Inactivation of DsbA, but not DsbC and DsbD, affects the intracellular survival and virulence of Shigella flexneri

The Balance between Protealysin and Its Substrate, the Outer Membrane Protein OmpX, Regulates Serratia proteamaculans Invasion

Serratia are opportunistic bacteria, causing infections in plants, insects, animals and humans under certain conditions. The development of bacterial infection in the human body involves several stages of host-pathogen interaction, including entry into non-phagocytic cells to evade host immune cells. The facultative pathogen Serratia proteamaculans is capable of penetrating eukaryotic cells. These bacteria synthesize an actin-specific metalloprotease named protealysin. After transformation with a plasmid carrying the protealysin gene, noninvasive E. coli penetrate eukaryotic cells. This suggests that protealysin may play a key role in S. proteamaculans invasion. This review addresses the mechanisms underlying protealysin's involvement in bacterial invasion, highlighting the main findings as follows. Protealysin can be delivered into the eukaryotic cell by the type VI secretion system and/or by bacterial outer membrane vesicles. By cleaving actin in the host cell, protealysin can mediate the reversible actin rearrangements required for bacterial invasion. However, inactivation of the protealysin gene leads to an increase, rather than decrease, in the intensity of S. proteamaculans invasion. This indicates the presence of virulence factors among bacterial protealysin substrates. Indeed, protealysin cleaves the virulence factors, including the bacterial surface protein OmpX. OmpX increases the expression of the EGFR and β1 integrin, which are involved in S. proteamaculans invasion. It has been shown that an increase in the invasion of genetically modified S. proteamaculans may be the result of the accumulation of full-length OmpX on the bacterial surface, which is not cleaved by protealysin. Thus, the intensity of the S. proteamaculans invasion is determined by the balance between the active protealysin and its substrate OmpX.

Making a chink in their armor: Current and next-generation antimicrobial strategies against the bacterial cell envelope

Gram-negative bacteria are uniquely equipped to defeat antibiotics. Their outermost layer, the cell envelope, is a natural permeability barrier that contains an array of resistance proteins capable of neutralizing most existing antimicrobials. As a result, its presence creates a major obstacle for the treatment of resistant infections and for the development of new antibiotics. Despite this seemingly impenetrable armor, in-depth understanding of the cell envelope, including structural, functional and systems biology insights, has promoted efforts to target it that can ultimately lead to the generation of new antibacterial therapies. In this article, we broadly overview the biology of the cell envelope and highlight attempts and successes in generating inhibitors that impair its function or biogenesis. We argue that the very structure that has hampered antibiotic discovery for decades has untapped potential for the design of novel next-generation therapeutics against bacterial pathogens.

Identification and characterization of two drug-like fragments that bind to the same cryptic binding pocket of Burkholderia pseudomallei DsbA

Disulfide-bond-forming proteins (Dsbs) play a crucial role in the pathogenicity of many Gram-negative bacteria. Disulfide-bond-forming protein A (DsbA) catalyzes the formation of the disulfide bonds necessary for the activity and stability of multiple substrate proteins, including many virulence factors. Hence, DsbA is an attractive target for the development of new drugs to combat bacterial infections. Here, two fragments, bromophenoxy propanamide (1) and 4-methoxy-N-phenylbenzenesulfonamide (2), were identified that bind to DsbA from the pathogenic bacterium Burkholderia pseudomallei, the causative agent of melioidosis. The crystal structures of oxidized B. pseudomallei DsbA (termed BpsDsbA) co-crystallized with 1 or 2 show that both fragments bind to a hydrophobic pocket that is formed by a change in the side-chain orientation of Tyr110. This conformational change opens a `cryptic' pocket that is not evident in the apoprotein structure. This binding location was supported by 2D-NMR studies, which identified a chemical shift perturbation of the Tyr110 backbone amide resonance of more than 0.05 p.p.m. upon the addition of 2 mM fragment 1 and of more than 0.04 p.p.m. upon the addition of 1 mM fragment 2. Although binding was detected by both X-ray crystallography and NMR, the binding affinity (Kd) for both fragments was low (above 2 mM), suggesting weak interactions with BpsDsbA. This conclusion is also supported by the crystal structure models, which ascribe partial occupancy to the ligands in the cryptic binding pocket. Small fragments such as 1 and 2 are not expected to have a high energetic binding affinity due to their relatively small surface area and the few functional groups that are available for intermolecular interactions. However, their simplicity makes them ideal for functionalization and optimization. The identification of the binding sites of 1 and 2 to BpsDsbA could provide a starting point for the development of more potent novel antimicrobial compounds that target DsbA and bacterial virulence.

Interplay between DsbA1, DsbA2 and C8J_1298 Periplasmic Oxidoreductases of Campylobacter jejuni and Their Impact on Bacterial Physiology and Pathogenesis

The bacterial proteins of the Dsb family catalyze the formation of disulfide bridges between cysteine residues that stabilize protein structures and ensure their proper functioning. Here, we report the detailed analysis of the Dsb pathway of Campylobacter jejuni. The oxidizing Dsb system of this pathogen is unique because it consists of two monomeric DsbAs (DsbA1 and DsbA2) and one dimeric bifunctional protein (C8J_1298). Previously, we showed that DsbA1 and C8J_1298 are redundant. Here, we unraveled the interaction between the two monomeric DsbAs by in vitro and in vivo experiments and by solving their structures and found that both monomeric DsbAs are dispensable proteins. Their structures confirmed that they are homologs of EcDsbL. The slight differences seen in the surface charge of the proteins do not affect the interaction with their redox partner. Comparative proteomics showed that several respiratory proteins, as well as periplasmic transport proteins, are targets of the Dsb system. Some of these, both donors and electron acceptors, are essential elements of the C. jejuni respiratory process under oxygen-limiting conditions in the host intestine. The data presented provide detailed information on the function of the C. jejuni Dsb system, identifying it as a potential target for novel antibacterial molecules.

Structural bioinformatic analysis of DsbA proteins and their pathogenicity associated substrates

The disulfide bond (DSB) forming system and in particular DsbA, is a key bacterial oxidative folding catalyst. Due to its role in promoting the correct assembly of a wide range of virulence factors required at different stages of the infection process, DsbA is a master virulence rheostat, making it an attractive target for the development of new virulence blockers. Although DSB systems have been extensively studied across different bacterial species, to date, little is known about how DsbA oxidoreductases are able to recognize and interact with such a wide range of substrates. This review summarizes the current knowledge on the DsbA enzymes, with special attention on their interaction with the partner oxidase DsbB and substrates associated with bacterial virulence. The structurally and functionally diverse set of bacterial proteins that rely on DsbA-mediated disulfide bond formation are summarized. Local sequence and secondary structure elements of these substrates are analyzed to identify common elements recognized by DsbA enzymes. This not only provides information on protein folding systems in bacteria but also offers tools for identifying new DsbA substrates and informs current efforts aimed at developing DsbA targeted anti-microbials.

Periplasmic Targets for the Development of Effective Antimicrobials against Gram-Negative Bacteria

Antibiotic resistance has emerged as a serious threat to global public health in recent years. Lack of novel antimicrobials, especially new classes of compounds, further aggravates the situation. For Gram-negative bacteria, their double layered cell envelope and an array of efflux pumps act as formidable barriers for antimicrobials to penetrate. While cytoplasmic targets are hard to reach, proteins in the periplasm are clearly more accessible, as the drug only needs to breach the outer membrane. In this review, we summarized recent efforts on the validation and testing of periplasmic proteins as potential antimicrobial targets and the development of related inhibitors that either inhibit the growth of a bacterial pathogen or reduce its virulence during interaction with host cells. We conclude that the periplasm contains a promising pool of novel antimicrobial targets that should be scrutinized more closely for the development of effective treatment against multidrug-resistant Gram-negative bacteria.

Protein Disulfide Exchange by the Intramembrane Enzymes DsbB, DsbD, and CcdA

The formation of disulfide bonds in proteins is an essential process in both prokaryotes and eukaryotes. In gram-negative bacteria including Escherichia coli, the proteins DsbA and DsbB mediate the formation of disulfide bonds in the periplasm. DsbA acts as the periplasmic oxidant of periplasmic substrate proteins. DsbA is reoxidized by transfer of reducing equivalents to the 4 TM helix membrane protein DsbB, which transfers reducing equivalents to ubiquinone or menaquinone. Multiple structural studies of DsbB have provided detailed structural information on intermediates in the process of DsbB catalyzed oxidation of DsbA. These structures and the insights gained are described. In proteins with more than one pair of Cys residues, there is the potential for formation of non-native disulfide bonds, making it necessary for the cell to have a mechanism for the isomerization of such non-native disulfide bonds. In E. coli, this is mediated by the proteins DsbC and DsbD. DsbC reduces mis-formed disulfide bonds. The eight-TM-helix protein DsbD reduces DsbC and is itself reduced by cytoplasmic thioredoxin. DsbD also contributes reducing equivalents for the reduction of cytochrome c to facilitate heme attachment. The DsbD functional homolog CcdA is a six-TM-helix membrane protein that provides reducing equivalents for the reduction of cytochrome c. A recent structure determination of CcdA has provided critical insights into how reducing equivalents are transferred across the membrane that likely also provides understanding how this is achieved by DsbD as well. This structure and the insights gained are described.

The Fragment-Based Development of a Benzofuran Hit as a New Class of Escherichia coli DsbA Inhibitors

A fragment-based drug discovery approach was taken to target the thiol-disulfide oxidoreductase enzyme DsbA from Escherichia coli (EcDsbA). This enzyme is critical for the correct folding of virulence factors in many pathogenic Gram-negative bacteria, and small molecule inhibitors can potentially be developed as anti-virulence compounds. Biophysical screening of a library of fragments identified several classes of fragments with affinity to EcDsbA. One hit with high mM affinity, 2-(6-bromobenzofuran-3-yl)acetic acid (6), was chemically elaborated at several positions around the scaffold. X-ray crystal structures of the elaborated analogues showed binding in the hydrophobic binding groove adjacent to the catalytic disulfide bond of EcDsbA. Binding affinity was calculated based on NMR studies and compounds 25 and 28 were identified as the highest affinity binders with dissociation constants (K_D) of 326 ± 25 and 341 ± 57 µM respectively. This work suggests the potential to develop benzofuran fragments into a novel class of EcDsbA inhibitors.

Engineering of the Dsb (disulfide bond) proteins - contribution towards understanding their mechanism of action and their applications in biotechnology and medicine

The Dsb protein family in prokaryotes catalyzes the generation of disulfide bonds between thiol groups of cysteine residues in nascent proteins, ensuring their proper three-dimensional structure; these bonds are crucial for protein stability and function. The first Dsb protein, Escherichia coli DsbA, was described in 1991. Since then, many details of the bond-formation process have been described through microbiological, biochemical, biophysical and bioinformatics strategies. Research with the model microorganism E. coli and many other bacterial species revealed an enormous diversity of bond-formation mechanisms. Research using Dsb protein engineering has significantly helped to reveal details of the disulfide bond formation. The first part of this review presents the research that led to understanding the mechanism of action of DsbA proteins, which directly transfer their own disulfide into target proteins. The second part concentrates on the mechanism of electron transport through the cell cytoplasmic membrane. Third and lastly, the review discusses the contribution of this research towards new antibacterial agents.

Geraniol as a novel antivirulence agent against bacillary dysentery-causing Shigella sonnei

Antimicrobial resistance has emerged as a major challenge to modern medicine and it has become urgent to seek alternative approaches to treat infections caused by fast-evolving multi-resistant clones of bacillary dysentery-causing Shigella sonnei. Here, we show that geraniol, a natural substance present in the essential oils of plants such as rose and lemongrass, can reduce S. sonnei proliferation inside host cells and protect Galleria mellonella larvae from killing by S. sonnei infection. We present evidence that geraniol competitively inhibits the catalytic activity of the master virulence regulator, DsbA, a periplasmic disulphide bond oxidoreductase required for Shigella survival in the host cell cytosol. Our observations suggest that geraniol holds a great therapeutic potential that should be further exploited.

Commensal-to-pathogen transition: One-single transposon insertion results in two pathoadaptive traits in Escherichia coli -macrophage interaction

Escherichia coli is both a harmless commensal in the intestines of many mammals, as well as a dangerous pathogen. The evolutionary paths taken by strains of this species in the commensal-to-pathogen transition are complex and can involve changes both in the core genome, as well in the pan-genome. One way to understand the likely paths that a commensal strain of E. coli takes when evolving pathogenicity is through experimentally evolving the strain under the selective pressures that it will have to withstand as a pathogen. Here, we report that a commensal strain, under continuous pressure from macrophages, recurrently acquired a transposable element insertion, which resulted in two key phenotypic changes: increased intracellular survival, through the delay of phagosome maturation and increased ability to escape macrophages. We further show that the acquisition of the pathoadaptive traits was accompanied by small but significant changes in the transcriptome of macrophages upon infection. These results show that under constant pressures from a key component of the host immune system, namely macrophage phagocytosis, commensal E. coli rapidly acquires pathoadaptive mutations that cause transcriptome changes associated to the host-microbe duet.

Targeting Bacterial Dsb Proteins for the Development of Anti-Virulence Agents

Recent years have witnessed a dramatic increase in bacterial antimicrobial resistance and a decline in the development of novel antibiotics. New therapeutic strategies are urgently needed to combat the growing threat posed by multidrug resistant bacterial infections. The Dsb disulfide bond forming pathways are potential targets for the development of antimicrobial agents because they play a central role in bacterial pathogenesis. In particular, the DsbA/DsbB system catalyses disulfide bond formation in a wide array of virulence factors, which are essential for many pathogens to establish infections and cause disease. These redox enzymes are well placed as antimicrobial targets because they are taxonomically widespread, share low sequence identity with human proteins, and many years of basic research have provided a deep molecular understanding of these systems in bacteria. In this review, we discuss disulfide bond catalytic pathways in bacteria and their significance in pathogenesis. We also review the use of different approaches to develop inhibitors against Dsb proteins as potential anti-virulence agents, including fragment-based drug discovery, high-throughput screening and other structure-based drug discovery methods.

Helicobacter pylori HP0231 Influences Bacterial Virulence and Is Essential for Gastric Colonization

The Dsb protein family is responsible for introducing disulfide bonds into nascent proteins in prokaryotes, stabilizing the structure of many proteins. Helicobacter pylori HP0231 is a Dsb-like protein, shown to catalyze disulfide bond formation and to participate in redox homeostasis. Notably, many H. pylori virulence factors are stabilized by the formation of disulfide bonds. By employing H. pylori HP0231 deficient strains we analyzed the effect of lack of this bacterial protein on the functionality of virulence factors containing putative disulfide bonds. The lack of H. pylori HP0231 impaired CagA translocation into gastric epithelial cells and reduced VacA-induced cellular vacuolation. Moreover, H. pylori HP0231 deficient bacteria were not able to colonize the gastric mucosa of mice, probably due to compromised motility. Together, our data demonstrate an essential function for H. pylori HP0231 in gastric colonization and proper function of bacterial virulence factors related to gastric pathology.

Morphological analysis of Francisella novicida epithelial cell infections in the absence of functional FipA

Francisella novicida is a surrogate pathogen commonly used to study infections by the potential bioterrorism agent, Francisella tularensis. One of the primary sites of Francisella infections is the liver where >90% of infected cells are hepatocytes. It is known that once Francisella enter cells it occupies a membrane-bound compartment, the Francisella-containing vacuole (FCV), from which it rapidly escapes to replicate in the cytosol. Recent work examining the Francisella disulfide bond formation (Dsb) proteins, FipA and FipB, have demonstrated that these proteins are important during the Francisella infection process; however, details as to how the infections are altered in epithelial cells have remained elusive. To identify the stage of the infections where these Dsbs might act during epithelial infections, we exploited a hepatocyte F. novicida infection model that we recently developed. We found that F. novicida ΔfipA-infected hepatocytes contained bacteria clustered within lysosome-associated membrane protein 1-positive FCVs, suggesting that FipA is involved in the escape of F. novicida from its vacuole. Our morphological evidence provides a tangible link as to how Dsb FipA can influence Francisella infections.

Virulence Gene Regulation in Shigella

Shigella species are the causative agents of bacillary dysentery in humans, an invasive disease in which the bacteria enter the cells of the epithelial layer of the large intestine, causing extensive tissue damage and inflammation. They rely on a plasmid-encoded type III secretion system (TTSS) to cause disease; this system and its regulation have been investigated intensively at the molecular level for decades. The lessons learned have not only deepened our knowledge of Shigella biology but also informed in important ways our understanding of the mechanisms used by other pathogenic bacteria to cause disease and to control virulence gene expression. In addition, the Shigella story has played a central role in the development of our appreciation of the contribution of horizontal DNA transfer to pathogen evolution.A 30-kilobase-pair "Entry Region" of the 230-kb virulence plasmid lies at the heart of the Shigella pathogenesis system. Here are located the virB and mxiE regulatory genes and most of the structural genes involved in the expression of the TTSS and its effector proteins. Expression of the virulence genes occurs in response to an array of environmental signals, including temperature, osmolarity, and pH.At the top of the regulatory hierarchy and lying on the plasmid outside the Entry Region isvirF, encoding an AraC-like transcription factor.Virulence gene expression is also controlled by chromosomal genes,such as those encoding the nucleoid-associated proteins H-NS, IHF, and Fis, the two-component regulators OmpR/EnvZ and CpxR/CpxA, the anaerobic regulator Fnr, the iron-responsive regulator Fur, and the topoisomerases of the cell that modulate DNA supercoiling. Small regulatory RNAs,the RNA chaperone Hfq,and translational modulation also affect the expression of the virulence phenotypetranscriptionally and/orposttranscriptionally.

Monitoring Oxidative Folding of a Single Protein Catalyzed by the Disulfide Oxidoreductase DsbA

Oxidative folding, the process by which proteins fold and acquire disulfide bonds concurrently, is of critical importance for a wide range of biological processes. Generally, this process is catalyzed by oxidoreductase enzymes that facilitate oxidation and also bear chaperone functionality. Although this process has been well described qualitatively, fine yet important details remain obscured by a limited quantitative perspective, arising from the limitations in the application of bulk biochemical methods to the study of oxidative folding. In this work, we have applied single molecule force spectroscopy techniques to monitor in real time the process of oxidative folding as catalyzed by DsbA, the enzyme solely responsible for the catalysis of oxidative folding in the bacterial periplasm. We provide a quantitative and detailed description of the catalytic mechanism utilized by DsbA that offers insight into the entire sequence of events that occurs in the periplasm from the unfolded-reduced state to the folded-oxidized protein. We have compared our results with those of protein disulfide-isomerase, the eukaryotic counterpart of DsbA, allowing us to devise a general mechanism for oxidative folding that also reflects upon the physiological functions and demands of these enzymes in vivo.

Structure of the Acinetobacter baumannii dithiol oxidase DsbA bound to elongation factor EF-Tu reveals a novel protein interaction site

The multidrug resistant bacterium Acinetobacter baumannii is a significant cause of nosocomial infection. Biofilm formation, that requires both disulfide bond forming and chaperone-usher pathways, is a major virulence trait in this bacterium. Our biochemical characterizations show that the periplasmic A. baumannii DsbA (AbDsbA) enzyme has an oxidizing redox potential and dithiol oxidase activity. We found an unexpected non-covalent interaction between AbDsbA and the highly conserved prokaryotic elongation factor, EF-Tu. EF-Tu is a cytoplasmic protein but has been localized extracellularly in many bacterial pathogens. The crystal structure of this complex revealed that the EF-Tu switch I region binds to the non-catalytic surface of AbDsbA. Although the physiological and pathological significance of a DsbA/EF-Tu association is unknown, peptides derived from the EF-Tu switch I region bound to AbDsbA with submicromolar affinity. We also identified a seven-residue DsbB-derived peptide that bound to AbDsbA with low micromolar affinity. Further characterization confirmed that the EF-Tu- and DsbB-derived peptides bind at two distinct sites. These data point to the possibility that the non-catalytic surface of DsbA is a potential substrate or regulatory protein interaction site. The two peptides identified in this work together with the newly characterized interaction site provide a novel starting point for inhibitor design targeting AbDsbA.

Crystal structure of the dithiol oxidase DsbA enzyme from proteus mirabilis bound non-covalently to an active site peptide ligand

The disulfide bond forming DsbA enzymes and their DsbB interaction partners are attractive targets for development of antivirulence drugs because both are essential for virulence factor assembly in Gram-negative pathogens. Here we characterize PmDsbA from Proteus mirabilis, a bacterial pathogen increasingly associated with multidrug resistance. PmDsbA exhibits the characteristic properties of a DsbA, including an oxidizing potential, destabilizing disulfide, acidic active site cysteine, and dithiol oxidase catalytic activity. We evaluated a peptide, PWATCDS, derived from the partner protein DsbB and showed by thermal shift and isothermal titration calorimetry that it binds to PmDsbA. The crystal structures of PmDsbA, and the active site variant PmDsbAC30S were determined to high resolution. Analysis of these structures allows categorization of PmDsbA into the DsbA class exemplified by the archetypal Escherichia coli DsbA enzyme. We also present a crystal structure of PmDsbAC30S in complex with the peptide PWATCDS. The structure shows that the peptide binds non-covalently to the active site CXXC motif, the cis-Pro loop, and the hydrophobic groove adjacent to the active site of the enzyme. This high-resolution structural data provides a critical advance for future structure-based design of non-covalent peptidomimetic inhibitors. Such inhibitors would represent an entirely new antibacterial class that work by switching off the DSB virulence assembly machinery.

Fusion of HPV L1 into Shigella surface IcsA: a new approach in developing live attenuated Shigella-HPV vaccine

Despite the success of L1 virus-like particles (VLPs) vaccines in prevention of high-risk human papillomavirus (HPV) infection and cervical cancer, extraordinary high cost for the complete vaccination has impeded widespread use of the vaccine in resource-poor countries, where cervical cancers impose greater challenge. Presentation of HPV L1 protein by attenuated pathogenic bacteria through natural infection provides a promising low-cost and convenient alternative. Here, we describe the construction and characterization of attenuated L1-expressing Shigella vaccine candidate, by fusion of L1 into the autotransporter of Shigella sonnei, IcsA, an essential virulence factor responsible for actin-based motility. The functional α domain of IcsA was replaced by codon-optimized L1 gene with independent open reading frames (ORFs) facilitated by suicide vector pJCB12. The L1 gene was stabilized in the genome of recombinant S. sonnei with protein expression and assembly of VLPs in the bacterial cytoplasm. Through conjunctival route vaccination in guinea pigs, L1-containing S. sonnei was able to elicit specific immune response to HPV16 L1 VLP as well as bacterial antigens. The results demonstrated the feasibility of the novel stratagem to develop prophylactic Shigella-HPV vaccines.

Rv2969c, essential for optimal growth in Mycobacterium tuberculosis, is a DsbA-like enzyme that interacts with VKOR-derived peptides and has atypical features of DsbA-like disulfide oxidases

The bacterial disulfide machinery is an attractive molecular target for developing new antibacterials because it is required for the production of multiple virulence factors. The archetypal disulfide oxidase proteins in Escherichia coli (Ec) are DsbA and DsbB, which together form a functional unit: DsbA introduces disulfides into folding proteins and DsbB reoxidizes DsbA to maintain it in the active form. In Mycobacterium tuberculosis (Mtb), no DsbB homologue is encoded but a functionally similar but structurally divergent protein, MtbVKOR, has been identified. Here, the Mtb protein Rv2969c is investigated and it is shown that it is the DsbA-like partner protein of MtbVKOR. It is found that it has the characteristic redox features of a DsbA-like protein: a highly acidic catalytic cysteine, a highly oxidizing potential and a destabilizing active-site disulfide bond. Rv2969c also has peptide-oxidizing activity and recognizes peptide segments derived from the periplasmic loops of MtbVKOR. Unlike the archetypal EcDsbA enzyme, Rv2969c has little or no activity in disulfide-reducing and disulfide-isomerase assays. The crystal structure of Rv2969c reveals a canonical DsbA fold comprising a thioredoxin domain with an embedded helical domain. However, Rv2969c diverges considerably from other DsbAs, including having an additional C-terminal helix (H8) that may restrain the mobility of the catalytic helix H1. The enzyme is also characterized by a very shallow hydrophobic binding surface and a negative electrostatic surface potential surrounding the catalytic cysteine. The structure of Rv2969c was also used to model the structure of a paralogous DsbA-like domain of the Ser/Thr protein kinase PknE. Together, these results show that Rv2969c is a DsbA-like protein with unique properties and a limited substrate-binding specificity.

Deletion of pic results in decreased virulence for a clinical isolate of Shigella flexneri 2a from China

Background

Shigella is a major pathogen responsible for bacillary dysentery, a severe form of shigellosis. Severity of the disease depends on the virulence of the infecting strain. Shigella pathogenicity is a multi-gene phenomenon, involving the participation of genes on an unstable large virulence plasmid and chromosomal pathogenicity islands.

Results

A multiplex PCR (mPCR) assay was developed to detect S. flexneri 2a from rural regions of Zhengding (Hebei Province, China). We isolated and tested 86 strains using our mPCR assay, which targeted the ipaH, ial and set1B genes. A clinical strain of S. flexneri 2a 51 (SF51) containing ipaH and ial, but lacking set1B was found. The virulence of this strain was found to be markedly decreased. Further testing showed that the SF51 strain lacked pic. To investigate the role of pic in S. flexneri 2a infections, a pic knockout mutant (SF301-∆ pic) and two complementation strains, SF301-∆ pic/pPic and SF51/pPic, were created. Differences in virulence for SF51, SF301-∆ pic, SF301-∆ pic/pPic, SF51/pPic and S. flexneri 2a 301 (SF301) were compared. Compared with SF301, both SF51 and SF301-∆ pic exhibited lower levels of Hela cell invasion and resulted in reduced keratoconjunctivitis, with low levels of tissue damage seen in murine eye sections. The virulence of SF301-∆ pic and SF51 was partially recovered in vitro and in vivo through the addition of a complementary pic gene.

Conclusions

The pic gene appears to be involved in an increase in pathogenicity of S. flexneri 2a. This gene assists with bacterial invasion into host cells and alters inflammatory reactions.

Requirement of the CXXC motif of novel Francisella infectivity potentiator protein B FipB, and FipA in virulence of F. tularensis subsp. tularensis

The lipoprotein encoded by the Francisella tularensis subsp. tularensis locus FTT1103 is essential for virulence; an FTT1103 deletion mutant is defective in uptake and intracellular survival, and mice survive high dose challenges of greater than 10⁸ bacteria. This protein has two conserved domains; one is found in a class of virulence proteins called macrophage infectivity potentiator (Mip) proteins, and the other in oxidoreductase Disulfide Bond formation protein A (DsbA)-related proteins. We have designated the protein encoded by FTT1103 as FipB for Francisellainfectivity potentiator protein B. The locus FTT1102 (fipA), which is upstream of fipB, also has similarity to same conserved Mip domain. Deletion and site-specific mutants of fipA and fipB were constructed in the Schu S4 strain, and characterized with respect to intracellular replication and in vivo virulence. A nonpolar fipA mutant demonstrated reduced survival in host cells, but was only slightly attenuated in vivo. Although FipB protein was present in a fipA mutant, the abundance of the three isoforms of FipB was altered, suggesting that FipA has a role in post-translational modification of FipB. Similar to many DsbA homologues, FipB contains a cysteine-any amino acid-any amino acid-cysteine (CXXC) motif. This motif was found to be important for FipB's role in virulence; a deletion mutant complemented with a gene encoding a FipB protein in which the first cysteine was changed to an alanine residue (AXXC) failed to restore intracellular survival or in vivo virulence. Complementation with a gene that encoded a CXXA containing FipB protein was significantly defective in intracellular growth; however, only slightly attenuated in vivo.

Natural products modulate Shigella-host-cell interaction

This study focused on identifying possible new options derived from natural sources for the treatment of bacterial infections. Several natural products were investigated for their potential in modulating Shigella-host-cell interactions. The proliferation of Shigella sonnei was effectively inhibited inside HEp-2 cells in the presence of 4-methoxycinnamic acid and propolin D. Propolin D also significantly reduced the apoptosis of infected macrophage-like U937 cells and moderately reduced the secretion of interleukin (IL)-1β and IL-18, which probably resulted from the inhibition of invasion plasmid antigen B secretion by this compound. Further characterization showed that propolin D did not prevent escape of Shigella from phagocytic vacuoles, as evidenced by actin-based motility and by the fact that addition of chloroquine did not further reduce the number of intracellular c.f.u. The role of propolin D in modulating autophagy could not be established under the experimental conditions used. As these compounds had no direct anti-Shigella activity in vitro, it was concluded that these compounds modulated Shigella-host-cell interactions by targeting yet-to-be defined mechanisms that provide benefits to host cells.

Structure and function of DsbA, a key bacterial oxidative folding catalyst

Since its discovery in 1991, the bacterial periplasmic oxidative folding catalyst DsbA has been the focus of intense research. Early studies addressed why it is so oxidizing and how it is maintained in its less stable oxidized state. The crystal structure of Escherichia coli DsbA (EcDsbA) revealed that the oxidizing periplasmic enzyme is a distant evolutionary cousin of the reducing cytoplasmic enzyme thioredoxin. Recent significant developments have deepened our understanding of DsbA function, mechanism, and interactions: the structure of the partner membrane protein EcDsbB, including its complex with EcDsbA, proved a landmark in the field. Studies of DsbA machineries from bacteria other than E. coli K-12 have highlighted dramatic differences from the model organism, including a striking divergence in redox parameters and surface features. Several DsbA structures have provided the first clues to its interaction with substrates, and finally, evidence for a central role of DsbA in bacterial virulence has been demonstrated in a range of organisms. Here, we review current knowledge on DsbA, a bacterial periplasmic protein that introduces disulfide bonds into diverse substrate proteins and which may one day be the target of a new class of anti-virulence drugs to treat bacterial infection.

Characterization of DsbD in Neisseria meningitidis

Proper periplasmic disulfide bond formation is important for folding and stability of many secreted and membrane proteins, and is catalysed by three DsbA oxidoreductases in Neisseria meningitidis. DsbD provides reducing power to DsbC that shuffles incorrect disulfide bond in misfolded proteins as well as to the periplasmic enzymes that reduce apo-cytochrome c (CcsX) or repair oxidative protein damages (MrsAB). The expression of dsbD, but not other dsb genes, is positively regulated by the MisR/S two-component system. Quantitative real-time PCR analyses showed significantly reduced dsbD expression in all misR/S mutants, which was rescued by genetic complementation. The direct and specific interaction of MisR with the upstream region of the dsbD promoter was demonstrated by electrophoretic mobility shift assay, and the MisR binding sequences were mapped. Further, the expression of dsbD was found to be induced by dithiothrietol (DTT), through the MisR/S regulatory system. Surprisingly, we revealed that inactivation of dsbD can only be achieved in a strain carrying an ectopically located dsbD, in the dsbA1A2 double mutant or in the dsbA1A2A3 triple mutant, thus DsbD is indispensable for DsbA-catalysed oxidative protein folding in N. meningitidis. The defects of the meningococcal dsbA1A2 mutant in transformation and resistance to oxidative stress were more severe in the absence of dsbD.

The secretome of Campylobacter concisus

A higher prevalence of Campylobacter concisus and higher levels of IgG antibodies specific to C. concisus in Crohn's disease patients than in controls were recently detected. In this study, 1D and 2D gel electrophoresis coupled with LTQ FT-MS and QStar tandem MS, respectively, were performed to characterize the secretome of a C. concisus strain isolated from a Crohn's disease patient. Two hundred and one secreted proteins were identified, of which 86 were bioinformatically predicted to be secreted. Searches were performed on the genome of C. concisus strain 13826, and 25 genes that have been associated with virulence or colonization in other organisms were identified. The zonula occludens toxin was found only in C. concisus among the Campylobacterales, although expanded searches revealed that this protein was present in two epsilon-proteobacterial species from extreme marine environments. Alignments and structural threading indicated that this toxin shared features with that of other virulent pathogens, including Neisseria meningitidis and Vibrio cholerae. Further comparative analyses identified several associations between the secretome of C. consisus and putative virulence factors of this bacterium. This study has identified several factors putatively associated with disease outcome, suggesting that C. concisus is a pathogen of the gastrointestinal tract.

Genome sequence of the endosymbiont Rickettsia peacockii and comparison with virulent Rickettsia rickettsii: identification of virulence factors

Rickettsia peacockii, also known as the East Side Agent, is a non-pathogenic obligate intracellular bacterium found as an endosymbiont in Dermacentor andersoni ticks in the western USA and Canada. Its presence in ticks is correlated with reduced prevalence of Rickettsia rickettsii, the agent of Rocky Mountain Spotted Fever. It has been proposed that a virulent SFG rickettsia underwent changes to become the East Side Agent. We determined the genome sequence of R. peacockii and provide a comparison to a closely related virulent R. rickettsii. The presence of 42 chromosomal copies of the ISRpe1 transposon in the genome of R. peacockii is associated with a lack of synteny with the genome of R. rickettsii and numerous deletions via recombination between transposon copies. The plasmid contains a number of genes from distantly related organisms, such as part of the glycosylation island of Pseudomonas aeruginosa. Genes deleted or mutated in R. peacockii which may relate to loss of virulence include those coding for an ankyrin repeat containing protein, DsbA, RickA, protease II, OmpA, ScaI, and a putative phosphoethanolamine transferase. The gene coding for the ankyrin repeat containing protein is especially implicated as it is mutated in R. rickettsii strain Iowa, which has attenuated virulence. Presence of numerous copies of the ISRpe1 transposon, likely acquired by lateral transfer from a Cardinium species, are associated with extensive genomic reorganization and deletions. The deletion and mutation of genes possibly involved in loss of virulence have been identified by this genomic comparison. It also illustrates that the introduction of a transposon into the genome can have varied effects; either correlating with an increase in pathogenicity as in Francisella tularensis or a loss of pathogenicity as in R. peacockii and the recombination enabled by multiple transposon copies can cause significant deletions in some genomes while not in others.

The structure of the bacterial oxidoreductase enzyme DsbA in complex with a peptide reveals a basis for substrate specificity in the catalytic cycle of DsbA enzymes

Oxidative protein folding in Gram-negative bacteria results in the formation of disulfide bonds between pairs of cysteine residues. This is a multistep process in which the dithiol-disulfide oxidoreductase enzyme, DsbA, plays a central role. The structure of DsbA comprises an all helical domain of unknown function and a thioredoxin domain, where active site cysteines shuttle between an oxidized, substrate-bound, reduced form and a DsbB-bound form, where DsbB is a membrane protein that reoxidizes DsbA. Most DsbA enzymes interact with a wide variety of reduced substrates and show little specificity. However, a number of DsbA enzymes have now been identified that have narrow substrate repertoires and appear to interact specifically with a smaller number of substrates. The transient nature of the DsbA-substrate complex has hampered our understanding of the factors that govern the interaction of DsbA enzymes with their substrates. Here we report the crystal structure of a complex between Escherichia coli DsbA and a peptide with a sequence derived from a substrate. The binding site identified in the DsbA-peptide complex was distinct from that observed for DsbB in the DsbA-DsbB complex. The structure revealed details of the DsbA-peptide interaction and suggested a mechanism by which DsbA can simultaneously show broad specificity for substrates yet exhibit specificity for DsbB. This mode of binding was supported by solution nuclear magnetic resonance data as well as functional data, which demonstrated that the substrate specificity of DsbA could be modified via changes at the binding interface identified in the structure of the complex.

Life on the inside: the intracellular lifestyle of cytosolic bacteria

Bacterial pathogens exploit a huge range of niches within their hosts. Many pathogens can invade non-phagocytic cells and survive within a membrane-bound compartment. However, only a small number of bacteria, including Listeria monocytogenes, Shigella flexneri, Burkholderia pseudomallei, Francisella tularensis and Rickettsia spp., can gain access to and proliferate within the host cell cytosol. Here, we discuss the mechanisms by which these cytosolic pathogens escape into the cytosol, obtain nutrients to replicate and subvert host immune responses.

Characterization of two homologous disulfide bond systems involved in virulence factor biogenesis in uropathogenic Escherichia coli CFT073

Disulfide bond (DSB) formation is catalyzed by disulfide bond proteins and is critical for the proper folding and functioning of secreted and membrane-associated bacterial proteins. Uropathogenic Escherichia coli (UPEC) strains possess two paralogous disulfide bond systems: the well-characterized DsbAB system and the recently described DsbLI system. In the DsbAB system, the highly oxidizing DsbA protein introduces disulfide bonds into unfolded polypeptides by donating its redox-active disulfide and is in turn reoxidized by DsbB. DsbA has broad substrate specificity and reacts readily with reduced unfolded proteins entering the periplasm. The DsbLI system also comprises a functional redox pair; however, DsbL catalyzes the specific oxidative folding of the large periplasmic enzyme arylsulfate sulfotransferase (ASST). In this study, we characterized the DsbLI system of the prototypic UPEC strain CFT073 and examined the contributions of the DsbAB and DsbLI systems to the production of functional flagella as well as type 1 and P fimbriae. The DsbLI system was able to catalyze disulfide bond formation in several well-defined DsbA targets when provided in trans on a multicopy plasmid. In a mouse urinary tract infection model, the isogenic dsbAB deletion mutant of CFT073 was severely attenuated, while deletion of dsbLI or assT did not affect colonization.

Identification of an essential Francisella tularensis subsp. tularensis virulence factor

Francisella tularensis, the highly virulent etiologic agent of tularemia, is a low-dose intracellular pathogen that is able to escape from the phagosome and replicate in the cytosol. Although there has been progress in identifying loci involved in the pathogenicity of this organism, analysis of the genome sequence has revealed few obvious virulence factors. We previously reported isolation of an F. tularensis subsp. tularensis strain Schu S4 transposon insertion mutant with a mutation in a predicted hypothetical lipoprotein, FTT1103, that was deficient in intracellular replication in HepG2 cells. In this study, a mutant with a defined nonpolar deletion in FTT1103 was created, and its phenotype, virulence, and vaccine potential were characterized. A phagosomal integrity assay and lysosome-associated membrane protein 1 colocalization revealed that DeltaFTT1103 mutant bacteria were defective in phagosomal escape. FTT1103 mutant bacteria were maximally attenuated in the mouse model; mice survived, without visible signs of illness, challenge by more than 10(10) CFU when the intranasal route was used and challenge by 10(6) CFU when the intraperitoneal, subcutaneous, or intravenous route was used. The FTT1103 mutant bacteria exhibited dissemination defects. Mice that were infected by the intranasal route had low levels of bacteria in their livers and spleens, and these bacteria were cleared by 3 days postinfection. Mutant bacteria inoculated by the subcutaneous route failed to disseminate to the lungs. BALB/c or C57BL/6 mice that were intranasally vaccinated with 10(8) CFU of FTT1103 mutant bacteria were protected against subsequent challenge with wild-type strain Schu S4. These experiments identified the FTT1103 protein as an essential virulence factor and also demonstrated the feasibility of creating defined attenuated vaccines based on a type A strain.

Francisella tularensis subsp. tularensis Schu S4 disulfide bond formation protein B, but not an RND-type efflux pump, is required for virulence

Francisella tularensis subsp. tularensis is a highly virulent bacterium that is a CDC select agent. Despite advancements in the understanding of its biology, details pertaining to virulence are poorly understood. In previous work, we identified a transposon insertion mutant in the FTT0107c locus that was defective in intracellular survival in HepG2 and J77A.1 cells. Here, we report that this mutant was also highly attenuated in vivo. The FTT0107c locus is predicted to encode an ortholog of the disulfide bond formation B protein (DsbB). This designation was confirmed by complementation of an Escherichia coli dsbB mutant. This dsbB mutant of Schu S4 was highly attenuated in mice, but unlike what has been reported for Francisella novicida, intranasal immunization with a sublethal dose did not induce protection against wild-type challenge. dsbB was found to be transcribed in an operon with acrA and acrB, which encode an RND-type efflux pump. However, this pump did not make a significant contribution to virulence because strains with nonpolar deletions in acrA and acrB behaved like wild-type strain Schu S4 with respect to intracellular growth and in vivo virulence. This result is in contrast to a report that an acrB mutant of a live vaccine strain of F. tularensis has decreased virulence in mice. Overall, these results demonstrate key differences between the virulence requirements of Schu S4 and less virulent subspecies of Francisella. We have shown that DsbB is a key participant in intracellular growth and virulence, and our results suggest that there are critical virulence factors that contain disulfide bonds.

Moraxella osloensis gene expression in the slug host Deroceras reticulatum

Background

The bacterium Moraxella osloensis is a mutualistic symbiont of the slug-parasitic nematode Phasmarhabditis hermaphrodita. In nature, P. hermaphrodita vectors M. osloensis into the shell cavity of the slug host Deroceras reticulatum in which the bacteria multiply and kill the slug. As M. osloensis is the main killing agent, genes expressed by M. osloensis in the slug are likely to play important roles in virulence. Studies on pathogenic interactions between bacteria and lower order hosts are few, but such studies have the potential to shed light on the evolution of bacterial virulence. Therefore, we investigated such an interaction by determining gene expression of M. osloensis in its slug host D. reticulatum by selectively capturing transcribed sequences.

Results

Thirteen M. osloensis genes were identified to be up-regulated post infection in D. reticulatum. Compared to the in vitro expressed genes in the stationary phase, we found that genes of ubiquinone synthetase (ubiS) and acyl-coA synthetase (acs) were up-regulated in both D. reticulatum and stationary phase in vitro cultures, but the remaining 11 genes were exclusively expressed in D. reticulatum and are hence infection specific. Mutational analysis on genes of protein-disulfide isomerase (dsbC) and ubiS showed that the virulence of both mutants to slugs was markedly reduced and could be complemented. Further, compared to the growth rate of wild-type M. osloensis, the dsbC and ubiS mutants showed normal and reduced growth rate in vitro, respectively.

Conclusion

We conclude that 11 out of the 13 up-regulated M. osloensis genes are infection specific. Distribution of these identified genes in various bacterial pathogens indicates that the virulence genes are conserved among different pathogen-host interactions. Mutagenesis, growth rate and virulence bioassays further confirmed that ubiS and dsbC genes play important roles in M. osloensis survival and virulence, respectively in D. reticulatum.

Potential role of thiol:disulfide oxidoreductases in the pathogenesis ofHelicobacter pylori

Helicobacter pylori infections are responsible for a sequence of molecular events which ultimately result in the development of gastric diseases. The pathogenesis of H. pylori has been studied extensively with strong focus on the identification of virulence factors. In contrast, the involvement of thiol:disulfide oxidoreductases in bacterial pathogenesis is less well understood. This paper provides a review of the current knowledge of H. pylori putative thiol:disulfide oxidoreductases, and their potential role in promoting virulence and colonization. Several bioinformatic analyses served to complete the information on these oxidoreductases of H. pylori.

Identification of transposon insertion mutants of Francisella tularensis tularensis strain Schu S4 deficient in intracellular replication in the hepatic cell line HepG2

Background

Francisella tularensis is a zoonotic intracellular bacterial pathogen that causes tularemia. The subspecies tularensis is highly virulent and is classified as a category A agent of biological warfare because of its low infectious dose by an aerosol route, and its ability to cause severe disease. In macrophages F. tularensis exhibits a rather novel intracellular lifestyle; after invasion it remains in a phagosome for three to six hours before escaping to, and replicating in the cytoplasm. The molecular mechanisms that allow F. tularensis to invade and replicate within a host cell have not been well defined.

Methods

We constructed a stable transposon mutagenesis library of virulent strain Schu S4 using a derivative of the EZ::TN transposon system^®. Approximately 2000 mutants were screened for the inability to invade, and replicate in the hepatic carcinoma cell line HepG2. These mutants were also tested for replication within the J774.1 macrophage-like cell line.

Results

Eighteen mutants defective in intracellular replication in HepG2 cells were identified. Eight of these mutants were auxotrophs; seven had mutations in nucleotide biosynthesis pathways. The remaining mutants had insertions in genes that were predicted to encode putative transporters, enzymes involved in protein modification and turnover, and hypothetical proteins. A time course of the intracellular growth of a pyrB mutant revealed that this mutant was only able to grow at low levels within HepG2 cells but grew like wild-type bacteria in J774.1 cells. This pyrB mutant was also attenuated in mice.

Conclusion

This is the first reported large-scale mutagenesis of a type A strain of F. tularensis and the first identification of mutants specifically defective in intracellular growth in a hepatic cell line. We have identified several genes and pathways that are key for the survival and growth of F. tularensis in a hepatic cell line, and a number of novel intracellular growth-defective mutants that have not been previously characterized in other pathogens. Further characterization of these mutants will help provide a better understanding of the pathogenicity of F. tularensis, and may have practical applications as targets for drugs or attenuated vaccines.

The Cpx envelope stress response affects expression of the type IV bundle-forming pili of enteropathogenic Escherichia coli

The Cpx envelope stress response mediates adaptation to potentially lethal envelope stresses in Escherichia coli. The two-component regulatory system consisting of the sensor kinase CpxA and the response regulator CpxR senses and mediates adaptation to envelope insults believed to result in protein misfolding in this compartment. Recently, a role was demonstrated for the Cpx response in the biogenesis of P pili, attachment organelles expressed by uropathogenic E. coli. CpxA senses misfolded P pilus assembly intermediates and initiates increased expression of both assembly and regulatory factors required for P pilus elaboration. In this report, we demonstrate that the Cpx response is also involved in the expression of the type IV bundle-forming pili of enteropathogenic E. coli (EPEC). Bundle-forming pili were not elaborated from an exogenous promoter in E. coli laboratory strain MC4100 unless the Cpx pathway was constitutively activated. Further, an EPEC cpxR mutant synthesized diminished levels of bundle-forming pili and was significantly affected in adherence to epithelial cells. Since type IV bundle-forming pili are very different from chaperone-usher-type P pili in both form and biogenesis, our results suggest that the Cpx envelope stress response plays a general role in the expression of envelope-localized organelles with diverse structures and assembly pathways.

Transcriptional adaptation of Shigella flexneri during infection of macrophages and epithelial cells: insights into the strategies of a cytosolic bacterial pathogen

Shigella flexneri, the etiologic agent of bacillary dysentery, invades epithelial cells as well as macrophages and dendritic cells and escapes into the cytosol soon after invasion. Dissection of the global gene expression profile of the bacterium in its intracellular niche is essential to fully understand the biology of Shigella infection. We have determined the complete gene expression profiles for S. flexneri infecting human epithelial HeLa cells and human macrophage-like U937 cells. Approximately one quarter of the S. flexneri genes showed significant transcriptional adaptation during infection; 929 and 1,060 genes were up- or down-regulated within HeLa cells and U937 cells, respectively. The key S. flexneri virulence genes, ipa-mxi-spa and icsA, were drastically down-regulated during intracellular growth. This theme seems to be common in bacterial infection, because the Ipa-Mxi-Spa-like type III secretion systems were also down-regulated during mammalian cell infection by Salmonella enterica serovar Typhimurium and Escherichia coli O157. The bacteria experienced restricted levels of iron, magnesium, and phosphate in both host cell types, as shown by up-regulation of the sitABCD system, the mgtA gene, and genes of the phoBR regulon. Interestingly, ydeO and other acid-induced genes were up-regulated only in U937 cells and not in HeLa cells, suggesting that the cytosol of U937 cells is acidic. Comparison with the gene expression of intracellular Salmonella serovar Typhimurium, which resides within the Salmonella-containing vacuole, indicated that S. flexneri is exposed to oxidative stress in U937 cells. This work will facilitate functional studies of hundreds of novel intracellularly regulated genes that may be important for the survival and growth strategies of Shigella in the human host.

Characterization of new DsbB-like thiol-oxidoreductases of Campylobacter jejuni and Helicobacter pylori and classification of the DsbB family based on phylogenomic, structural and functional criteria

In Gram-negative bacterial cells, disulfide bond formation occurs in the oxidative environment of the periplasm and is catalysed by Dsb (disulfide bond) proteins found in the periplasm and in the inner membrane. In this report the identification of a new subfamily of disulfide oxidoreductases encoded by a gene denoted dsbI, and functional characterization of DsbI proteins from Campylobacter jejuni and Helicobacter pylori, as well as DsbB from C. jejuni, are described. The N-terminal domain of DsbI is related to DsbB proteins and comprises five predicted transmembrane segments, while the C-terminal domain is predicted to locate to the periplasm and to fold into a β-propeller structure. The dsbI gene is co-transcribed with a small ORF designated dba ( dsbI-accessory). Based on a series of deletion and complementation experiments it is proposed that DsbB can complement the lack of DsbI but not the converse. In the presence of DsbB, the activity of DsbI was undetectable, hence it probably acts only on a subset of possible substrates of DsbB. To reconstruct the principal events in the evolution of DsbB and DsbI proteins, sequences of all their homologues identifiable in databases were analysed. In the course of this study, previously undetected variations on the common thiol-oxidoreductase theme were identified, such as development of an additional transmembrane helix and loss or migration of the second pair of Cys residues between two distinct periplasmic loops. In conjunction with the experimental characterization of two members of the DsbI lineage, this analysis has resulted in the first comprehensive classification of the DsbB/DsbI family based on structural, functional and evolutionary criteria.

Role of the two-component regulator CpxAR in the virulence of Salmonella enterica serotype Typhimurium

The CpxAR (Cpx) two-component regulator controls the expression of genes in response to a variety of environmental cues. The Cpx regulator has been implicated in the virulence of several gram-negative pathogens, although a role for Cpx in vivo has not been demonstrated directly. Here we investigate whether positive or negative control of gene expression by Cpx is important for the pathogenesis of Salmonella enterica serotype Typhimurium. The Cpx signal pathway in serotype Typhimurium was disrupted by insertional inactivation of the cpxA and cpxR genes. We also constitutively activated the Cpx pathway by making an internal in-frame deletion in cpxA (a cpxA* mutation). Activation of the Cpx pathway inhibited induction of the envelope stress response pathway controlled by the alternative sigma factor sigma(E) (encoded by rpoE). Conversely, the Cpx pathway was highly up-regulated (>40-fold) in a serotype Typhimurium rpoE mutant. The cpxA* mutation, but not the cpxA or the cpxR mutation, significantly reduced the capacity of serotype Typhimurium to adhere to and invade eucaryotic cells, although intracellular replication was not affected. The cpxA and cpxA* mutations significantly impaired the ability of serotype Typhimurium to grow in vivo in mice. To our knowledge, this is the first demonstration that the Cpx system is important for a bacterial pathogen in vivo.

The role of the Shigella flexneri yihE gene in LPS synthesis and virulence

Previously, the authors have shown that inactivation of Shigella flexneri yihE, a gene of unknown function upstream of dsbA, which encodes a periplasmic disulphide catalyst, results in a global change of gene expression. Among the severely down-regulated genes are galETKM, suggesting that the yihE mutant, Sh54, may inefficiently produce the UDP-glucose and UDP-galactose required for LPS synthesis. This paper demonstrates that LPS synthesis in Sh54 is impaired. As a result, Sh54 is unable to polymerize host cell actin, due to aberrant localization of IcsA, or to cause keratoconjunctivitis in guinea pigs. Furthermore, Sh54 is more sensitive to some antimicrobial agents, and exhibits epithelial cytotoxicity characteristic of neither wild-type nor dsbA mutants. Supplying galETK in trans restores LPS synthesis and corrects all the defects. Hence, it is clear that the Shigella yihE gene is important not only in regulating global gene expression, as shown previously, but also in virulence through LPS synthesis via regulating the expression of the galETK operon.

DsbA of Pseudomonas aeruginosa is essential for multiple virulence factors

DsbA is a periplasmic thiol:disulfide oxidoreductase which contributes to the process of protein folding by catalyzing the formation of disulfide bonds. In this study, we demonstrate that the dsbA gene is required for the expression of the type III secretion system under low-calcium inducing conditions, intracellular survival of P. aeruginosa upon infection of HeLa cells, and twitching motility. The diverse phenotypes of the dsbA mutant are likely due to its defect in the folding of proteins that are involved in various biological processes, such as signal sensing, protein secretion, and defense against host clearing. In light of its effect on various virulence factors, DsbA could be an important target for the control of P. aeruginosa infections.

Salmonella enterica serovar typhimurium rdoA is growth phase regulated and involved in relaying Cpx-induced signals

The disulfide oxidoreductase, DsbA, mediates disulfide bond formation in proteins as they enter or pass through the periplasm of gram-negative bacteria. Although DsbA function has been well characterized, less is known about the factors that control its expression. Previous studies with Escherichia coli demonstrated that dsbA is part of a two-gene operon that includes an uncharacterized, upstream gene, yihE, that is positively regulated via the Cpx stress response pathway. To clarify the role of the yihE homologue on dsbA expression in Salmonella enterica serovar Typhimurium, the effect of this gene (termed rdoA) on the regulation of dsbA expression was investigated. Transcriptional assays assessing rdoA promoter activity showed growth phase-dependent expression with maximal activity in stationary phase. Significant quantities of rdoA and dsbA transcripts exist in serovar Typhimurium, but only extremely low levels of rdoA-dsbA cotranscript were detected. Activation of the Cpx system in serovar Typhimurium increased synthesis of both rdoA- and dsbA-specific transcripts but did not significantly alter the levels of detectable cotranscript. These results indicate that Cpx-mediated induction of dsbA transcription in serovar Typhimurium does not occur through an rdoA-dsbA cotranscript. A deletion of the rdoA coding region was constructed to definitively test the relevance of the rdoA-dsbA cotranscript to dsbA expression. The absence of RdoA affects DsbA expression levels when the Cpx system is activated, and providing rdoA in trans complements this phenotype, supporting the hypothesis that a bicistronic mechanism is not involved in serovar Typhimurium dsbA regulation. The rdoA null strain was also shown to be altered in flagellar phase variation. First it was found that induction of the Cpx stress response pathway switched flagellar synthesis to primarily phase 2 flagellin, and this effect was then found to be abrogated in the rdoA null strain, suggesting the involvement of RdoA in mediating Cpx-related signaling.

Identification of a disulfide isomerase protein of Leishmania major as a putative virulence factor

Several approaches have been previously used to elucidate the genetic basis of Leishmania virulence. In general, they were based on laboratory Leishmania clones genetically modified or grown in the presence of selecting agents. In a previous study, we demonstrated that Leishmania major freshly isolated from human cutaneous lesions showed significant differences in the severity of the experimental disease induced in BALB/c mice. Here, using the mRNA differential display technique, we analyzed gene expression in L. major promastigotes showing different levels of virulence. We have identified a novel Leishmania gene encoding a 477-amino-acid protein exhibiting two distinct regions that are identical to the putative active-site sequence (CGHC) of the eukaryotic protein disulfide isomerase (PDI). The recombinant protein displayed a specific PDI enzymatic activity. This L. major disulfide isomerase protein (LmPDI) is predominantly expressed, at both the mRNA and protein levels, in highly virulent strains. Specific PDI inhibitors abolished the enzymatic activity of the recombinant protein and profoundly affected parasite growth. These findings suggest that LmPDI may play an important role in Leishmania natural pathogenicity and may constitute a new target for anti-Leishmania chemotherapy.

Pathoadaptive mutations that enhance virulence: genetic organization of the cadA regions of Shigella spp

Pathoadaptive mutations improve the fitness of pathogenic species by modification of traits that interfere with factors (virulence and ancestral) required for survival in host tissues. A demonstrated pathoadaptive mutation is the loss of lysine decarboxylase (LDC) expression in Shigella species that have evolved from LDC-expressing Escherichia coli. Previous studies demonstrated that the product of LDC activity, cadaverine, blocks the action of Shigella enterotoxins and that the gene encoding LDC, cadA, was abolished by large chromosomal deletions in each Shigella species. To better understand the nature and evolution of these pathoadaptive mutations, remnants of the cad region were sequenced from the four Shigella species. These analyses reveal novel gene arrangements in this region of the pathogens' chromosomes. Insertion sequences, a phage genome, and/or loci from different positions on the ancestral E. coli chromosome displaced the cadA locus to form distinct genetic linkages that are unique to each Shigella species. Hybridization studies, using an E. coli K-12 microarray, indicated that the genes displaced to form the novel linkages still remain in the Shigella genomes. None of these novel gene arrangements were observed in representatives of all E. coli phylogenies. Collectively, these observations indicate that inactivation of the cadA antivirulence gene occurred independently in each Shigella species. The convergent evolution of these pathoadaptive mutations demonstrates that, following evolution from commensal E. coli, strong pressures in host tissues selected Shigella clones with increased fitness and virulence through the loss of an ancestral trait (LDC). These observations strongly support the role of pathoadaptive mutation as an important pathway in the evolution of pathogenic organisms.

Influence of the yihE gene of Shigella flexneri on global gene expression: on analysis using DNA arrays

Inactivation of dsbA (disulfide bond formation), either by an insertion (Sh4, dsbA::kan) or by alteration of the active site (Sh42, dsbA33G), renders Shigella flexneri avirulent. However, Sh4 and Sh42 behave differently in many ways in vitro and in vivo. A gene of unknown function, yihE, up-stream and cotranscribed with dsbA, is thought to differentiate Sh4 and Sh42 as the kan insertion may result in a truncated unstable yihE-dsbA mRNA in Sh4. To gain insight into the function of yihE, DNA array hybridization was performed to study the genomic expression in Sh4, Sh42, and a newly constructed yihE mutant (Sh54). Compared to the wild-type, M90TS, Sh4, and Sh54 demonstrated significantly changed transcription levels of about 100 genes, of which many involved in energy metabolism and stress response were down- and up-regulated, respectively. In contrast, Sh42 showed altered transcription levels of only 20 genes. The results argue that yihE is principally responsible for the changed genomic expression in Sh4 and Sh54. Given the fact that the transcription of yihE-dsbA is regulated by the CpxRA two-component signal transduction system, yihE is probably involved in the extracytoplasmic stress response in a manor deserving further studies.

Inactivation of DsbA alters the behaviour of Shigella flexneri towards murine and human-derived macrophage-like cells

The mutants of Shigella flexneri, Sh4 (dsbA::kan) and Sh42 (dsbA33G), behave differently towards murine and human-derived macrophage-like cells in vitro. Sh4 was trapped in the phagocytic vacuoles of the murine J774 cells as evidenced by its colony forming units plus and minus chloroquine exposure in a gentamicin protection assay, and by light and transmission electron microscopy (TEM). Sh42, similar to the wild-type M90TS, was able to escape from the vacuoles and kill host cells presumably by inducing apoptosis. In U937 cells, unlike M90TS that was free in the cytosol, both Sh4 and Sh42 grew poorly. TEM revealed that Sh4 and Sh42 were trapped within the U937 phagocytic vacuoles. Furthermore, the two mutants induced different patterns of interleukin-1beta and tumour necrosis factor-alpha expression, which might explain why they possess different immunogenic properties in vivo.