The Salmonella ompC gene: structure and use as a carrier for heterologous sequences

In silico designing and characterization of outer membrane protein (OmpC) gene from Salmonella enterica and its expression in Nicotiana tabacum for developing a plant-based vaccine against salmonellosis

Salmonella, a gram-negative bacteria, is the leading cause of foodborne illness globally. Two serovars of Salmonella, S. enteritidis and S. typhimurium are responsible for the majority of human salmonellosis. Prolonged salmonellosis caused by Salmonella species leads to the development of colon cancer, which is 3rd most common cancer in the world. Porins in the outer membrane of Salmonella can be used to elicit immune response. The production of plant-based vaccine against salmonellosis and the subsequent colon cancer using outer membrane proteins can be helpful for the people of developing countries. In this study, OmpC protein from Salmonella enteritidis was subjected to various bioinformatics tools which exhibited OmpC vaccine construct to be sufficiently immunogenic, non-allergenic, non-toxic and non-homologous to human proteins. Docking analysis showed strong interaction of OmpC vaccine model with TLR-4. After in silico analysis, this vaccine construct was expressed in tobacco plants via Agrobacterium-mediated transformation. Gateway® cloning was used to clone OmpC gene. Transformation and integration of transgene within tobacco plants was confirmed through conventional PCR. qRT-PCR was done for expression analysis and copy number calculated was 2. The expressed OmpC protein accumulated up to 0.42 % of total soluble protein. Immunization of mice with total soluble protein (TSP) and purified OmpC protein generated significant level of anti-OmpC antibodies. The vaccine candidate also demonstrated significant protective effect in mice upon challenging with Salmonella typhimurium. To the best of our knowledge, this is the first study reporting the expression of OmpC antigen in plants for potential use as vaccine against salmonellosis.

Microbial profile of broiler carcasses processed at a university scale mobile poultry processing unit

Chicken and chicken products have been associated with foodborne pathogens such as Salmonella, Campylobacter, and Escherichia coli (E. coli). Poultry comprises an important segment of the agricultural economy (75 million birds processed as of 2019) in West Virginia (WV). The risk of pathogens on processed chickens has risen with the increased popularity of mobile poultry processing units (MPPUs). This study evaluated the microbial safety of broilers processed in a MPPU in WV. This study assessed aerobic plate counts (APCs), E. coli counts and the presence/absence of Salmonella and Campylobacter on 96 broiler carcasses following each MPPU step of scalding, eviscerating, and chilling. Samples were either chilled in ice water only (W) or ice water with 5 ppm chlorine (CW). The highest number of bacteria recovered from carcasses were APCs (4.21 log10CFU/mL) and E. coli (3.77 log10CFU/mL; P = 0.02). A total reduction of 0.30 (P = 0.10) and 0.63 (P = 0.01) log10CFU/mL for APCs and E. coli, respectively, occurred from chilling carcasses in CW. Overall, results show that E. coli, Salmonella, and Campylobacter were significantly (P < 0.05) reduced from the initial scalding to the chilling step. However, Salmonella frequency doubled (15.63-34.38%) after the evisceration step, indicating that washing carcasses after evisceration may be a critical control point in preventing cross-contamination by Salmonella. Proper chilling is also an important microbial mitigation step in MPPU processing. Results indicate that Campylobacter was more resistant to chilling than Salmonella. Campylobacter was not completely inactivated until carcasses were chilled in CW, whereas W was sufficient to reduce Salmonella on carcasses. The results led to the conclusion that although 5 ppm chlorine (Cl2) achieved more bacterial reductions than water alone, the reductions were not always significant (P > 0.05). Further MPPU studies are needed to verify more effective chilling and processing strategies.

Antimicrobial Lasso Peptide Cloacaenodin Utilizes a Unique TonB-Dependent Transporter to Access Susceptible Bacteria

The development of new antimicrobial agents effective against Gram-negative bacteria remains a major challenge in drug discovery. The lasso peptide cloacaenodin has potent antimicrobial activity against multiple strains in the Enterobacter genus, one of the ESKAPE pathogens. Here, we show that cloacaenodin uses a previously uncharacterized TonB-dependent transporter, which we name CloU, to cross the outer membrane (OM) of susceptible bacteria. Inner membrane transport is mediated by the protein SbmA. CloU is distinct from the known OM transporters (FhuA and PupB) utilized by other antimicrobial lasso peptides and thus offers important insight into the spectrum of activity of cloacaenodin. Using knowledge of the transport pathway to predict other cloacaenodin-susceptible strains, we demonstrate the activity of cloacaenodin against clinical isolates of Enterobacter and of a Kluyvera strain. Further, we use molecular dynamics simulations and mutagenesis of CloU to explain the variation in cloacaenodin susceptibility observed across different strains of Enterobacter. This work expands the currently limited understanding of lasso peptide uptake and advances the potential of cloacaenodin as an antibiotic.

S. Typhi derived vaccines and a proposal for outer membrane vesicles (OMVs) as potential vaccine for typhoid fever

Typhoid fever is a serious systemic infection caused by Salmonella Typhi (S. Typhi), spread by the feco-oral route and closely associated with poor food hygiene and inadequate sanitation. Nearly 93% of S. Typhi strains have acquired antibiotic resistance against most antibiotics. Vaccination is the only promising way to prevent typhoid fever. This review covers the nature and composition of S. Typhi, pathogenecity and mode of infection, epidemiology, and nature of drug resistance. Several components (Vi-polysaccharides, O-antigens, flagellar antigens, full length OMPs, and short peptides from OMPs) of S. Typhi have been utilized for vaccine design for protection against typhoid fever. Vaccine delivery systems also contribute to efficacy of the vaccines. In this study, we propose to develop S. Typhi derived OMVs as vaccine for protection against typhoid fevers.

The CRISPR-Cas System Is Involved in OmpR Genetic Regulation for Outer Membrane Protein Synthesis in Salmonella Typhi

The CRISPR-Cas cluster is found in many prokaryotic genomes including those of the Enterobacteriaceae family. Salmonella enterica serovar Typhi (S. Typhi) harbors a Type I-E CRISPR-Cas locus composed of cas3, cse1, cse2, cas7, cas5, cas6e, cas1, cas2, and a CRISPR1 array. In this work, it was determined that, in the absence of cas5 or cas2, the amount of the OmpC porin decreased substantially, whereas in individual cse2, cas6e, cas1, or cas3 null mutants, the OmpF porin was not observed in an electrophoretic profile of outer membrane proteins. Furthermore, the LysR-type transcriptional regulator LeuO was unable to positively regulate the expression of the quiescent OmpS2 porin, in individual S. Typhi cse2, cas5, cas6e, cas1, cas2, and cas3 mutants. Remarkably, the expression of the master porin regulator OmpR was dependent on the Cse2, Cas5, Cas6e, Cas1, Cas2, and Cas3 proteins. Therefore, the data suggest that the CRISPR-Cas system acts hierarchically on OmpR to control the synthesis of outer membrane proteins in S. Typhi.

Recombinant outer membrane protein C of Aeromonas salmonicida subsp. masoucida, a potential vaccine candidate for rainbow trout (Oncorhynchus mykiss)

Aeromonas salmonicida subsp. masoucida (ASM) is an important bacterial pathogen of salmonid fish, which can cause huge economic losses to the fish farming industry. In order to screen effective vaccine candidate proteins, four outer membrane proteins of ASM, including OmpA, OmpC, OmpK and OmpW, were selected and recombinantly expressed in Escherichia coli. The result of western blotting showed that these four recombinant proteins could be recognized by rainbow trout anti-ASM antibodies. The immune protective effects of the four rOMPs were also investigated, and the relative percentage survival (RPS) of rOmpA, rOmpC, rOmpK and rOmpW were 71.1%, 81.6%, 55.3% and 42.1%, respectively. The RPS of rOmpC was significantly higher than the other three rOMPs, so the immune responses of rainbow trout induced by rOmpC were further investigated. The results showed that vaccination with rOmpC could significantly induced the production of specific serum antibodies and proliferation of sIg + lymphocytes in peripheral blood. Meanwhile, RT-qPCR analysis showed that rOmpC could significantly enhance the expression of the MHC-II, TCR, CD4, CD8, IL-8 and IgM genes compared with the BSA immunized group. These results demonstrated that rOmpC could induce strong humoral immune response in rainbow trout and provided effective immune protection against ASM challenge, which indicated that OmpC is a promising vaccine candidate against Aeromonas salmonicida infection.

Conservation of the OmpC Porin Among Typhoidal and Non-Typhoidal Salmonella Serovars

Salmonella enterica infections remain a challenging health issue, causing significant morbidity and mortality worldwide. Current vaccines against typhoid fever display moderate efficacy whilst no licensed vaccines are available for paratyphoid fever or invasive non-typhoidal salmonellosis. Therefore, there is an urgent need to develop high efficacy broad-spectrum vaccines that can protect against typhoidal and non-typhoidal Salmonella. The Salmonella outer membrane porins OmpC and OmpF, have been shown to be highly immunogenic antigens, efficiently eliciting protective antibody, and cellular immunity. Furthermore, enterobacterial porins, particularly the OmpC, have a high degree of homology in terms of sequence and structure, thus making them a suitable vaccine candidate. However, the degree of the amino acid conservation of OmpC among typhoidal and non-typhoidal Salmonella serovars is currently unknown. Here we used a bioinformatical analysis to classify the typhoidal and non-typhoidal Salmonella OmpC amino acid sequences into different clades independently of their serological classification. Further, our analysis determined that the porin OmpC contains various amino acid sequences that are highly conserved among both typhoidal and non-typhoidal Salmonella serovars. Critically, some of these highly conserved sequences were located in the transmembrane β-sheet within the porin β-barrel and have immunogenic potential for binding to MHC-II molecules, making them suitable candidates for a broad-spectrum Salmonella vaccine. Collectively, these findings suggest that these highly conserved sequences may be used for the rational design of an effective broad-spectrum vaccine against Salmonella.

Single-nucleotide polymorphism typing analysis for molecular subtyping of Salmonella Tennessee isolates associated with the 2007 nationwide peanut butter outbreak in the United States

Background

In 2007, a nationwide Salmonella Tennessee outbreak occurred via contaminated peanut butter. Here, we developed a single-nucleotide polymorphism (SNP)-typing method for S. Tennessee to determine the clonal subtypes of S. Tennessee that were associated with the peanut butter outbreak.

Methods and results

One seventy-six S. Tennessee isolates from various sources, including humans, animals, food, and the environment, were analyzed by using the SNP technique. Eighty-four representative SNP markers were selected by comparing the sequences of three representative S. Tennessee strains with different multi-locus sequence typing and variable number tandem repeats from our collection. The set of eighty-four SNP markers showed 100% typeability for the 176 strains, with the nucleotide diversity ranging from 0.011 to 0.107 (mean = 0.049 ± 0.018, median = 0.044) for each marker. Among the four clades and nine subtypes generated by the SNP typing, subtype 1, which comprised 142 S. Tennessee strains, was the most predominant. The dominance of single-strain clones in subtype 1 revealed that S. Tennessee is highly clonal regardless of outbreak-association, source, or period of isolation, suggesting the presence of an S. Tennessee strain prototype. Notably, a minimum 18 SNP set was able to determine clonal S. Tennessee strains with similar discrimination power, potentially allowing more rapid and economic strain genotyping for both outbreaks and sporadic cases.

Conclusions

The SNP-typing method described here might aid the investigation of the epidemiology and microevolution of pathogenic bacteria by discriminating between outbreak-related and sporadic clinical cases. In addition, this approach enables us to understand the population structure of the bacterial subtypes involved in the outbreak.

Electronic supplementary material

The online version of this article (doi:10.1186/s13099-017-0176-y) contains supplementary material, which is available to authorized users.

Characterization of Salmonella typhi OmpC and OmpF porins engineered with HIV-gp41 epitope on the surface loops

Porins form trimers in the outer membrane and help transport nutrients and waste products across the bacterial cell membrane. Porin loops are suitable candidates as display systems due to their high immunogenicity and presentation at the bacterial cell surface. In this study, Salmonella typhi porins (OmpC and OmpF) engineered with the Kennedy peptide from gp41 of HIV were characterised. The chimeric OmpC carrying the Kennedy peptide in loop7 did not trimerise, whereas the chimeric OmpF with the epitope in loop5 formed trimers and also was recognised by the antibodies in the HIV patient serum. The results suggest that chimeric S. typhi OmpF may be taken further as a potential candidate to develop as an epitope display system. Proteins 2017; 85:657-664. © 2016 Wiley Periodicals, Inc.

Edwardsiella tarda Outer Membrane Protein C: An Immunogenic Protein Induces Highly Protective Effects in Flounder (Paralichthys olivaceus) against Edwardsiellosis

Outer membrane protein C of Edwardsiella tarda is a major cell surface antigen and it was identified to be an immunogenic protein by Western blot using flounder (Paralichthys olivaceus) anti-recombinant OmpC (rOmpC), and anti-E. tarda antibodies. rOmpC tested the immune protective effect against E. tarda challenge in a flounder model and produced a relative percentage of survival rate of 85%. The immune response of flounder induced by rOmpC was investigated, and the results showed that: (1) the levels of specific serum antibodies induced by rOmpC were significantly higher than the control group after the second week after immunization, and the peak level occurred at week five after immunization; (2) rOmpC could induce the proliferation of sIg+ lymphocytes, and the peak levels of sIg+ lymphocytes in blood, spleen, and pronephros occurred at 4–5 weeks after immunization; and (3) the MHCIIα, CD4-1, IL-1β, IL-6 and TNF-α genes were significantly induced after being injected with rOmpC. Taken together, these results demonstrated that rOmpC could evoke highly protective effects against E. tarda challenge and induce strong innate immune response and humoral immune response of flounder, which indicated that OmpC was a promising vaccine candidate against E. tarda infection.

The Salmonella enterica serovar Typhi ltrR-ompR-ompC-ompF genes are involved in resistance to the bile salt sodium deoxycholate and in bacterial transformation

A characterization of the LtrR regulator, an S. Typhi protein belonging to the LysR family is presented. Proteomics, outer membrane protein profiles and transcriptional analyses demonstrated that LtrR is required for the synthesis of OmpR, OmpC and OmpF. DNA-protein interaction analysis showed that LtrR binds to the regulatory region of ompR and then OmpR interacts with the ompC and ompF promoters inducing porin synthesis. LtrR-dependent and independent ompR promoters were identified, and both promoters are involved in the synthesis of OmpR for OmpC and OmpF production. To define the functional role of the ltrR-ompR-ompC-ompF genetic network, mutants in each gene were obtained. We found that ltrR, ompR, ompC and ompF were involved in the control of bacterial transformation, while the two regulators and ompC are necessary for the optimal growth of S. Typhi in the presence of one of the major bile salts found in the gut, sodium deoxycholate. The data presented establish the pivotal role of LtrR in the regulatory network of porin synthesis and reveal new genetic strategies of survival and cellular adaptation to the environment used by Salmonella.

Identification of an iron acquisition machinery in Flavobacterium columnare

Flavobacterium columnare, a fastidious Gram-negative pathogen and the causative agent of columnaris disease, is one of the most harmful pathogens in the freshwater fish-farming industry. Nevertheless the virulence mechanisms of F. columnare are not well understood. Bacterial iron uptake from the host during infection is an important mechanism of virulence. Here we identified and analyzed part of the iron uptake machinery of F. columnare. Under iron-limited conditions during in vitro growth, synthesis of an outer membrane protein of ~86 kDa was upregulated. This protein was identified as a TonB-dependent ferrichrome-iron receptor precursor (FhuA). Synthesis of siderophores in F. columnare was corroborated by chrome azurol S assays. A putative ferric uptake regulator (Fur) protein was also identified in the F. columnare genome. Structural analysis of the F. columnare Fur protein revealed that it was similar to Fur proteins involved in iron uptake regulation of other bacteria. Furthermore, Salmonella enterica serovar Typhimurium (S. Typhimurium) Δfur mutants were partially complemented by the F. columnare fur gene. We conclude that a siderophore-mediated iron uptake system exists in F. columnare, and fur from F. columnare could partially complement S. Typhimurium Δfur mutant.

Cell surface display of a β-glucosidase employing the type V secretion system on ethanologenic Escherichia coli for the fermentation of cellobiose to ethanol

We used the autodisplay system AIDA-I, which belongs to the type V secretion system (TVSS), to display the β-glucosidase BglC from Thermobifida fusca on the outer membrane of the ethanologenic Escherichia coli strain MS04 (MG1655 ∆pflB, ∆adhE, ∆frdA, ∆xylFGH, ∆ldhA, PpflB::pdc (Zm)-adhB (Zm)). MS04 that was transformed with the plasmid pAIDABglCRHis showed cellobiase activity (171 U/g(CDW)) and fermented 40 g/l cellobiose in mineral medium in 60 h with an ethanol yield of 81 % of the theoretical maximum. Whole-cell protease treatment, SDS-PAGE, and Western-blot analysis demonstrated that BglC was attached to the external surface of the outer membrane of MS04. When attached to the cells, BglC showed 93.3 % relative activity in the presence of 40 g/l ethanol and retained 100 % of its activity following 2 days of incubation at 37 °C with the same ethanol concentration. This study shows the potential of the TVSS (AIDA-I) and BglC as tools for the production of lignocellulosic bio-commodities.

Asymmetric pore occupancy in crystal structure of OmpF porin from Salmonella typhi

OmpF is a major general diffusion porin of Salmonella typhi, a Gram-negative bacterium, which is an obligatory human pathogen causing typhoid. The structure of S. typhi Ty21a OmpF (PDB Id: 3NSG) determined at 2.8 Å resolution by X-ray crystallography shows a 16-stranded β-barrel with three β-barrel monomers associated to form a trimer. The packing observed in S. typhi Ty21a rfOmpF crystals has not been observed earlier in other porin structures. The variations seen in the loop regions provide a starting point for using the S. typhi OmpF for structure-based multi-valent vaccine design. Along one side of the S. typhi Ty21a OmpF pore there exists a staircase arrangement of basic residues (20R, 60R, 62K, 65R, 77R, 130R and 16K), which also contribute, to the electrostatic potential in the pore. This structure suggests the presence of asymmetric electrostatics in the porin oligomer. Moreover, antibiotic translocation, permeability and reduced uptake in the case of mutants can be understood based on the structure paving the way for designing new antibiotics.

Cloning, sequencing and in silico analysis of omp C of salmonella typhimurium

Salmonella Typhimurium is an important pathogen having a broad host range. In human population it causes mostly gastroenteritis but there are reports in which it was found to be responsible to cause several lethal diseases like endocarditis and meningitis. Poultry products are the major sources of this organism in India as these are consumed at various stages of cooking. The available vaccines have their own limitations such as short-term immunity. Outer membrane proteins have shown some promising potential, so in the present study Omp C of Salmonella Typhimurium was cloned and sequenced to explore the possibility of development of r-DNA vaccine against Salmonella Typhimurium for poultry. The sequence of Omp C was studied for antigenic indexing, epitope mapping, and MHC mapping using various bioinformatic tools. The ORF analysis revealed a complete coding region of approximately 1000 bp. Five major and 13 minor B-cell epitopes were identified having an antigenic index of 1.7. The sequences also showed major histocompatibility complex (MHC) class I and class II binding region indicating a potential of eliciting cell-mediated immune response. The findings indicate that Omp C may be proven as promising candidate for development of r-DNA vaccine against Salmonella Typhimurium.

Palladium-mediated cell-surface labeling

Benign C-C bond formation at various sites in cell-surface channels has been achieved through Suzuki-Miyaura coupling of genetically positioned unnatural amino acids containing aryl halide side chains. This enabled site-selective cell surface manipulation of Escherichia coli ; the phosphine-free catalyst caused no cell death at required Pd loadings, suggesting future in vivo application of catalytic metal-mediated bond formation in more complex organisms.

Phenotype, virulence and immunogenicity of Edwardsiella ictaluri cyclic adenosine 3',5'-monophosphate receptor protein (Crp) mutants in catfish host

Edwardsiella ictaluri is an Enterobacteriaceae that causes lethal enteric septicemia in catfish. Being a mucosal facultative intracellular pathogen, this bacterium is an excellent candidate to develop immersion-oral live attenuated vaccines for the catfish aquaculture industry. Deletion of the cyclic 3',5'-adenosine monophosphate (cAMP) receptor protein (crp) gene in several Enterobacteriaceae has been utilized in live attenuated vaccines for mammals and birds. Here we characterize the crp gene and report the effect of a crp deletion in E. ictaluri. The E. ictaluri crp gene and encoded protein are similar to other Enterobacteriaceae family members, complementing Salmonella enterica Δcrp mutants in a cAMP-dependent fashion. The E. ictaluri Δcrp-10 in-frame deletion mutant demonstrated growth defects, loss of maltose utilization, and lack of flagella synthesis. We found that the E. ictaluri Δcrp-10 mutant was attenuated, colonized lymphoid tissues, and conferred immune protection against E. ictaluri infection to zebrafish (Danio rerio) and catfish (Ictalurus punctatus). Evaluation of the IgM titers indicated that bath immunization with the E. ictaluri Δcrp-10 mutant triggered systemic and skin immune responses in catfish. We propose that deletion of the crp gene in E. ictaluri is an effective strategy to develop immersion live attenuated antibiotic-sensitive vaccines for the catfish aquaculture industry.

Reconstitution and engineering of apoptotic protein interactions on the bacterial cell surface

The interactions between pro- and anti-apoptotic members of the Bcl-2 class of proteins control whether a cell lives or dies, and the study of these protein-protein interactions has been an area of intense research. In this report, we describe a new tool for the study and engineering of apoptotic protein interactions that is based on the flow cytometric detection of these interactions on the surface of Escherichia coli. After validation of the assay with the well-studied interaction between the Bak(72-87) peptide and the anti-apoptotic protein Bcl-x(L), the effect of both increasing and decreasing Bak peptide length on Bcl-x(L) binding was investigated. Previous work demonstrated that the Bak(72-87) peptide also binds to the anti-apoptotic protein Bcl-2, albeit with lower binding affinity compared to Bcl-x(L). Here, we demonstrate that a slightly longer Bak peptide corresponding to amino acids 72-89 of Bak binds Bcl-x(L) and Bcl-2 equally well. Approximate binding affinity calculations on these peptide-protein complexes confirm the experimental observations. The flow cytometric assay was also used to screen a saturation mutagenesis library of Bak(72-87) variants for improved affinity to Bcl-x(L). The best variants obtained from this library exhibit an apparent K(d) to Bcl-x(L) 4-fold lower than that of wild-type Bak(72-87).

Diminished redundancy of outer membrane factor proteins in rhizobiales: a nodT homolog is essential for free-living Rhizobium etli

Rhizobium etli is a gram-negative soil bacterium that induces nitrogen-fixing nodules on common bean roots (Phaseolus vulgaris). R. etli encodes two genes homologous to nodT of Rhizobium leguminosarum. nodTch is chromosomal and forms an operon with new genes resembling a multi-drug efflux pump of the resistance-nodulation-cell division (RND) family. nodTch is the last gene of this operon and can also be independently transcribed; the gene product is located in the bacterial outer membrane. Cell survival requires nodTch under all conditions tested. A second nodT gene, nodTpc, is encoded by plasmid c; it is constitutively transcribed but does not complement the essential function encoded by nodTch. NodT proteins belong to the outer membrane efflux proteins of the TolC superfamily. The number of duplications in the tolC gene family positively correlates with genome size in gram-negative bacteria. Nonetheless, some alpha-proteobacteria, including R. etli, encode fewer outer membrane factor exporters than expected suggesting further roles in addition to detoxification.

Functional assay of Salmonella typhi OmpC using reconstituted large unilamellar vesicles: a general method for characterization of outer membrane proteins

The immunodominant trimeric beta-barrel outer membrane protein OmpC from Salmonella typhi, the causative agent of typhoid, has been functionally characterized here. The activity in the vesicle environment was studied in vitro using OmpC reconstituted into proteoliposomes. Passage of polysaccharides and polyethyleneglycols through OmpC has been examined to determine the permeability properties. The relative rate of neutral solute flux yields a radius of 1.1 nm for the S. typhi OmpC pore. This is almost double the pore size of Escherichia coli. This provides an example of large pore size present in the porins that form trimers as in the general bacterial porin family. The method used in this study provides a good membrane model for functional studies of porins.

Salmonella porins induce a sustained, lifelong specific bactericidal antibody memory response

We examined the ability of porins from Salmonella enterica serovar typhi to induce a long-term antibody response in BALB/c mice. These porins triggered a strong lifelong production of immunoglobulin G (IgG) antibody in the absence of exogenous adjuvant. Analysis of the IgG subclasses produced during this antibody response revealed the presence of the subclasses IgG2b, IgG1, IgG2a and weak IgG3. Despite the high homology of porins, the long-lasting anti-S. typhi porin sera did not cross-react with S. typhimurium. Notably, the antiporin sera showed a sustained lifelong bactericidal-binding activity to the wild-type S. typhi strain, whereas porin-specific antibody titres measured by enzyme-linked immunosorbent assay (ELISA) decreased with time. Because our porin preparations contained the outer membrane proteins C and F (OmpC and OmpF), we evaluated the individual contribution of each porin to the long-lasting antibody response. OmpC and OmpF induced long-lasting antibody titres, measured by ELISA, which were sustained for 300 days. In contrast, although OmpC induced sustained high bactericidal antibody titres for 300 days, postimmunization, the bactericidal antibody titre induced by OmpF was not detected at day 180. These results indicate that OmpC is the main protein responsible for the antibody-mediated memory bactericidal response induced by porins. Taken together, our results show that porins are strong immunogens that confer lifelong specific bactericidal antibody responses in the absence of added adjuvant.

Overexpression, refolding, and purification of the major immunodominant outer membrane porin OmpC from Salmonella typhi: characterization of refolded OmpC

The major immunodominant integral outer membrane protein C (OmpC) from Salmonella typhi Ty21a was overexpressed, without the signal peptide, in Escherichia coli. The protein aggregates as inclusion bodies (IBs) in the cytoplasm. OmpC from IBs was solubilized with 4 M urea and refolded. This involved rapid dilution of unfolded OmpC into a refolding buffer containing polyoxyethylene-9-lauryl ether (C(12)E(9)) and glycerol. The refolded OmpC (rfOmpC) was shown to be structurally similar to the native OmpC by SDS-PAGE, Western blotting, tryptic digestion, ultrafiltration, circular dichroism, and fluorescence spectroscopic techniques. Crystals of rfOmpC were obtained in preliminary crystallization trials. The rfOmpC also sets a stage for rational design by recombinant DNA technology for vaccine design and high resolution structure determination.

Conformational epitope mapping of OmpC, a major cell surface antigen from Salmonella typhi

The outer membrane protein OmpC, a trimer made of 16 stranded beta-barrel monomers, is a major cell surface antigen from the human pathogen Salmonella typhi. The relative stability of the epitopes recognising a Salmonella specific MAb (referred as MPN5) and an Enterobacteria specific MAb (referred as P7D8) and the role of the trimeric organisation has been probed using gel electrophoresis and monoclonal antibodies. The assembly of the trimer and the stability of the beta-barrel are found to be important for epitope presentation. The Salmonella specific conformational epitope is found to be more stable than the Enterobacteria specific one. The important residues of the Salmonella specific (Asp 25 of loop 1, Asp 340 of loop 8, Lys 334 of loop 8, and Tyr 210 of loop 5) and the Enterobacteria specific (Asp 25 of loop 1, Tyr 210 of loop 5, and Lys 152 of loop 4) conformational epitope have been identified using monoclonal antibodies, chemical modification, and solid phase binding methods.

OmpR and LeuO positively regulate the Salmonella enterica serovar Typhi ompS2 porin gene

The Salmonella enterica serovar Typhi ompS2 gene codes for a 362-amino-acid outer membrane protein that contains motifs common to the porin superfamily. It is expressed at very low levels compared to the major OmpC and OmpF porins, as observed for S. enterica serovar Typhi OmpS1, Escherichia coli OmpN, and Klebsiella pneumoniae OmpK37 quiescent porins. A region of 316 bp, between nucleotides -413 and -97 upstream of the transcriptional start point, is involved in negative regulation, as its removal resulted in a 10-fold increase in ompS2 expression in an S. enterica serovar Typhi wild-type strain. This enhancement in expression was not observed in isogenic mutant strains, which had specific deletions of the regulatory ompB (ompR envZ) operon. Furthermore, ompS2 expression was substantially reduced in the presence of the OmpR D55A mutant, altered in the major phosphorylation site. Upon random mutagenesis, a mutant where the transposon had inserted into the upstream regulatory region of the gene coding for the LeuO regulator, showed an increased level of ompS2 expression. Augmented expression of ompS2 was also obtained upon addition of cloned leuO to the wild-type strain, but not in an ompR isogenic derivative, consistent with the notion that the transposon insertion had increased the cellular levels of LeuO and with the observed dependence on OmpR. Moreover, LeuO and OmpR bound in close proximity, but independently, to the 5' upstream regulatory region. Thus, the OmpR and LeuO regulators positively regulate ompS2.

Intestinal colonisation-inhibition and virulence of Salmonella phoP, rpoS and ompC deletion mutants in chickens

Administration of live Salmonella strains to day-old chicks provides profound protection against superinfection with a related strain within a matter of hours by a colonisation-inhibition mechanism, which is primarily a bacterial physiological process. Although currently available, commercial, live attenuated Salmonella vaccines induce protection by adaptive immunity, none of them is able to induce protection against Salmonella organisms by colonisation-inhibition and, therefore, they are unable to protect newly-hatched birds immediately after oral vaccination. In this study, mutants of Salmonella Typhimurium and Enteritidis with deletions in phoP and rpoS, either alone or in combination with ompC, were characterised and tested for their level of attenuation and their ability to inhibit the intestinal colonisation of the isogenic parent strains in chickens. Mutants with deletions only in rpoS demonstrated an unaffected potential to inhibit the intestinal colonisation of the challenge strain but were still fully virulent for the chickens. Mutants with deletions in phoP, either alone or in combination with rpoS, resulted in a high level of attenuation, unimpaired ability to colonise the gut and a nearly unaffected potential to inhibit the challenge strain from caecal colonisation. Mutants with an additional deletion in ompC revealed a reduced capacity of intestinal colonisation-inhibition when compared to the control strains and both the single rpoS and the phoP deletion mutants. Mutations in phoP- or phoP-regulated genes may therefore be used for the development of live attenuated Salmonella vaccines possessing these novel characteristics.

A Salmonella typhi OmpC fusion protein expressing the CD154 Trp140-Ser149 amino acid strand binds CD40 and activates a lymphoma B-cell line

CD154 is a type II glycoprotein member of the tumour necrosis factor (TNF) ligand family, which is expressed mainly on the surface of activated T lymphocytes. The interaction with its receptor CD40, plays a central role in the control of several functions of the immune system. Structural models based on the homology of CD154 with TNF and lymphotoxin indicate that binding to CD40 involves three regions surrounding amino acids K143, R203 and Q220, and that strands W140-S149 and S198-A210 are critical for such interactions. Also, it has been reported that two recombinant CD154 fragments, including amino acid residues Y45-L261 or E108-L261 are biologically active, whereas other polypeptides, including S149-L261, are not. Therefore, we decided to construct a fusion protein inserting the W140-S149 amino acid strand (WAEKGYYTMS) in an external loop of the outer membrane protein C (OmpC) from Salmonella enterica serovar Typhi and assess its ability to bind CD40 and activate B cells. The sodium dodecyl sulphate-polyacrylamide gel electrophoresis demonstrated that the chimeric OmpC-gp39 protein conserved its ability to form trimers. Binding to CD40 was established by three variants of enzyme-linked immunosorbent assay, a direct binding assay by coating plates with a recombinant CD40-Fc protein and through two competition assays between OmpC-gp39 and recombinant CD154 or soluble CD40-Fc. Flow cytometry analysis demonstrated that OmpC-gp39 increased the expression levels of major histocompatibility complex II, CD23, and CD80, in Raji human B-cell lymphoma similarly to an antibody against CD40. These results further support that the CD154/CD40 interaction is similar to the TNF/TNF receptor. This is the first report of a bacterial fusion protein containing a small amino acid strand form a ligand that is able to activate its cognate receptor.

Evaluation of the use of PCR and reverse transcriptase PCR for detection of pathogenic bacteria in biosolids from anaerobic digestors and aerobic composters

A PCR-based method and a reverse transcriptase PCR (RT-PCR)-based method were developed for the detection of pathogenic bacteria in organic waste, using Salmonella spp., Listeria monocytogenes, Yersinia enterocolitica, and Staphylococcus aureus as model organisms. In seeded organic waste samples, detection limits of less than 10 cells per g of organic waste were achieved after one-step enrichment of bacteria, isolation, and purification of DNA or RNA before PCR or RT-PCR amplification. To test the reproducibility and reliability of the newly developed methods, 46 unseeded samples were collected from diverse aerobic (composting) facilities and anaerobic digestors and analyzed by both culture-based classical and newly developed PCR-based procedures. No false-positive but some false-negative results were generated by the PCR- or RT-PCR-based methods after one-step enrichment when compared to the classical detection methods. The results indicated that the level of activity of the tested bacteria in unseeded samples was very low compared to that of freshly inoculated cells, preventing samples from reaching the cell density required for PCR-based detection after one-step enrichment. However, for Salmonella spp., a distinct PCR product could be obtained for all 22 nonamended samples that tested positive for Salmonella spp. by the classical detection procedure when a selective two-step enrichment (20 h in peptone water at 37 degrees C and 24 h in Rappaport Vassiliadis medium at 43 degrees C) was performed prior to nucleic acid extraction and PCR. Hence, the classical procedure was shortened, since cell plating and further differentiation of isolated colonies can be omitted, substituted for by highly sensitive and reliable detection based on nucleic acid extraction and PCR. Similarly, 2 of the 22 samples in which Salmonella spp. were detected also tested positive for Listeria monocytogenes according to a two-step enrichment procedure followed by PCR, compared to 3 samples that tested positive when classical isolation procedures were followed. The study shows that selective two-step enrichment is useful when very low numbers of bacterial pathogens must be detected in organic waste materials, such as biosolids. There were no false-positive results derived from DNA of dead cells in the waste sample, suggesting that it is not necessary to perform RT-PCR analyses when PCR is combined with selective enrichment. Large numbers of added nontarget bacteria did not affect detection of Salmonella spp., L. monocytogenes, and Y. enterocolitica but increased the detection limit of Staphylococcus aureus from <10 to 10(4) CFU/g of organic waste. Overall, the detection methods developed using seeded organic waste samples from one waste treatment facility (WTF) needed to be modified for satisfactory detection of pathogens in samples from other WTFs, emphasizing the need for extensive field testing of laboratory-derived PCR protocols. A survey of 13 WTFs in Germany revealed that all facilities complied with the German Biowaste Ordinance, which mandates that the end product after anaerobic digestion or aerobic composting be free of Salmonella In addition, all biosolids were free of L. monocytogenes, Staphylococcus aureus, and Y. enterocolitica, as evidenced by both classical and PCR-based detection methods.

Therapeutic efficacy of an E coli strain carrying an ovalbumin allergenic peptide as a fused protein to OMPC in a murine model of allergic airway inflammation

An Escherichia coli strain expressing the ovalbumin (OVA) 323-329 allergenic peptide on the bacterial surface was evaluated for its ability to reduce the lung inflammatory response in mice allergic to OVA. BALB/c mice were rendered allergic by means of two intraperitoneal injections of OVA suspended in alum 5 days apart, and one intratracheal boost 1 week later. The mice were then treated with two intranasal, 1 week apart, doses of 4x10(9) E. coli-UH302 transformed with plasmids pST13 or pST13-OVA(323-339), which bear the OmpC porin from Salmonella enterica serovar Typhi or the OmpC with the OVA allergenic 323-339 amino acid sequence inserted in the external loop 5. The allergic inflammatory reaction was evaluated on day 31, finding that mice treated with E. coli-UH302-pST13-OVA reduced four to seven times perivascular and peribronchial infiltrates, mucus production, goblet cell hyperplasia and eosinophils when compared with mice treated with E. coli-UH302-pST13 or saline solution. These results were consistent with a significant decrease of IL-5 mRNA and induction of IFN-gamma mRNA in cells from bronchio-alveolar lavages (BAL). Specific serum IgE anti-OVA was also reduced, although the decrease did not reach statistical significance. These results demonstrate that the bacterial live vector bearing an allergenic peptide successfully moderated two important components of allergy, pulmonary inflammation and mucus overproduction.

Outer membrane proteins as surface display systems

Outer membrane proteins (OMPs) of gram-negative bacteria can be used as carrier proteins to present foreign peptide epitopes on the bacterial cell surface. They all have a common structural motif of a beta-barrel that is composed of a variable number of transmembrane beta-strands connected at the periplasmic side with short turns and at the outside with long surface-accessible loops. Outer membrane proteins occur as monomers like OmpA, or assemble into trimers like the porins. Foreign gene products have been fused to surface-accessible regions of several outer membrane proteins including the porins OmpC, PhoE and LamB, lipoproteins as well as the OmpA protein. Short epitopes that are inserted into outer membrane proteins induce epitope-specific antibody responses, and are thus appealing candidates for live recombinant vaccines. Also large insertions, of more than 100 amino acids, are in some cases tolerated and do not affect the overall conformation of the carrier protein. The possible applications for outer membrane display include recombinant vaccines, peptide library screening, development of biocatalysts or whole-cell adsorbents, and adhesin-receptor interaction studies. It is expected that in the near future, development of new display systems will still increase the utilization of this emerging exciting technology.

Homology model of surface antigen OmpC from Salmonella typhi and its functional implications

Homology based 3D structural model of the immunodominant major surface antigen OmpC from Salmonella typhi, an obligatory human pathogen, was built to understand the possible unique conformational features of its antigenic loops with respect to other immunologically cross reacting porins. The homology model was built based on the known crystal structures of the E. coli porins OmpF and PhoE. Structure based sequence alignment helped to define the structurally conserved regions (SCRs). The SCR regions of OmpC were modelled using the coordinates of corresponding regions from reference proteins. Surface exposed variable regions were modelled based on the sequence similarity and loop search in PDB. Structural refinement based on symmetry restrained energy minimization resulted in an agreeable model for the trimer of OmpC. The resulting model was compared with other porin structures, having b-barrel fold with 16 transmembrane beta-strands, and found that the variable regions are unique in terms of sequence and structure. A ranking of the loops taking into account the antigenic index, the sequence variability, the surface accessibility in the context of the trimer, and the structural variability suggests that loop 4 (151-172), loop 5 (194-218) and loop 6 (237-264) are the best ranked B-cell epitopes. The model provides possible explanations for the functional and unique immunological properties associated with the surface exposed regions and outlines the implications for structure based experimental design.

Purification of integral outer-membrane protein OmpC, a surface antigen from Salmonella typhi for structure-function studies: a method applicable to enterobacterial major outer-membrane protein

Extraction of the outer-membrane porin, OmpC, from Salmonella typhi Ty21a was done by using a modified salt-extraction procedure. It was possible to extract only the major outer-membrane protein (OMP) from the crude membrane using this method. Aberrant lipopolysaccharide (LPS) production in the galE mutant Ty21a has resulted in more isoforms of OmpC and subsequently led to anomalous mobility in SDS-PAGE. The purity of the preparation was confirmed by denaturing urea SDS-PAGE and N-terminal sequencing. The major OMP extracts had LPS of both bound and free forms. The free form of LPS could be removed by gel filtration and the bound form, largely, was removed using ion-exchange chromatography and by passing through ultrafiltration devices. This method has been used to extract the native trimer of OmpC, the major OMP, in a large scale, for structure-function studies. S. typhi Ty21a OmpC preparation yielded reproducible diffraction-quality crystals. Extracts of porin from wild-type Escherichia coli HB101, grown under high osmolarity conditions, showed a single species of OMP on SDS-PAGE. This suggests the possible application of the method to other gram-negative bacterial porins.

Salmonella enteritidis fimbriae displaying a heterologous epitope reveal a uniquely flexible structure and assembly mechanism

Two distinct Salmonella fimbrins, AgfA and SefA, comprising thin aggregative fimbriae SEF17 and SEF14, respectively, were each genetically engineered to carry PT3, an alpha-helical 16-amino acid Leishmania T-cell epitope derived from the metalloprotease gp63. To identify regions within AgfA and SefA fimbrins amenable to replacement with this epitope, PCR-generated chimeric fimbrin genes were constructed and used to replace the native chromosomal agfA and sefA genes in Salmonella enteritidis. Immunoblot analysis using anti-SEF17 and anti-PT3 sera demonstrated that all ten AgfA chimeric fimbrin proteins were expressed by S. enteritidis under normal growth conditions. Immunoelectron microscopy confirmed that eight of the AgfA::PT3 proteins were effectively assembled into cell surface-exposed fimbriae. The PT3 replacements in AgfA altered Congo red (CR) binding, cell-cell adhesion and cell surface properties of S. enteritidis to varying degrees. However, these chimeric fimbriae were still highly stable, being resistant to proteinase K digestion and requiring harsh formic acid treatment for depolymerization. In marked contrast to AgfA, none of the chimeric SefA proteins were expressed or assembled into fimbriae. Since each PT3 replacement constituted over 10% of the AgfA amino acid sequence and all ten replacements collectively represented greater than 75% of the entire AgfA primary sequence, the ability of AgfA to accept large sequence substitutions and still assemble into fibers is unique among fimbriae and other structural proteins. This structural flexibility may be related to the novel fivefold repeating sequence of AgfA and its recently proposed structure Proper formation of chimeric fimbrial fibers suggests an unusual assembly mechanism for thin aggregative fimbriae which tolerates aberrant structures. This study opens a range of possibilities for Salmonella thin aggregative fimbriae as a carrier of heterologous epitopes and as an experimental model for studies of protein structure.

Characterization of adhesive epitopes with the OmpS display system

OmpS is an outer membrane protein of Vibrio cholerae where it forms trimeric pores that function in the uptake of maltose and maltodextrins. Based on sequence similarity to LamB proteins, a model of OmpS folding in the outer membrane has been constructed. According to this model, OmpS contains 18 transmembrane beta-strands and nine surface-accessible loops. Adhesive epitopes can, when inserted into surface-accessible loop 4 (L4) and expressed in Escherichia coli, retain their functional characteristics. We inserted three D-repeats from the Staphylococcus aureus fibronectin-binding protein FnBPA into L4 of OmpS and showed that E. coli cells expressing these hybrids bind fibronectin. DNA fragments covering the N-terminal half of the globoside-binding P-fimbrial adhesin class II PapG of E. coli were cloned into the same surface accessible loop (L4) of OmpS. Fragments of papG encoding 53 or 186 amino acids from the N-terminal end of class II PapG adhesin were found to confer bacterial adhesiveness to globoside. Removal of 23 amino acids from the N-terminus of PapG did not affect receptor binding, but removal of 31 amino acids abolished it. The newly developed night sky image technique was also used to demonstrate the binding properties of membrane vesicles carrying the hybrid proteins. We raised antibodies against the purified hybrid protein containing 53 amino acids from PapG. This antiserum recognized the P-fimbriae on E. coli cells. These data provide evidence that the N-terminal first 53 amino acids of class II PapG contain the receptor-binding domain.

Display of polyhistidine peptides on the Escherichia coli cell surface by using outer membrane protein C as an anchoring motif

A novel cell surface display system was developed by employing Escherichia coli outer membrane protein C (OmpC) as an anchoring motif. Polyhistidine peptides consisting of up to 162 amino acids could be successfully displayed on the seventh exposed loop of OmpC. Recombinant cells displaying polyhistidine could adsorb up to 32.0 micromol of Cd(2+) per g (dry weight) of cells.

The porin OmpC ofSalmonella typhimuriummediates adherence to macrophages

Macrophages recognize, adhere to, and phagocytose Salmonella typhimurium. The major outer membrane protein OmpC is a candidate ligand for macrophage recognition. To confirm this we used transposon mutagenesis to develop an ompC-deficient mutant in a known virulent strain of S. typhimurium; mutant and wild type were compared in macrophage adherence and association assays. Radiolabeled wild type S. typhimurium bound to macrophages at five-fold higher levels than did the ompC mutant. In association assays, macrophages in monolayers bound and internalized three-fold more wild type than mutant, while macrophages in suspension bound and internalized 40-fold more wild type than mutant. The ompC gene of our test strain of S. typhimurium contains several discrete differences compared with the ompC genes of Salmonella typhi and Escherichia coli. The deduced OmpC amino acid sequence of S. typhimurium shares 77 and 98% identity with OmpC amino acid sequences of E. coli and S. typhi, respectively. Evidence from this study supports a role for the OmpC protein in initial recognition by macrophages and distinguishes regions of this protein that potentially participate in host-cell recognition of bacteria by phagocytic cells.Key words: Salmonella, porin, macrophage, outer membrane protein, DNA sequencing.

Crystallization of the immunodominant outer membrane protein OmpC; the first protein crystals from Salmonella typhi, a human pathogen

OmpC, a surface antigen of Salmonella typhi was crystallized after several attempts, using PEG 3350. Well shaped hexagonal crystals were grown from vapor diffusion method using octyl glucoside and C12E9 as detergents. Crystals are sensitive to X-ray and diffract weakly up to 7 A. Porin isoforms, due to the bound lipopolysaccharides, could be the cause for poor diffraction. Crystal quality depends largely on the purification method, and in case of LPS contamination, the genetic background of the bacteria. Crystallization and initial data collection suggest optimum conditions and the method of choice for OmpC crystallization.

Negative and positive regulation of the non-osmoregulated ompS1 porin gene in Salmonella typhi: a novel regulatory mechanism that involves OmpR

The Salmonella typhi ompS1 gene codes for an outer membrane protein of the OmpC/OmpF porin family. It is expressed at very low levels, relative to the major porins. However, deletion analysis of the 5' regulatory region showed that the gradual removal of nucleotides -310 to -88, upstream from the P1 major transcriptional start-point, resulted in a stepwise increase in expression, reaching levels 10-fold above those for the ompC major porin gene. Hence, this 222 bp segment contains cis-acting regulatory elements involved in negative control. Primer extension analysis revealed the presence of three promoters: P1 activity was OmpR dependent; P2 was expressed at a lower level in the absence of OmpR; and P3 had a minor constitutive activity. OmpR bound preferentially to box II, an 18 bp F1/C1 canonical site, the removal (-88 to -66) of which resulted in a decrease in expression thus supporting its role in positive control. Expression of ompS1 was not induced by a set of stress conditions, including a shift in osmolarity, nor was the IHF regulator involved in negative control. An ompS1 homologue was found in E. coli K-12, which contains a nonsense codon and a shift in the reading frame, whereas Salmonella typhimurium contains an open reading frame in this region. Thus, S. typhi ompS1 provides novel features in OmpR regulation.

Distinctive IS200 insertion between gyrA and rcsC genes in Salmonella typhi

While probing the vicinity of ompC, a copy of the IS200 insertion element was found between the gyrA and rcsC genes of Salmonella typhi, the causal agent of typhoid fever. This distinctive feature was conserved throughout 63 S. typhi isolates of different geographical origins and was absent from 46 other Salmonella serotypes, including those most associated with human infections, as well as from 19 other enteric bacteria. Furthermore, the location of this IS200 copy corresponds to a constant band, present throughout the 14 PstI S. typhi IS200 fingerprints, encompassing several Vi phage types. Interestingly, an apparently unrelated serotype not frequently associated with human disease, Salmonella weltevreden, contained an IS200 copy at the same position, although it was accompanied by an additional segment of cryptic DNA. On the basis of these findings, PCR assays were designed for molecular typing of S. typhi, and these are potentially useful in studying the epidemiology of typhoid fever.

Antigenic determinants of the OmpC porin from Salmonella typhimurium

The antigenic determinants of Salmonella typhimurium OmpC were investigated by the analysis of cyanogen bromide (CNBr)-generated porin peptides with antiporin monoclonal antibodies (MAbs). We identified six bands (f1 to f6) with estimated molecular masses of 35.5, 31.0, 25.0, 22.5, 13.8, and 10.0 kDa, respectively. In addition, two small fragments (f7 and f8; 3.0 to 6.0 kDa) were detected only infrequently. The OmpC monomer or its CNBr-generated peptides were electrophoretically transferred to a polyvinylidene difluoride membrane and then subjected to amino acid composition analysis and N-terminal sequencing. A comparison of the amino acid composition data with known compositions of Escherichia coli and Salmonella typhi OmpC showed some differences; however, the amino acid sequences of 71 residues identified in S. typhimurium showed 88 and 98% identity with OmpC from E. coli and S. typhi, respectively. The screening of CNBr peptides with the 12 anti-(S. typhimurium) OmpC MAbs by Western blot (immunoblot), in conjunction with the prediction of the OmpC folding pattern based on the known three-dimensional structure of E. coli OmpF, showed that four MAbs reacted with surface-exposed epitopes on loops L2, L8, and L4 to L7, four MAbs reacted with a region in the eyelet structure on loop L3, and four MAbs reacted with the buried epitopes on transmembrane beta strands. The MAbs reacting with surface-exposed loops showed no cross-reaction with E. coli OmpC, whose sequence has diverged extensively from that of S. typhi and (probably) S. typhimurium OmpC only in regions of the externally exposed loops. In contrast, MAbs reacting with transmembrane beta strands, whose sequence is strongly conserved, showed strong cross-reaction with E. coli OmpC. These results show that comparison with the E. coli OmpF structure predicts the folding pattern of S. typhimurium OmpC rather accurately and that evolutionary divergence in sequences is confined to the external loops. The possible roles of these surface-exposed and buried epitopes as potentially useful antigenic regions for diagnostic assays and vaccine development are discussed.

Isolation and characterization of ompS1, a novel Salmonella typhi outer membrane protein-encoding gene

We have isolated a novel outer membrane protein (OMP)-encoding gene from Salmonella typhi (St), termed ompS1, using the ompF gene of Escherichia coli (Ec) as a heterologous probe. The structural ompS1 gene codes for an OmpS1 polypeptide that consists of 373 amino acids (aa) in the mature product, with a putative 21-aa leader sequence, containing highly conserved aa residues that have been implicated in pore formation. Mature OmpS1 (41 kDa) is larger than the OmpC, OmpF and PhoE St and Ec porins. In contrast to the major porins, it is undetectable in Coomassie-stained OMP preparations; although, when ompS1 was cloned into a high-copy-number plasmid under the control of the inducible tac promoter, it was detectable along with major OMPs. The 5' regulatory region of ompS1 has five putative binding sites for OmpR, a positive transcriptional regulator. The ompS1 gene shows restriction-fragment length polymorphism (RFLP) among Salmonellae.