thrives extracellularly in a milieu of professional phagocytes and other antibacterial components of the innate and adaptive immune responses. This review summarizes our understanding of the interplay between this pathogen and its host that leads to development and persistence of disease.

RECENT FINDINGS

H. ducreyi expresses key virulence mechanisms to resist host defenses. The secreted LspA proteins are tyrosine-phosphorylated by host kinases, which may contribute to their antiphagocytic effector function. The serum resistance and adherence functions of DsrA map to separate domains of this multifunctional virulence factor. An influx transporter protects H. ducreyi from killing by the antimicrobial peptide LL37. Regulatory genes have been identified that may coordinate virulence factor expression during disease. Dendritic cells and natural killer cells respond to H. ducreyi and may be involved in determining the differential outcomes of infection observed in humans.

SUMMARY

A human model of H. ducreyi infection has provided insights into virulence mechanisms that allow this human-specific pathogen to survive immune pressures. Components of the human innate immune system may also determine the ultimate fate of H. ducreyi infection by driving either clearance of the organism or an ineffective response that allows disease progression.

Fis is essential for capsule production in Pasteurella multocida and regulates expression of other important virulence factors

P. multocida is the causative agent of a wide range of diseases of animals, including fowl cholera in poultry and wild birds. Fowl cholera isolates of P. multocida generally express a capsular polysaccharide composed of hyaluronic acid. There have been reports of spontaneous capsule loss in P. multocida, but the mechanism by which this occurs has not been determined. In this study, we identified three independent strains that had spontaneously lost the ability to produce capsular polysaccharide. Quantitative RT-PCR showed that these strains had significantly reduced transcription of the capsule biosynthetic genes, but DNA sequence analysis identified no mutations within the capsule biosynthetic locus. However, whole-genome sequencing of paired capsulated and acapsular strains identified a single point mutation within the fis gene in the acapsular strain. Sequencing of fis from two independently derived spontaneous acapsular strains showed that each contained a mutation within fis. Complementation of these strains with an intact copy of fis, predicted to encode a transcriptional regulator, returned capsule expression to all strains. Therefore, expression of a functional Fis protein is essential for capsule expression in P. multocida. DNA microarray analysis of one of the spontaneous fis mutants identified approximately 30 genes as down-regulated in the mutant, including pfhB_2, which encodes a filamentous hemagglutinin, a known P. multocida virulence factor, and plpE, which encodes the cross protective surface antigen PlpE. Therefore these experiments define for the first time a mechanism for spontaneous capsule loss in P. multocida and identify Fis as a critical regulator of capsule expression. Furthermore, Fis is involved in the regulation of a range of other P. multocida genes including important virulence factors.

Pasteurella multocida is an animal pathogen of worldwide economic significance. It causes fowl cholera in wild birds and poultry, hemorrhagic septicemia in ungulates, and atrophic rhinitis in swine. The major virulence factor in fowl cholera-causing isolates is the polysaccharide capsule, which is composed of hyaluronic acid. Although there have been reports of spontaneous capsule loss in some strains, to date there has been no systematic investigation into the molecular mechanisms of this phenomenon. In this study, we describe for the first time the underlying transcriptional mechanisms required for the expression of capsule in P. multocida, and identify a transcriptional regulator required for capsule production.

Haemophilus ducreyi SapA contributes to cathelicidin resistance and virulence in humans

Haemophilus ducreyi is an extracellular pathogen of human epithelial surfaces that resists human antimicrobial peptides (APs). The organism's genome contains homologs of genes sensitive to antimicrobial peptides (sap operon) in nontypeable Haemophilus influenzae. In this study, we characterized the sap-containing loci of H. ducreyi 35000HP and demonstrated that sapA is expressed in broth cultures and H. ducreyi-infected tissue; sapA is also conserved among both class I and class II H. ducreyi strains. We constructed a nonpolar sapA mutant of H. ducreyi 35000HP, designated 35000HPsapA, and compared the percent survival of wild-type 35000HP and 35000HPsapA exposed to several human APs, including alpha-defensins, beta-defensins, and the cathelicidin LL-37. Unlike an H. influenzae sapA mutant, strain 35000HPsapA was not more susceptible to defensins than strain 35000HP was. However, we observed a significant decrease in the survival of strain 35000HPsapA after exposure to LL-37, which was complemented by introducing sapA in trans. Thus, the Sap transporter plays a role in resistance of H. ducreyi to LL-37. We next compared mutant strain 35000HPsapA with strain 35000HP for their ability to cause disease in human volunteers. Although both strains caused papules to form at similar rates, the pustule formation rate at sites inoculated with 35000HPsapA was significantly lower than that of sites inoculated with 35000HP (33.3% versus 66.7%; P = 0.007). Together, these data establish that SapA acts as a virulence factor and as one mechanism for H. ducreyi to resist killing by antimicrobial peptides. To our knowledge, this is the first demonstration that an antimicrobial peptide resistance mechanism contributes to bacterial virulence in humans.