Characterization of invasion plasmid antigen genes (ipaBCD) from Shigella flexneri

Shigella Mutant with Truncated O-Antigen as an Enteric Multi-Pathogen Vaccine Platform

Background/Objectives: Rising antibiotic resistance underscores the urgent need for effective vaccines against shigellosis. Our previous research identified the Shigella flexneri 2a truncated mutant (STM), a wzy gene knock-out strain cultivated in shake-flasks, as a promising broadly protective Shigella vaccine candidate. Expanding on this finding, our current study explores the feasibility of transitioning to a fermentor-grown STM as a vaccine candidate for further clinical development. Methods: The STM and STM-Cj, engineered to express the conserved Campylobacter jejuni N-glycan antigen, were grown in animal-free media, inactivated with formalin, and evaluated for key antigen retention and immunogenicity in mice. Results: The fermentor-grown STM exhibited significantly increased production yields and retained key antigens after inactivation. Immunization with the STM, particularly along with the double-mutant labile toxin (dmLT) adjuvant, induced robust immune responses to the conserved proteins IpaB, IpaC, and PSSP-1. Additionally, it provided protection against homologous and heterologous Shigella challenges in a mouse pulmonary model. The STM-Cj vaccine elicited antibody responses specific to the N-glycan while maintaining protective immune responses against Shigella. These findings underscore the potential of the fermentor-grown STM as a safe and immunogenic vaccine platform for combating shigellosis and possibly other gastrointestinal bacterial infections. Conclusions: Further process development to optimize growth and key antigen expression as well as expanded testing in additional animal models for the assessment of protection against Shigella and Campylobacter are needed to build on these encouraging initial results. Ultimately, clinical trials are essential to evaluate the efficacy and safety of STM-based vaccines in humans.

REV7: a small but mighty regulator of genome maintenance and cancer development

REV7, also known as MAD2B, MAD2L2, and FANCV, is a HORMA-domain family protein crucial to multiple genome stability pathways. REV7's canonical role is as a member of polymerase ζ, a specialized translesion synthesis polymerase essential for DNA damage tolerance. REV7 also ensures accurate cell cycle progression and prevents premature mitotic progression by sequestering an anaphase-promoting complex/cyclosome activator. Additionally, REV7 supports genome integrity by directing double-strand break repair pathway choice as part of the recently characterized mammalian shieldin complex. Given that genome instability is a hallmark of cancer, it is unsurprising that REV7, with its numerous genome maintenance roles, is implicated in multiple malignancies, including ovarian cancer, glioma, breast cancer, malignant melanoma, and small-cell lung cancer. Moreover, high REV7 expression is associated with poor prognoses and treatment resistance in these and other cancers. Promisingly, early studies indicate that REV7 suppression enhances sensitivity to chemotherapeutics, including cisplatin. This review aims to provide a comprehensive overview of REV7's myriad roles in genome maintenance and other functions as well as offer an updated summary of its connections to cancer and treatment resistance.

Bioactivity and efficacy of a hyperimmune bovine colostrum product- Travelan, against shigellosis in a non-Human primate model (Macaca mulatta)

Infectious diarrhea is a World Health Organization public health priority area due to the lack of effective vaccines and an accelerating global antimicrobial resistance crisis. New strategies are urgently needed such as immunoprophylactic for prevention of diarrheal diseases. Hyperimmune bovine colostrum (HBC) is an established and effective prophylactic for infectious diarrhea. The commercial HBC product, Travelan® (Immuron Ltd, Australia) targets multiple strains of enterotoxigenic Escherichia coli (ETEC) is highly effective in preventing diarrhea in human clinical studies. Although Travelan® targets ETEC, preliminary studies suggested cross-reactivity with other Gram-negative enteric pathogens including Shigella and Salmonella species. For this study we selected an invasive diarrheal/dysentery-causing enteric pathogen, Shigella, to evaluate the effectiveness of Travelan®, both in vitro and in vivo. Here we demonstrate broad cross-reactivity of Travelan® with all four Shigella spp. (S. flexneri, S. sonnei, S. dysenteriae and S. boydii) and important virulence factor Shigella antigens. Naïve juvenile rhesus macaques (NJRM) were randomized, 8 dosed with Travelan® and 4 with a placebo intragastrically twice daily over 6 days. All NJRM were challenged with S. flexneri 2a strain 2457T on the 4th day of treatment and monitored for diarrheal symptoms. All placebo-treated NJRM displayed acute dysentery symptoms within 24-36 hours of challenge. Two Travelan®-treated NJRM displayed dysentery symptoms and six animals remained healthy and symptom-free post challenge; resulting in 75% efficacy of prevention of shigellosis (p = 0.014). These results strongly indicate that Travelan® is functionally cross-reactive and an effective prophylactic for shigellosis. This has positive implications for the prophylactic use of Travelan® for protection against both ETEC and Shigella spp. diarrheal infections. Future refinement and expansion of pathogens recognized by HBC including Travelan® could revolutionize current management of gastrointestinal infections and outbreaks in travelers' including military, peacekeepers, humanitarian workers and in populations living in endemic regions of the world.

Characterization of Functional B-Cell Epitopes at the Amino Terminus of Shigella Invasion Plasmid Antigen B (IpaB)

Shigella invasion plasmid antigen B (IpaB) plays an important role in causing shigellosis. While IpaB's protein structure, contribution to disease mechanism, and protective immunity against Shigella infection have been well studied, the significance of individual antigenic domains, especially at the N terminus, has not been systematically characterized. In an attempt to identify IpaB protein functional epitopes and to construct an optimized polyvalent multiepitope fusion antigen (MEFA) immunogen for development of a protein-based cross protective Shigella vaccine, in this study, we in silico identified immunodominant B-cell epitopes from the IpaB N terminus, fused each epitope to carrier protein CsaB (the major subunit of enterotoxigenic Escherichia coli CS4 adhesin) for epitope fusion proteins, immunized mice with each epitope fusion protein, examined IpaB-specific antibody responses, and assessed antibody functional activity against Shigella bacterial invasion. A total of 10 B-cell continuous epitopes were identified from IpaB N terminus, and after being fused to carrier protein CsaB, each epitope induced anti-IpaB IgG responses in the intramuscularly immunized mice. While in vitro antibody invasion inhibition assays demonstrated that antibodies derived from each identified epitope were functional, epitopes 1 (LAKILASTELGDNTIQAA), 2 (HSTSNILIPELKAPKSL), and 4 (QARQQKNLEFSDKI) induced antibodies to inhibit Shigella sonnei and Shigella flexneri invasion at levels similar to those of recombinant IpaB protein, suggesting that these three IpaB epitopes can be used potentially as IpaB-representing antigens to induce protective anti-IpaB antibodies and for construction of an epitope-based polyvalent MEFA protein immunogen for Shigella vaccine development. IMPORTANCE Currently, there are no effective measures for control or prevention of Shigella infection, the most common cause of diarrhea in children 3 to 5 years of age in developing countries. Challenges in developing Shigella vaccines include virulence heterogeneity among species and serotypes. To overcome virulence heterogeneity challenge and to develop a protein-based multivalent Shigella vaccine, we targeted a panel of virulence factors, including invasion plasmid antigens, identified functional antigenic domains or epitopes as representative antigens, and applied the novel epitope- and structure-based vaccinology platform multiepitope fusion antigen (MEFA) to integrate functional antigenic domains or epitopes into a backbone immunogen to produce a polyvalent immunogen for cross protective antibodies. Identification of functional IpaB epitopes from this study enhances our understanding of IpaB immunogenicity and allows us to directly utilize IpaB epitopes for construction of a cross protective polyvalent Shigella immunogen and to accelerate development of a protein-based Shigella vaccine.

Burden, Antibiotic Resistance, and Clonality of Shigella spp. Implicated in Community-Acquired Acute Diarrhoea in Lilongwe, Malawi

Although numerous studies have investigated diarrhoea aetiology in many sub-Saharan African countries, recent data on Shigella species’ involvement in community-acquired acute diarrhoea (CA-AD) in Malawi are scarce. This study investigated the incidence, antibiotic susceptibility profile, genotypic characteristics, and clonal relationships of Shigella flexneri among 243 patients presenting with acute diarrhoea at a District Hospital in Lilongwe, Malawi. Shigella spp. were isolated and identified using standard microbiological and serological methods and confirmed by identifying the ipaH gene using real-time polymerase chain reaction. The isolates’ antibiotic susceptibility to 20 antibiotics was determined using the VITEK 2 system according to EUCAST guidelines. Genes conferring resistance to sulfamethoxazole (sul1, sul2 and sul3), trimethoprim (dfrA1, dfrA12 and dfrA17) and ampicillin (oxa-1 and oxa-2), and virulence genes (ipaBCD, sat, ial, virA, sen, set1A and set1B) were detected by real-time PCR. Clonal relatedness was assessed using ERIC-PCR. Thirty-four Shigella flexneri isolates were isolated (an overall incidence of 14.0%). All the isolates were fully resistant to sulfamethoxazole/trimethoprim (100%) and ampicillin (100%) but susceptible to the other antibiotics tested. The sul1 (79%), sul2 (79%), sul3 (47%), dfrA12 (71%) and dfrA17 (56%) sulfonamide and trimethoprim resistance genes were identified; Oxa-1, oxa-2 and dfrA1 were not detected. The virulence genes ipaBCD (85%), sat (85%), ial (82%), virA (76%), sen (71%), stx (71%), set1A (26%) and set1B (18%) were detected. ERIC-PCR profiling revealed that the Shigella isolates were genetically distinct and clonally unrelated, indicating the potential involvement of genetically distinct S. flexneri in CA-AD in Malawi. The high percentage resistance to ampicillin and sulfamethoxazole/trimethoprim and the presence of several virulence determinants in these isolates emphasises a need for continuous molecular surveillance studies to inform preventive measures and management of Shigella-associated diarrhoeal infections in Malawi.

Cross-Protective Shigella Whole-Cell Vaccine With a Truncated O-Polysaccharide Chain

Shigella is a highly prevalent bacterium causing acute diarrhea and dysentery in developing countries. Shigella infections are treated with antibiotics but Shigellae are increasingly resistant to these drugs. Vaccination can be a countermeasure against emerging antibiotic-resistant shigellosis. Because of the structural variability in Shigellae O-antigen polysaccharides (Oag), cross-protective Shigella vaccines cannot be derived from single serotype-specific Oag. We created an attenuated Shigella flexneri 2a strain with one rather than multiple Oag units by disrupting the Oag polymerase gene (Δwzy), which broadened protective immunogenicity by exposing conserved surface proteins. Inactivated Δwzy mutant cells combined with Escherichia coli double mutant LT(R192G/L211A) as adjuvant, induced potent antibody responses to outer membrane protein PSSP-1, and type III secretion system proteins IpaB and IpaC. Intranasal immunization with the vaccine preparation elicited cross-protective immunity against S. flexneri 2a, S. flexneri 3a, S. flexneri 6, and Shigella sonnei in a mouse pneumonia model. Thus, S. flexneri 2a Δwzy represents a promising candidate strain for a universal Shigella vaccine.

Shigella outer membrane protein PSSP-1 is broadly protective against Shigella infection

In developing countries, Shigella is a primary cause of diarrhea in infants and young children. Although antibiotic therapy is an effective treatment for shigellosis, therapeutic options are narrowing due to the emergence of antibiotic resistance. Thus, preventive vaccination could become the most efficacious approach for controlling shigellosis. We have identified several conserved protein antigens that are shared by multiple Shigella serotypes and species. Among these, one antigen induced cross-protection against experimental shigellosis, and we have named it pan-Shigella surface protein 1 (PSSP-1). PSSP-1-induced protection requires a mucosal administration route and coadministration of an adjuvant. When PSSP-1 was administered intranasally, it induced cross-protection against Shigella flexneri serotypes 2a, 5a, and 6, Shigella boydii, Shigella sonnei, and Shigella dysenteriae serotype 1. Intradermally administered PSSP-1 induced strong serum antibody responses but failed to induce protection in the mouse lung pneumonia model. In contrast, intranasal administration elicited efficient local and systemic antibody responses and production of interleukin 17A and gamma interferon. Interestingly, blood samples from patients with recent-onset shigellosis showed variable but significant mucosal antibody responses to other conserved Shigella protein antigens but not to PSSP-1. We suggest that PSSP-1 is a promising antigen for a broadly protective vaccine against Shigella.

Characterization of molten globule PopB in absence and presence of its chaperone PcrH

The TTSS encoding "translocator operon" of Pseudomonas aeruginosa consists of a major translocator protein PopB, minor translocator protein PopD and their cognate chaperone PcrH. Far-UV CD spectra and secondary structure prediction servers predict an α-helical model for PopB, PcrH and PopB-PcrH complex. PopB itself forms a single species of higher order oligomer (15 mer) as seen from AUC, but in complex with PcrH, both monomeric (1:1) and oligomeric form exist. PopB has large solvent-exposed hydrophobic patches and exists as an unordered molten globule in its native state, but on forming complex with PcrH it gets transformed into an ordered molten globule. Tryptophan fluorescence spectrum indicates that PopB interacts with the first TPR region of dimeric PcrH to form a stable PopB-PcrH complex that has a partial rigid structure with a large hydrodynamic radius and few tertiary contacts. The pH-dependent studies of PopB, PcrH and complex by ANS fluorescence, urea induced unfolding and thermal denaturation experiments prove that PcrH not only provides structural support to the ordered molten globule PopB in complex but also undergoes conformational change to assist PopB to pass through the needle complex of TTSS and form pores in the host cell membrane. ITC experiments show a strong affinity (K(d) ~ 0.37 μM) of PopB for PcrH at pH 7.8, which reduces to ~0.68 μM at pH 5.8. PcrH also loses its rigid tertiary structure at pH 5 and attains a molten globule conformation. This indicates that the decrease in pH releases PopB molecules and thus triggers the TTSS activation mechanism for the formation of a functional translocon.

Vinculin activators target integrins from within the cell to increase melanoma sensitivity to chemotherapy

Metastatic melanoma is an aggressive skin disease for which there are no effective therapies. Emerging evidence indicates that melanomas can be sensitized to chemotherapy by increasing integrin function. Current integrin therapies work by targeting the extracellular domain, resulting in complete gains or losses of integrin function that lead to mechanism-based toxicities. An attractive alternative approach is to target proteins, such as vinculin, that associate with the integrin cytoplasmic domains and regulate its ligand-binding properties. Here, we report that a novel reagent, denoted vinculin-activating peptide or VAP, increases integrin activity from within the cell, as measured by elevated (i) numbers of active integrins, (ii) adhesion of cells to extracellular matrix ligands, (iii) numbers of cell-matrix adhesions, and (iv) downstream signaling. These effects are dependent on both integrins and a key regulatory residue A50 in the vinculin head domain. We further show that VAP dramatically increases the sensitivity of melanomas to chemotherapy in clonal growth assays and in vivo mouse models of melanoma. Finally, we show that the increase in chemosensitivity results from increases in DNA damage-induced apoptosis in a p53-dependent manner. Collectively, these findings show that integrin function can be manipulated from within the cell and validate integrins as a new therapeutic target for the treatment of chemoresistant melanomas.

Biophysical characterization of Chlamydia trachomatis CT584 supports its potential role as a type III secretion needle tip protein

Chlamydia are obligate intracellular bacterial pathogens that cause a variety of diseases. Like many Gram-negative bacteria, they employ type III secretion systems (T3SS) for invasion, establishing and maintaining their unique intracellular niche, and possibly cellular exit. Computational structure prediction indicated that ORF CT584 is homologous to other T3SS needle tip proteins. Tip proteins have been shown to be localized to the extracellular end of the T3SS needle and play a key role in controlling secretion of effector proteins. We have previously demonstrated that T3SS needle tip proteins from different bacteria share many biophysical characteristics. To support the hypothesis that CT584 is a T3SS needle tip protein, biophysical properties of CT584 were explored as a function of pH and temperature, using spectroscopic techniques. Far-UV circular dichroism, Fourier transform infrared spectroscopy, UV absorbance spectroscopy, ANS extrinsic fluorescence, turbidity, right angle static light scattering, and analytical ultracentrifugation were all employed to monitor the secondary, tertiary, quaternary, and aggregation behavior of this protein. An empirical phase diagram approach is also employed to facilitate such comparisons. These analyses demonstrate that CT584 shares many biophysical characteristics with other T3SS needle tip proteins. These data support the hypothesis that CT584 is a member of the same functional family, although future biologic analyses are required.

Phenotypic and genotypic characterization of Shigella spp. with reference to its virulence genes and antibiogram analysis from river Narmada

Water samples of the river Narmada from the source to the mouth were analyzed for the presence of shigellae and the Shigella isolates from 180 water samples were characterized by biotyping, serotyping and molecular typing. Out of all the 40 isolates, 23 were identified as S. flexneri, 10 as S. sonnei and 7 as S. dysenteriae. Serotyping was found to be the better identification method than biotyping. In the present investigation, amplified ribosomal DNA restriction analysis (ARDRA) with a probe complementary to 16S rRNA was performed. Repeated ARDRA analysis established the similarities between the isolates and thus suggested ARDRA as authentic and precise detection protocol. The isolates were also analyzed for the presence of virulence genes including ipaBCD, ipaH and stx1. All the 40 isolates of Shigella were found to be positive for the ipaH gene. The plasmid encoded invasion-associated genes ipaBCD were present only in S. flexneri and the stx1 gene was found only in S. dysenteriae. This study demonstrated the existence of Shigella in the river Narmada and the dispersion of different virulence genes among the isolates, which appear to constitute an environmental reservoir of Shigella-specific virulence genes.

Phenotypic and genotypic characterization of Shigella spp. with reference to its virulence genes and antibiogram analysis from river Narmada

Water samples of the river Narmada from origin to end were analyzed for the presence of shigellae. Analysis of 40 water samples by biotyping, serotyping, and molecular typing were done. Out of all 40 isolates, 23 were identified as Shigella flexneri, 10 as Shigella sonnei, and seven as Shigella dysenteriae. Serotyping was found to be a better identification method than biotyping since biotyping was not found to be very sharp. In the present investigation, amplified ribosomal DNA restriction analysis (ARDRA) with a probe complementary to 16S rRNA was performed. Repeated ARDRA analysis establishes the similarities between the isolates and thus suggested ARDRA as authentic and precise detection protocol. The isolates were also analyzed for the presence of virulence genes including ipaBCD, ipaH, and stx1 and all the 40 isolates of Shigella showed positive result for ipaH gene while the plasmid encoded invasion-associated genes ipaBCD was present only in S. flexneri and stx1 gene was present only in S. dysenteriae. This study demonstrated the existence of Shigella in the river Narmada and dispersion of different virulence genes among these isolates, which appear to constitute an environmental reservoir of Shigella-specific virulence genes.

Safety and colonization of two novel VirG(IcsA)-based live Shigella sonnei vaccine strains in rhesus macaques (Macaca mulatta)

Shigella are gram-negative bacterium that cause bacillary dysentery (shigellosis). Symptoms include diarrhea and discharge of bloody mucoid stools, accompanied by severe abdominal pain, nausea, vomiting, malaise, and fever. Persons traveling to regions with poor sanitation and crowded conditions become particularly susceptible to shigellosis. Currently a vaccine for Shigella has not been licensed in the United States, and the organism quickly becomes resistant to medications. During the past 10 y, several live attenuated oral Shigella vaccines, including the strain WRSS1, have been tested in humans with considerable success. These Phase I vaccines lack the gene for the protein VirG also known as IcsA, which enables the organism to disseminate in the host target tissue. However, 5% to 20% of the vaccinated volunteers developed mild fever and brief diarrhea, and the removal of additional virulence-associated genes from the vaccine strain may reduce or eliminate these side effects. We administered 2 Shigella sonnei vaccines, WRSs2 and WRSs3, along with WRSS1 to compare their rates of colonization and clinical safety in groups of 5 rhesus macaques. The primate model provides the most physiologically relevant animal system to test the validity and efficacy of vaccine candidates. In this pilot study using a gastrointestinal model of infection, the vaccine candidates WRSs2 and WRSs3, which have additional deletions in the enterotoxin and LPS modification genes, provided better safety and comparable immunogenicity to those of WRSS1.

IpaA targets beta1 integrins and rho to promote actin cytoskeleton rearrangements necessary for Shigella entry

Shigella invasion into the colonic epithelium involves many steps including the formation of large membrane protrusions by the epithelial cells that facilitate bacterial engulfment. IpaA, a Shigella protein secreted into target cells upon cell contact induces a loss of actin stress fibers in cells and promotes the reorganization of actin at the site of entry. The mechanism for this is not known but is thought to involve recruitment of the focal adhesion protein vinculin to IpaA. Here we have examined the mechanism for the effects of IpaA on the actin cytoskeleton. We show that IpaA-induced loss of actin stress fibers and cell rounding do not require vinculin expression or an intact vinculin binding site on IpaA. Rather, we find that cells expressing IpaA exhibited elevated Rho activity and increased myosin light chain phosphorylation. In addition, IpaA decreases integrin affinity for extracellular matrix ligands by interfering with talin recruitment to the integrin cytoplasmic tail. The combination of these two effects, namely weakened adhesion and increased contractility, account for the loss of actin stress fibers and cell rounding observed in cells exposed to IpaA.

Shigella dysenteriae type 1-specific bacteriophage from environmental waters in Bangladesh

Shigella dysenteriae type 1 is the causative agent of the most severe form of bacillary dysentery, which occurs as epidemics in many developing countries. We isolated a bacteriophage from surface water samples from Bangladesh that specifically lyses strains of S. dysenteriae type 1. This phage, designated SF-9, belongs to the Podoviridae family and has a 41-kb double-stranded DNA genome. Further screening of water samples for the prevalence of the phage revealed 9 of 71 (12.6%) water samples which were positive for the phage. These water samples were also positive in PCR assays for one or more S. dysenteriae type 1-specific genes, including ipaBCD and stx1, and live S. dysenteriae type 1 was isolated from three phage-positive samples. The results of this study suggest that phage SF-9 may have epidemiological applications in tracing the presence of S. dysenteriae type 1 in environmental waters.

Determination of the InvE binding site required for expression of IpaB of the Shigella sonnei virulence plasmid: involvement of a ParB boxA-like sequence

The InvE protein positively regulates the expression of virulence genes ipaBCD in Shigella sonnei. The InvE has significant homology with ParB of plasmid P1, which is known as a plasmid partitioning factor with DNA binding ability. Although the DNA binding activity of InvE has been predicted, it is not known whether the DNA binding activity is necessary for type III secretion system-associated gene expression. In this study, we determined the transcription start site of the icsB-ipaBCD operon (ipa operon) and constructed a series of deletions of the icsB promoter region in the Escherichia coli K-12 background. The deletion study revealed that an 86-bp region upstream of the icsB transcription start site was essential for expression of the ipa operon, where the ParB binding motif (ParB BoxA-like sequence) was observed. Purified glutathione S-transferase-InvE fusion protein bound directly to the -93 to -54 region (designating the icsB transcription start site as nucleotide +1) containing the ParB BoxA-like sequence. These results indicated that InvE bound directly to the promoter region.

Identification of RanBMP interacting with Shigella flexneri IpaC invasin by two-hybrid system of yeast

AIM: Bacillary dysentery caused by Shigella flexneri is still a threat to human health. Of four invasion plasmid antigen proteins (IpaA, B, C and D), IpaC plays an important role in the pathogenicity of this pathogen. The purpose of this study was to investigate the proteins interacting with IpaC in the host cell during the pathogenic process of this disease.

METHODS: By applying two-hybrid system, the bait plasmid containing ipaC gene was constructed and designated pGBKT-ipaC. The bait plasmid was transformed AH109, and proved to express IpaC and then HeLa cDNA library plasmids were introduced into the above transformed AH109. The transformation mixture was plated on medium lacking Trp, Leu, and His in the initial screen, then restreaked on medium lacking Trp, Leu, His and Ade. Colonies growing on the selection medium were further assayed for β-galactosidase activity. BLAST was carried out in the database after sequencing the inserted cDNA of the positive library plasmid.

RESULTS: Among the 2 × 10⁶ transformants, 64 positive clones were obtained as determined by activation of His, Ade and LacZ reporter genes. Sequence analysis revealed that cDNA inserts of two colonies were highly homologous to a known human protein, RanBPM.

CONCLUSION: These results provide evidence that IpaC may be involved in the invasion process of S. flexneri by interacting with RanBPM, and RanBPM is most likely to be the downstream target of IpaC in the cascade events of S. flexneri infection.

Structural characterization of the N terminus of IpaC from Shigella flexneri

The primary effector for Shigella invasion of epithelial cells is IpaC, which is secreted via a type III secretion system. We recently reported that the IpaC N terminus is required for type III secretion and possibly other functions. In this study, mutagenesis was used to identify an N-terminal secretion signal and to determine the functional importance of the rest of the IpaC N terminus. The 15 N-terminal amino acids target IpaC for secretion by Shigella flexneri, and placing additional amino acids at the N terminus does not interfere with IpaC secretion. Furthermore, amino acid sequences with no relationship to the native IpaC secretion signal can also direct its secretion. Deletions introduced beyond amino acid 20 have no effect on secretion and do not adversely affect IpaC function in vivo until they extend beyond residue 50, at which point invasion function is completely eliminated. Deletions introduced at amino acid 100 and extending toward the N terminus reduce IpaC's invasion function but do not eliminate it until they extend to the N-terminal side of residue 80, indicating that a region from amino acid 50 to 80 is critical for IpaC invasion function. To explore this further, the ability of an IpaC N-terminal peptide to associate in vitro with its translocon partner IpaB and its chaperone IpgC was studied. The N-terminal peptide binds tightly to IpaB, but the IpaC central hydrophobic region also appears to participate in this binding. The N-terminal peptide also associates with the chaperone IpgC and IpaB is competitive for this interaction. Based on additional biophysical data, we propose that a region between amino acids 50 and 80 is required for chaperone binding, and that the IpaB binding domain is located downstream from, and possibly overlapping, this region. From these data, we propose that the secretion signal, chaperone binding region, and IpaB binding domain are located at the IpaC N terminus and are essential for presentation of IpaC to host cells during bacterial entry; however, IpaC effector activity may be located elsewhere.

Identification of substrates and chaperone from the Yersinia enterocolitica 1B Ysa type III secretion system

All pathogenic Yersinia enterocolitica strains carry the pYV plasmid encoding the Ysc-Yop type III secretion (TTS) system, which operates at 37 degrees C. In addition, biovar 1B Y. enterocolitica strains possess a second, chromosomally encoded, TTS system called Ysa, which operates, at least in vitro, under low-temperature and high-salt (LTHS) conditions. Six open reading frames, sycB, yspB, yspC, yspD, yspA, and acpY, neighbor the ysa genes encoding the Ysa TTS apparatus. Here we show that YspA, YspB, YspC, and YspD are secreted by the Ysa TTS system under LTHS conditions. SycB is a chaperone for YspB and YspC and stabilizes YspB. YspB, YspC, and SycB share some similarity with TTS substrates and the chaperone encoded by the Mxi-Spa locus of Shigella flexneri and SPI-1 of Salmonella enterica. In addition, Ysa also secretes the pYV-encoded YopE under LTHS conditions, indicating that YopE is a potential effector of both Y. enterocolitica TTS systems. YspC could also be secreted by S. flexneri, but no functional complementation of ipaC was observed, which indicates that despite their similarity the Ysa and the Mxi-Spa systems are not interchangeable. When expressed from the yopE promoter, YspB and YspC could also be secreted via the Ysc injectisome. However, they could not form detectable pores in eukaryotic target cells and could not substitute for YopB and YopD for translocation of Yop effectors.

Clinical presentation in relation to diversity within the Helicobacter pylori cag pathogenicity island

OBJECTIVE

This study investigated the genetic diversity of the cag pathogenicity island (PAI) in Helicobacter pylori (H. pylori) in relation to clinical outcome and interleukin (IL)-8 production.

METHODS

Seven genes in the cag PAI (cagA, cagE, cagG, cagM, cagT, open reading frame 13 and 10) were examined by polymerase chain reaction and Southern blot hybridization using H. pylori from 120 patients with different presentations (duodenal ulcer, gastric cancer, gastritis alone). IL-8 production from AGS cells (gastric cancer cell line) cocultured with H. pylori was measured by ELISA.

RESULTS

An intact cag PAI was present in 104 (87%) isolates, and five (4%) had deletions within the cag PAI; 11 (9%) lacked the entire cag PAI. Clinical isolates containing the complete cag PAI induced a greater secretion of IL-8 as compared with those without the cag PAI (3048 +/- 263 vs 480 +/- 28 pg/ml, p < 0.001). Deletion of only cagG reduced IL-8 secretion by two thirds. Deletions of more than one locus reduced IL-8 secretion to background. A similar proportion of H. pylori from patients with gastritis, duodenal ulcer, or gastric cancer had intact cag PAI (88%, 88%, and 85%, respectively). Although the presence of cagG was a better predictor of the presence of an intact cag PAI than cagA or cagE, the presence or absence of any of these genes had no association with clinical presentation.

CONCLUSION

Although the cag PAI plays an important role in IL-8 production, clinical presentation cannot be predicted by the presence of an intact cag PAI or any of these seven cag PAI genes.

Isolation of Shigella dysenteriae type 1 and S. flexneri strains from surface waters in Bangladesh: comparative molecular analysis of environmental Shigella isolates versus clinical strains

Bacillary dysentery caused by Shigella species is a public health problem in developing countries including Bangladesh. Although, shigellae-contaminated food and drinks are often the source of the epidemic's spread, the possible presence of the pathogen and transmission of it through environmental waters have not been adequately examined. We analyzed surface waters collected in Dhaka, Bangladesh, for the presence of shigellae by a combination of PCR assays followed by concentration and culturing of PCR-positive samples. Analysis of 128 water samples by PCR assays for Shigella-specific virulence genes including ipaBCD, ipaH, and stx1 identified 14 (10.9%) samples which were positive for one or more of these virulence genes. Concentration of the PCR-positive samples by filtration followed by culturing identified live Shigella species in 11 of the 14 PCR-positive samples. Analysis of rRNA gene restriction patterns (ribotype) showed that the environmental isolates shared ribotypes with a collection of clinical isolates, but in contrast to the clinical isolates, 10 of the 11 environmental isolates were either negative or carried deletions in the plasmid-encoded invasion-associated genes ipaB, ipaC, and ipaD. However, all environmental Shigella isolates were positive for the chromosomal multicopy invasion-associated gene ipaH and all Shigella dysenteriae type 1 isolates were positive for the stx1 gene in addition to ipaH. This study demonstrated the presence of Shigella in the aquatic environment and dispersion of different virulence genes among these isolates which appear to constitute an environmental reservoir of Shigella-specific virulence genes. Since critical virulence genes in Shigella are carried by plasmids or mobile genetic elements, the environmental gene pool may contribute to an optimum combination of genes, causing the emergence of virulent Shigella strains which is facilitated in particular by close contact of the population with surface waters in Bangladesh.

Complete DNA sequence and analysis of the large virulence plasmid of Shigella flexneri

The complete sequence analysis of the 210-kb Shigella flexneri 5a virulence plasmid was determined. Shigella spp. cause dysentery and diarrhea by invasion and spread through the colonic mucosa. Most of the known Shigella virulence determinants are encoded on a large plasmid that is unique to virulent strains of Shigella and enteroinvasive Escherichia coli; these known genes account for approximately 30 to 35% of the virulence plasmid. In the complete sequence of the virulence plasmid, 286 open reading frames (ORFs) were identified. An astonishing 153 (53%) of these were related to known and putative insertion sequence (IS) elements; no known bacterial plasmid has previously been described with such a high proportion of IS elements. Four new IS elements were identified. Fifty putative proteins show no significant homology to proteins of known function; of these, 18 have a G+C content of less than 40%, typical of known virulence genes on the plasmid. These 18 constitute potentially unknown virulence genes. Two alleles of shet2 and five alleles of ipaH were also identified on the plasmid. Thus, the plasmid sequence suggests a remarkable history of IS-mediated acquisition of DNA across bacterial species. The complete sequence will permit targeted characterization of potential new Shigella virulence determinants.

Identification of functional regions within invasion plasmid antigen C (IpaC) of Shigella flexneri

Shigella flexneri causes bacillary dysentery with symptoms resulting from the inflammation that accompanies bacterial entry into the cells of the colonic epithelium. The effectors of S. flexneri invasion are the Ipa proteins, particularly IpaB and IpaC, which are secreted at the host-pathogen interface following bacterial contact with a host cell. Of the purified Ipa proteins, only IpaC has been shown to possess quantifiable in vitro activities that are related to cellular invasion. In this study, ipaC deletion mutants were generated to identify functional regions within the IpaC protein. From these data, we now know that the N-terminus and an immunogenic central region are not required for IpaC-dependent enhancement of cellular invasion by S. flexneri. However, to restore invasiveness to an ipaC null mutant of S. flexneri, the N-terminus is essential, because IpaC mutants lacking the N-terminus are not secreted by the bacterium. Deletion of the central hydrophobic region eliminates IpaC's ability to interact with phospholipid membranes, and fusion of this region to a modified form of green fluorescent protein converts it into an efficient membrane-associating protein. Meanwhile, deletion of the C-terminus eliminates the mutant protein's ability to establish protein-protein contacts with full-length IpaC. Interestingly, the mutant form of ipaC that restores partial invasiveness to the S. flexneri ipaC null mutant also restores full contact-mediated haemolysis activity to this bacterium. These data support a model in which IpaC possesses a distinct functional organization that is important for bacterial invasion. This information will be important in defining the precise role of IpaC in S. flexneri pathogenesis and in exploring the potential effects of purified IpaC at mucosal surfaces.

Shigella flexneri IpaH(7.8) facilitates escape of virulent bacteria from the endocytic vacuoles of mouse and human macrophages

The behavior of Shigella flexneri ipaH mutants was studied in human monocyte-derived macrophages (HMDM), in 1-day-old human monocytes, and in J774 mouse macrophage cell line. In HMDM, strain pWR700, an ipaH(7.8) deletion mutant of S. flexneri 2a strain 2457T, behaved like the wild-type strain 2457T. This strain caused rapid host cell death by oncosis, and few bacterial CFU were recovered after incubation in the presence of gentamicin as previously described for 2457T-infected HMDM. However, analysis of bacterial compartmentalization within endocytic vacuoles with gentamicin and chloroquine indicated that more pWR700 than 2457T was present within the endocytic vacuoles of HMDM, suggesting that ipaH(7.8) deletion mutant transited more slowly from the vacuoles to the cytoplasm. In contrast to findings with HMDM, CFU recovered from pWR700-infected mouse J774 cells were 2 to 3 logs higher than CFU from 2457T-infected J774 cells. These values exceeded CFU recovered after infection of J774 cells with plasmid-cured avirulent strain M4243A1. Incubation with gentamicin and chloroquine clearly showed that pWR700 within J774 cells was mostly present within the endocytic vacuoles. This distribution pattern was similar to that seen with M4243A1 and contrasted with the pattern seen with 2457T. Complementation of pWR700 with a recombinant clone expressing ipaH(7. 8) restored the intracellular distribution of bacteria to that seen with the wild-type strain. Strains with deletions in ipaH(4.5) or ipaH(9.8), however, behaved like 2457T in both HMDM and J774 cells. The distribution profile of pWR700 in 1-day-old monocytes was similar to that seen in J774 cells. Like infected J774 cells, 1-day-old human monocytes demonstrated apoptosis upon infection with virulent Shigella. These results suggest that a role of the ipaH(7. 8) gene product is to facilitate the escape of the virulent bacteria from the phagocytic vacuole of monocytes and macrophages.

Shigella flexneri YSH6000 induces two types of cell death, apoptosis and oncosis, in the differentiated human monoblastic cell line U937

Shigella flexneri, but not a non-invasive mutant derivative rapidly induced cell death in human monoblastic U937 cells as well as in differentiated cells pretreated with interferon-gamma (IFN gamma) or retinoic acid (RA). We investigated the morphological and biochemical characteristics of bacterial invasion-induced cell death in these differentiated U937 cells. IFN gamma-differentiated cells showed morphological changes typical of apoptosis and their DNA was cleaved giving a ladder-like electrophoretic pattern after infection by Shigellae. In contrast, swelling of the cytoplasm and blebbing of the plasma membrane were observed in RA-differentiated and undifferentiated cells invaded by the bacteria. No condensation of nuclei was observed in these cells by light microscopy, and no internucleosomal fragmentation of DNA was detected on agarose gels, which resembled the features of oncosis. Furthermore, cleavage of poly(ADP-ribose) polymerase, a substrate for apoptotic caspases, was seen only in IFN gamma-pretreated cells but not in RA-pretreated or undifferentiated cells. These findings suggested that virulent Shigella flexneri induces distinct types of cell death in U937 cells depending on their differentiation state.

Clinical outcome after infection with Helicobacter pylori does not appear to be reliably predicted by the presence of any of the genes of the cag pathogenicity island

BACKGROUND

The development of clinical disease after infection with Helicobacter pylori has been reported to be associated with expression of the cagA gene. Recently, it has been shown that cagA is part of a multigene locus, described as the cag pathogenicity island (PAI). The role of this region in determining clinical outcome remains to be established.

AIMS

To investigate whether the presence of cagA is always associated with the presence of the complete cag PAI and to evaluate the distribution of selected cag genes in 73 H pylori strains isolated from patients in France.

METHODS

Clinical strains of H pylori were screened for selected genes of the cag PAI by polymerase chain reaction and colony hybridisation.

RESULTS

Of 64 strains that harboured the cagA gene, 57 (89%) also contained the entire cag PAI. The entire cag PAI was found in 85% (48/56) and 53% (9/17) of duodenal ulcer and non-ulcer dyspepsia isolates, respectively. Eight strains had deletions within the cag PAI, including deletion of the cagA gene in one isolate; the deletions were not associated with the insertion sequence IS605. Of eight strains lacking the cag PAI, four were isolated from patients with duodenal ulcer.

CONCLUSION

The cag PAI is not a uniform, conserved entity. Although the presence of the cag PAI is highly associated with duodenal ulcer, the clinical outcome of infection with H pylori is not reliably predicted by any gene of the cag PAI.

Identification of cpxR as a positive regulator essential for expression of the Shigella sonnei virF gene

virF is the master regulator which activates the virulence determinant genes of Shigella spp. such as ipaBCD and virG. We previously reported that expression of virF itself is regulated in a pH-dependent manner and that cpxA, a sensor of a two-component regulatory system, is involved in this regulation (S. Nakayama and H. Watanabe, J. Bacteriol. 177:5062-5069, 1995). Disruption of cpxR, which has been thought to be the cognate response regulator of cpxA (J. Dong, S. Iuchi, H.-S. Kwan, Z. Lue, and E. C. C. Lin, Gene 136:227-230, 1993), abolished virF expression almost completely. Purified CpxR bound directly to the upstream region of virF. Binding capacity was enhanced when CpxR was phosphorylated by coincubation with acetyl phosphate in vitro. Furthermore, we observed that phosphorylated CpxR could activate virF transcription in vitro. These results clearly indicated that CpxR was an essential activator for virF expression and strongly suggested that the binding of phosphorylated CpxR to the target site upstream of the virF gene induced a direct activation of virF transcription.

Delivery of the non-membrane-permeative antibiotic gentamicin into mammalian cells by using Shigella flexneri membrane vesicles

We developed a model to test whether non-membrane-permeative therapeutic agents such as gentamicin could be delivered into mammalian cells by means of bacterial membrane vesicles. Many gram-negative bacteria bleb off membrane vesicles (MVs) during normal growth, and the quantity of these vesicles can be increased by brief exposure to gentamicin (J. L. Kadurugamuwa and T. J. Beveridge, J. Bacteriol. 177:3998-4008, 1995), which can be entrapped within the MVs. Gentamicin-induced MVs (g-MVs) were isolated from Shigella flexneri and contained 85 +/- 2 ng of gentamicin per microgram of MV protein. Immunogold electron microscopic labeling of thin sections with antibodies specific to S. flexneri lipopolysaccharide (LPS) demonstrated the adherence and subsequent engulfment of MVs by the human Henle 407 intestinal epithelial cell line. Further incubation of g-MVs with S. flexneri-infected Henle cells revealed that the g-MVs penetrated throughout the infected cells and reduced the intracellular pathogen by approximately 1.5 log10 CFU in the first hour of incubation. Antibiotic was detected in the cytoplasms of host cells, indicating the intracellular placement of the drug following the penetration of g-MVs. Soluble antibiotic, added as a fluid to the tissue culture growth medium, had no effect on intracellular bacterial growth, confirming the impermeability of the cell membranes of the tissue to gentamicin. Western blot analysis of MVs with S. flexneri Ipa-specific antibodies demonstrated that the invasion protein antigens IpaB, IpaC, and IpaD were present in MVs. Being bilayered, with outer faces composed of LPS and Ipa proteins, these MVs were readily engulfed by the otherwise impermeable membranes and eventually liberated their contents into the cytoplasmic substance of the host tissue.

Type III protein secretion systems in bacterial pathogens of animals and plants

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

Identification of epitope and surface-exposed domains of Shigella flexneri invasion plasmid antigen D (IpaD)

Transport and surface expression of the invasion plasmid antigens (Ipa proteins) is an essential trait in the pathogenicity of Shigella spp. In addition to the type III protein secretion system encoded by the mxi/spa loci on the large virulence plasmid, transport of IpaB and IpaC into the surrounding medium is modulated by IpaD. To characterize the structural topography of IpaD, the Geysen epitope-mapping system was used to identify epitopes recognized by surface-reactive monoclonal and polyclonal antibodies produced against purified recombinant IpaD or synthetic IpaD peptides. Surface-exposed epitopes of IpaD were confined to the first 180 amino acid residues, whereas epitopes in the carboxyl-terminal half were not exposed on the Shigella surface. By using convalescent-phase sera from 10 Shigella flexneri-infected monkeys, numerous epitopes were mapped within a surface-exposed region of IpaD between amino acid residues 14 and 77. Epitopes were also identified in the carboxyl-terminal half of IpaD with a few convalescent-phase sera. Comparison of IpaD epitope sequences with Salmonella SipD sequences indicated that very similar epitopes may exist in the carboxyl-terminal region of each protein whereas the IpaD epitopes in the surface-exposed amino-terminal region were unique for the Shigella protein. Although the IpaD and SipD homologs may play similar roles in transport, the dominant serum antibody response to IpaD is against the unique region of this protein exposed on the surface of the pathogen.

Environmental regulation of Salmonella typhi invasion-defective mutants

Salmonella typhi is the etiologic agent of human typhoid. During infection, S. typhi adheres to and invades epithelial and M cells that line the distal ileum. To survive in the human host, S. typhi must overcome numerous complex extracellular and intracellular environments. Since relatively little is known about S. typhi pathogenesis, studies were initiated to identify S. typhi genes involved in the early steps of interaction with the host and to evaluate the environmental regulation of these genes. In the present study, TnphoA mutagenesis was used to study these early steps. We isolated 16 Salmonella typhi TnphoA mutants that were defective for both adherence and invasion of the human small intestinal epithelial cell line Int407. Twelve of sixteen mutations were identified in genes homologous to the S. typhimurium invG and prgH genes, which are known to be involved in the type III secretion pathway of virulence proteins. Two additional insertions were identified in genes sharing homology with the cpxA and damX genes from Escherichia coli K-12, and two uncharacterized invasion-deficient mutants were nonmotile. Gene expression of TnphoA fusions was examined in response to environmental stimuli. We found that the cpxA, invG, and prgH genes were induced when grown under conditions of high osmolarity (0.3 M NaCl). Expression of invG and prgH genes was optimal at pH 6.5 and strongly reduced at low pH (5.0). Transcription of both invG and prgH TnphoA gene fusions was initiated during the late logarithmic growth phase and was induced under anaerobic conditions. Finally, we show that both invG and prgH genes appear to be regulated by DNA supercoiling, a mechanism influenced by environmental factors. These results are the first to demonstrate that in S. typhi, (i) the prgH and cpxA genes are osmoregulated, (ii) the invG gene is induced under low oxygen conditions, (iii) the invG gene is pH regulated and growth phase dependent, and (iv) the prgH gene appears to be regulated by DNA supercoiling. Since our experimental conditions were designed to mimic the in vivo environmental milieu, our results suggest that specific environmental conditions act as signals to induce the expression of S. typhi invasion genes.

Identification and molecular characterization of a 27 kDa Shigella flexneri invasion plasmid antigen, IpaJ

Shigella species and enteroinvasive Escherichia coli contain a core set of virulence genes whose coordinated expression results in the invasion of host colonic epithelial cells and the dysenteric syndrome. A number of virulence determinants are carried by the 230 kb invasion plasmid found in all virulent strains of Shigellae. Many of these invasion plasmid genes encode immunogens that are recognized by convalescent serum, including proteins that mediate the invasion (IpaB, IpaC, IpaD) and cell spreading (VirG or IcsA and IcsB) phenotypes. In this report, we describe the molecular characterization of a novel invasion plasmid antigen from Shigella flexneri, designated IpaJ. The ipaJ gene encodes a 780 bp open reading frame (ORF), separated from the ipaR (virB) stop codon by 944 bp. The predicted amino acid sequence for IpaJ revealed a consensus signal peptide for protein export. TnphoA mutagenesis of the ipaJ ORF confirmed the presence of export signal sequences in IpaJ. Unlike ipaBCDA genes, transcription analysis of ipaJ indicated that the gene is not expressed in a temperature-dependent fashion. The IpaJ protein was expressed and purified as a His6-tagged fusion protein that reacted with convalescent sera in Western blot analyses, confirming its identification as a Shigella immunogen. Construction and phenotypic characterization of ipaJ mutants in two serotypes of S. flexneri showed that the mutants were not compromised in their ability to invade cultured epithelial cells or to form plaques on BHK cell monolayers. In addition, the ipaJ mutants were Sereny positive indicating a capacity for intercellular dissemination; however, in the limited number of guinea-pigs tested, the keratoconjunctivitis reaction appeared attenuated.

The modified nucleoside 2-methylthio-N6-isopentenyladenosine in tRNA of Shigella flexneri is required for expression of virulence genes

The virulence of the human pathogen Shigella flexneri is dependent on both chromosome- and large-virulence-plasmid-encoded genes. A kanamycin resistance cassette mutation in the miaA gene (miaA::Km Sma), which encodes the tRNA N6-isopentyladenosine (i6A37) synthetase and is involved in the first step of the synthesis of the modified nucleoside 2-methylthio-N6-isopentenyladenosine (ms2i6A), was transferred to the chromosome of S. flexneri 2a by phage P1 transduction. In the wild-type bacterium, ms2i6A37 is present in position 37 (next to and 3' of the anticodon) in a subset of tRNA species-reading codons starting with U (except tRNA(Ser) species SerI and SerV). The miaA::Km Sma mutant of S. flexneri accordingly lacked ms2i6A37 in its tRNA. In addition, the mutant strains showed reduced expression of the virulence-related genes ipaB, ipaC, ipaD, virG, and virF, accounting for sixfold-reduced contact hemolytic activity and a delayed response in the focus plaque assay. A cloned sequence resulting from PCR amplification of the wild-type Shigella chromosome and exhibiting 99% homology with the nucleotide sequence of the Escherichia coli miaA gene complemented the virulence-associated phenotypes as well as the level of the modified nucleoside ms2i6A in the tRNA of the miaA mutants. In the miaA mutant, the level of the virulence-associated protein VirF was reduced 10-fold compared with the wild type. However, the levels of virF mRNA were identical in the mutant and in the wild type. These findings suggest that a posttranscriptional mechanism influenced by the presence of the modified nucleoside ms2i6A in the tRNA is involved in the expression of the virF gene. The role of the miaA gene in the virulence of other Shigella species and in enteroinvasive E. coli was further generalized.

Effect of shigella enterotoxin 1 (ShET1) on rabbit intestine in vitro and in vivo

BACKGROUND

Shigella enterotoxin 1 is a novel enterotoxin elaborated by Shigella flexneri 2a that causes fluid accumulation in rabbit ileal loops and a rise in short circuit current in Ussing chambers.

AIMS

To gain insights into the mechanism of action of shigella enterotoxin 1.

METHODS

Supernatants from genetically engineered clones either overexpressing shigella enterotoxin 1 or producing deletion mutants of the toxin were tested in rabbit ileum both in vitro and in vivo.

RESULTS

In rabbit ileum shigella enterotoxin 1 induced an irreversible rise in short circuit current that was not mediated by any of the recognised intracellular mediators of secretion. Deletion of 90% of the A subunit of the holotoxin ablated its enterotoxicity. In the in vivo perfusion model, the toxin induced a time dependent decrease in water absorption, whereas no changes were detected in the segment perfused with supernatants obtained from the deletion mutant. Finally, partially purified toxin induced a dose dependent increment in short circuit current that reached its plateau at a toxin concentration of 4 x 10(-6) M.

CONCLUSIONS

Shigella enterotoxin 1 induces a time and dose dependent intestinal secretion in the rabbit animal model, suggesting that it may be responsible for the watery phase of Shigella flexneri 2a infection.

Secretion of Shigella flexneri Ipa invasins on contact with epithelial cells and subsequent entry of the bacterium into cells are growth stage dependent

Upon contact with the surface of epithelial cells, Shigella flexneri secretes Ipa proteins through the Mxi-Spa type III secretion apparatus. Among the Ipa proteins, IpaB and IpaC form a soluble complex in the bacterial supernatant which appears to be sufficient to initiate the cellular rearrangements necessary to achieve bacterial entry. Here, we provide the first evidence that efficiency of bacterial entry into cells depends on the stage of bacterial growth. Bacteria in the early phase of exponential growth are six times more invasive than those in the stationary phase. The entry efficiency of the bacteria present on the cell surface appears to correlate with the percentage of those that are able to secrete their invasins. This suggests that the capacity to activate the Mxi-Spa apparatus is a major factor in the regulation of bacterial entry efficiency. Consistent with these observations, we have further shown that bacteria which have reached the stage of division secrete Ipa proteins more often than those that have not. Also, initial secretion occurs essentially in the area of the septation furrow. The Ipa proteins, secreted in the vicinity of the septation furrow, seem to initiate the early stages of reorganization of the host cell cytoskeleton.

Soluble invasion plasmid antigen C (IpaC) from Shigella flexneri elicits epithelial cell responses related to pathogen invasion

Shigella flexneri invades colonic epithelial cells by pathogen-induced phagocytosis. The three proposed effectors of S. flexneri internalization are invasion plasmid antigens B (IpaB), IpaC, and IpaD, which are encoded on the pathogen's 230-kb virulence plasmid and translocated to the extracellular milieu via the Mxi-Spa translocon. To date, there are no definitive functional data for any purified Ipa protein. Here, we describe the first characterization of highly purified recombinant IpaC, which elicits numerous epithelial cell responses related to events that take place during pathogen invasion. 125I-labeled IpaC binds cultured Henle 407 intestinal cells with an apparent dissociation constant in the low micromolar range. Moreover, incubation of epithelial cells with IpaC results in general changes in cellular phosphoprotein content, demonstrating this protein's ability to influence cellular protein kinase activities. These results contrast dramatically with those seen for recombinant IpaD, which does not bind to or induce detectable changes in the normal activities of cultured epithelial cells. In addition to influencing host cell activities, preincubation of epithelial cells with purified IpaC enhances uptake of S. flexneri by host cells. A similar result is seen when the cells are preincubated with a highly concentrated water extract of virulent S. flexneri 2a (strain 2457T). Interestingly, soluble IpaC also appears to promote uptake of the noninvasive S. flexneri 2a strain BS103. Purified IpaD failed to enhance the uptake of virulent S. flexneri and did not facilitate uptake of BS103. Taken together, the data suggest that IpaC is a potential effector of the host cell biological activities and may be responsible for entry of S. flexneri into target cells.

Identification of a Shigella flexneri criR gene increasing ipa genes expression: a novel member of response regulators of the two-component signal transduction family

A genetic locus named cri, which enhanced the expression of ipa genes, was cloned into Escherichia coli K-12 from Shigella flexneri 1b chromosomal DNA. Subcloning and Tn5-Tc1 transposon experiments showed that cri locus was located on a 2.6-kb HindIII fragment. Nucleotide sequence analysis of the region revealed at least three open reading frames (ORF), one of which, named criR, encoded a protein of 226 amino-acid residues and transcriptionally increased the ipaB expression. The deduced regulatory protein CriR shared a significant homology with bacterial transcriptional activators of the two-component signal transduction family. A homologue of the criR gene was present in genomic DNA of Shigella spp. and E. coli strains, and mapped at the 14.6-min region of E. coli K-12 chromosomal DNA. These results indicate that criR is a new member of response regulators.

Identification and characterization of phoN-Sf, a gene on the large plasmid of Shigella flexneri 2a encoding a nonspecific phosphatase

A gene encoding a nonspecific phosphatase, named PhoN-Sf, was identified on the large virulence plasmid (pMYSH6000) of Shigella flexneri 2a YSH6000. The phosphatase activity in YSH6000 was observed under high-phosphate conditions. However, it was found that low-phosphate conditions induced a slightly higher level of activity. The nucleotide sequence of the phoN-Sf region cloned from pMYSH6000 possessing the phoN-Sf gene encoded 249 amino acids with a typical signal sequence at the N terminus. The deduced amino acid sequence of the PhoN-Sf protein revealed significant homology to sequences of nonspecific acid phosphatases of other bacteria, such as Providencia stuartii (PhoN, 83.2%), Morganella morganii (PhoC, 80.6%), Salmonella typhimurium (PhoN, 47.8%), and Zymomonas mobilis (PhoC, 34.8%). The PhoN-Sf protein was purified, and its biochemical properties were characterized. The apparent molecular mass of the protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was calculated to be 27 kDa. The 20 amino acids at the N terminus corresponded to the 20 amino acid residues following the putative signal sequence of PhoN-Sf protein deduced from the nucleotide sequence. The PhoN-Sf activity had a pH optimum of 6.6, and the optimum temperature was 37 degrees C. The enzymatic activity was inhibited by diisopropyl fluorophosphate, N-bromosuccinimide, or dithiothreitol but not by EDTA. The subcellular localization of the PhoN-Sf protein in YSH6000 revealed that the protein was found predominantly in the periplasm. Examination of Shigella and enteroinvasive Escherichia coli strains for PhoN-Sf production by immunoblotting with the PhoN-specific antibody and for the presence of phoN-Sf DNA by using a phoN-Sf probe indicated that approximately one-half of the strains possessed the phoN-Sf gene on the large plasmid and expressed the PhoN-Sf protein. The Tn5 insertion mutants of YSH6000 possessing phoN-Sf::Tn5 still retained wild-type levels of invasiveness, as well as the subsequent spreading capacity in MK2 epithelial cell monolayers, thus suggesting that the PhoN-Sf activity is not involved in expression of the virulence phenotypes of Shigella strains under in vitro conditions.

Antibody response of monkeys to invasion plasmid antigen D after infection with Shigella spp

The antigen preparation most often used for determining the levels of antibodies to virulence-associated proteins of Shigella spp. consists of a mixture of proteins (including IpaB, IpaC, IpaD, and VirG*) extracted from virulent shigellae with water (water extract). To overcome the lack of specificity for individual antigens in the water-extract enzyme-linked immunosorbent assay (ELISA), the ipaD gene from S. flexneri has been cloned, expressed to a high level, and purified for use in a new ELISA for the determination of the levels of antibody against IpaD in monkeys and humans challenged with shigellae. The IpaD ELISA for serum immunoglobulins G and A correlated well with the water-extract ELISA in that monkeys infected with S. flexneri or S. sonnei responded with high serum antibody titers in both assays. The IpaD assay required less antigen per well, had much lower background levels, and did not require correction with antigens from an avirulent organism. In conjunction with the water-extract ELISA, it was possible to identify infected animals that did not respond to IpaD but did produce antibodies that reacted in the water-extract ELISA. This indicates that even though IpaB, IpaC, and IpaD are essential for the invasiveness phenotype, the infected host does not always produce antibodies against all components of the invasiveness apparatus.

Interaction of Ipa proteins of Shigella flexneri with alpha5beta1 integrin promotes entry of the bacteria into mammalian cells

Shigella is a genus of highly adapted bacterial pathogens that cause bacillary dysentery in humans. Bacteria reaching the colon invade intestinal epithelial cells by a process of bacterial-directed endocytosis mediated by the Ipa proteins: IpaB, IpaC, and IpaD of Shigella. The invasion of epithelial cells is thought to be a receptor-mediated phenomenon, although the cellular components of the host that interact with the Ipa proteins have not yet been identified. We report here that in a Shigella flexneri invasive system and Chinese hamster ovary (CHO) cell monolayers, the Ipa proteins were capable of interacting directly with alpha5beta1 integrin. The invasive capacity of S. flexneri for CHO cells increased as levels of alpha5beta1 integrin were elevated. When CHO cells were infected with S. flexneri, the tyrosine phosphorylation both of pp 125FAK, an integrin-regulated 125 K focal adhesion kinase, and of paxillin was stimulated. In contrast, an isogenic strain of S. flexneri that was defective in invasion owing to a mutation in its spa32 gene failed to induce such phosphorylation. Under in vitro and in vivo conditions, the released IpaB, IpaC, and IpaD proteins bound to alpha 5 beta 1 integrin in a manner different from that of soluble fibronectin but similar to that of the tissue form of fibronectin. At the site of attachment of S. flexneri to CHO cells, alpha5beta1 integrin converged with polymerization of actin. These data thus suggest that the capacity of Ipa proteins to interact with alpha5beta1 integrin may be an important Shigella factor in triggering the reorganization of actin cytoskeletons.

Recognition of three epitopic regions on invasion plasmid antigen C by immune sera of rhesus monkeys infected with Shigella flexneri 2a

The invasive ability of Shigella spp. is correlated with the expression of several plasmid-encoded proteins, including invasion plasmid antigen C (IpaC). By characterizing the antigenic structure of IpaC with monoclonal antibodies and convalescent-phase sera, it may be possible to determine the physical location of specific epitopes as well as the involvement of epitopes in a protective immune response or the host's susceptibility to disease. By using overlapping octameric synthetic peptides, which together represent the entire IpaC protein, the precise linear sequence of four surface-exposed epitopes was defined for four IpaC monoclonal antibodies. Furthermore, 17 unique peptide epitopes of IpaC were mapped by using 9-day-postinfection serum samples from 13 rhesus monkeys challenged with Shigella flexneri 2a. Each individual recognized a somewhat different array of IpaC peptide epitopes after infection with shigellae. However, the epitopes were clustered within three regions of the protein: region I (between amino acid residues 1 and 61), region II (between amino acid residues 177 and 258), and region III (between amino acid residues 298 and 307). Region II was recognized by 92% of S. flexneri-infected individuals and was considered to be a highly immunogenic region. Animals asymptomatic for shigellosis after challenge with S. flexneri recognized peptide epitopes within all three epitopic regions of IpaC, whereas symptomatic animals recognized peptides in only one or two of the epitopic regions. Antibody from monkeys challenged with S. sonnei recognized IpaC peptide epitopes which fell within and outside the three S. flexneri epitopic regions. While numerous potential epitopes exist on the IpaC protein, the identification of three regions in which epitopes are clustered suggests that these regions are significant with respect to the immune response and to subsequent pathogenesis postinfection.

Involvement of cpxA, a sensor of a two-component regulatory system, in the pH-dependent regulation of expression of Shigella sonnei virF gene

In Shigella species, IpaBCD proteins encoded on the virulence plasmid direct the entry of this bacterium into host epithelial cells. Expression of the ipaBCD genes is under the control of several environmental conditions, such as temperature and osmolarity. Extracellular pH also controlled the the expression of the genes, and this regulation occurred mainly at the step of expression of virF, a plasmid-encoded positive regulator of ipaBCD. The expression of virF was activated at high pH (pH 7.4) and repressed at low pH (pH 6.0). We isolated a Tn10 transposon mutant in Escherichia coli K-12 which altered this regulation at the transcriptional level. The Tn10 in the mutant inserted within a reading frame of the cpxA gene, whose product belongs to a family of sensor proteins of two-component signal transduction systems. Complementation analysis showed that cpxA was involved in the pH-dependent regulation of virF gene expression. A gene homologous to cpxA was conserved in Shigella spp. as well as in E. coli. These results may indicate that CpxA senses directly or indirectly a change in extracellular pH and influences the expression of virF in E. coli and Shigella spp.

Relationship between evolutionary rate and cellular location among the Inv/Spa invasion proteins of Salmonella enterica

For 21 strains of Salmonella enterica, nucleotide sequences were obtained for three invasion genes, spaO, spaP, and spaQ, of the chromosomal inv/spa complex, the products of which form a protein export system required for entry of the bacteria into nonphagocytic host cells. These genes are present in all eight subspecies of the salmonellae, and homologues occur in a variety of other bacteria, including the enteric pathogens Shigella and Yersinia, in which they are plasmid borne. Evolutionary diversification of the invasion genes among the subspecies of S. enterica has been generally similar in pattern and average rate to that of housekeeping genes. However, the range of variation in evolutionary rate among the invasion genes is unusually large, and there is a relationship between the evolutionary rate and cellular location of the invasion proteins, possibly reflecting diversifying selection on exported proteins in adaptation to variable host factors in extracellular environments. The SpaO protein, which is hypervariable in S. enterica and exhibits only 24% sequence identity with its homologues in Shigella and Yersinia, is secreted. In contrast, the membrane-associated proteins SpaP, SpaQ, and InvA are weakly polymorphic and have > 60% sequence identity with the corresponding proteins of other enteric bacteria. Acquisition of the inv/spa genes may have been a key event in the evolution of the salmonellae as pathogens, following which the invention of flagellar phase shifting facilitated niche expansion to include warm-blooded vertebrates.

The evolution of invasion by enteric bacteria

Despite differences in disease pathologies and host range, many enteric pathogens, including Salmonella and Shigella spp., utilize a remarkably similar machinery to secrete proteins that promote their entry into host cells. Analogous structures are required for the export of virulence proteins in other animal and plant pathogens. While the structure and organization of the gene complexes specifying these secretory pathways are broadly conserved, their phylogenetic distribution and genomic locations suggest that these sequences arose independently in divergent pathogens.

Shigella enterotoxin 1: an enterotoxin of Shigella flexneri 2a active in rabbit small intestine in vivo and in vitro

Culture filtrates of Shigella flexneri 2a strain M4243 grown in iron-depleted medium, caused significant fluid accumulation in rabbit ileal loops. Also, when tested in Ussing chambers, a greater rise in potential difference and short circuit current was seen with such filtrates compared with the medium control. Analogous filtrates from two M4243 derivatives lacking the 140-MD invasiveness plasmid (either M4243avir or BS103) retained 60-65% of the wild-type enterotoxic activity. Ultrafiltration and gel exclusion size fractionation of M4243 filtrate revealed that the activity was approximately 60 kD. SDS-PAGE performed on this fraction showed 18 bands, 5 of which reacted with human convalescent sera. Genes encoding this enterotoxin, named ShET1 for Shigella enterotoxin 1, were cloned from the S. flexneri 2a chromosome, and two separate open reading frames of 534 and 186 bp were sequenced. These observations suggest that S. flexneri 2a elaborates two distinct enterotoxins: ShET1, encoded by genes located on the chromosome, and ShET2, encoded by a gene on the 140-MD invasiveness plasmid. ShET1, which is composed of two distinct subunits and is elaborated in vivo, where it elicits an immune response, may be important in the pathogenesis of diarrheal illness due to S. flexneri 2a.