Intramuscular immunization with genetically inactivated (ghosts) Actinobacillus pleuropneumoniae serotype 9 protects pigs against homologous aerosol challenge and prevents carrier state

A bacterial ghost vaccine against Aeromonas salmonicida infection in turbot (Scophthalmus maximus)

Aeromonas salmonicida is one of the most prevalent pathogens that causes huge economic losses to aquaculture. Effective vaccination is the first choice for preventing infection. Bacterial ghost (BG), an empty bacterial shell devoid of cytoplasm, is a promising vaccine antigen with distinct advantages. Herein, we established strategies for producing a substantial yield of A. salmonicida ghost (ASG) and investigated the immune-protective properties of it. As a result, 2.84 mg/ml NaOH was discovered to be capable of inducing considerable amounts of ASG. Furthermore, the ASG vaccine elicited adaptive immunity in turbots after rapid activation of innate immunity. Even though formalin-killed cells (FKC) produced a few more antibodies than ASG, ASG ultimately provided a much stronger immune protection effect because it strengthened cellular immunity, with a relative percentage survival (RPS) of 50.1 % compared to FKC. These findings demonstrated that ASG effectively activated cell-mediated immunity, which helped get rid of microorganisms inside cells. Therefore, this study presented novel perspectives for future research on furunculosis vaccine products based on ASG as an antigen.

Serotype diversity and antimicrobial susceptibility profiles of Actinobacillus pleuropneumoniae isolated in Italian pig farms from 2015 to 2022

Actinobacillus pleuropneumoniae (APP) is a bacterium frequently associated with porcine pleuropneumonia. The acute form of the disease is highly contagious and often fatal, resulting in significant economic losses for pig farmers. Serotype diversity and antimicrobial resistance (AMR) of APP strains circulating in north Italian farms from 2015 to 2022 were evaluated retrospectively to investigate APP epidemiology in the area. A total of 572 strains isolated from outbreaks occurring in 337 different swine farms were analysed. The majority of isolates belonged to serotypes 9/11 (39.2%) and 2 (28.1%) and serotype diversity increased during the study period, up to nine different serotypes isolated in 2022. The most common resistances were against tetracycline (53% of isolates) and ampicillin (33%), followed by enrofloxacin, florfenicol and trimethoprim/sulfamethoxazole (23% each). Multidrug resistance (MDR) was common, with a third of isolates showing resistance to more than three antimicrobial classes. Resistance to the different classes and MDR varied significantly depending on the serotype. In particular, the widespread serotype 9/11 was strongly associated with florfenicol and enrofloxacin resistance and showed the highest proportion of MDR isolates. Serotype 5, although less common, showed instead a concerning proportion of trimethoprim/sulfamethoxazole resistance. Our results highlight how the typing of circulating serotypes and the analysis of their antimicrobial susceptibility profile are crucial to effectively manage APP infection and improve antimicrobial stewardship.

Actinobacillus pleuropneumoniae encodes multiple phase-variable DNA methyltransferases that control distinct phasevarions

Actinobacillus pleuropneumoniae is the cause of porcine pleuropneumonia, a severe respiratory tract infection that is responsible for major economic losses to the swine industry. Many host-adapted bacterial pathogens encode systems known as phasevarions (phase-variable regulons). Phasevarions result from variable expression of cytoplasmic DNA methyltransferases. Variable expression results in genome-wide methylation differences within a bacterial population, leading to altered expression of multiple genes via epigenetic mechanisms. Our examination of a diverse population of A. pleuropneumoniae strains determined that Type I and Type III DNA methyltransferases with the hallmarks of phase variation were present in this species. We demonstrate that phase variation is occurring in these methyltransferases, and show associations between particular Type III methyltransferase alleles and serovar. Using Pacific BioSciences Single-Molecule, Real-Time (SMRT) sequencing and Oxford Nanopore sequencing, we demonstrate the presence of the first ever characterised phase-variable, cytosine-specific Type III DNA methyltransferase. Phase variation of distinct Type III DNA methyltransferase in A. pleuropneumoniae results in the regulation of distinct phasevarions, and in multiple phenotypic differences relevant to pathobiology. Our characterisation of these newly described phasevarions in A. pleuropneumoniae will aid in the selection of stably expressed antigens, and direct and inform development of a rationally designed subunit vaccine against this major veterinary pathogen.

Analysis of Salmonella lineage-specific traits upon cell sorting

Microbial cell individuality is receiving increasing interest in the scientific community. Individual cells within clonal populations exhibit noticeable phenotypic heterogeneity. The advent of fluorescent protein technology and advances in single-cell analysis has revealed phenotypic cell variant in bacterial populations. This heterogeneity is evident in a wide range of phenotypes, for example, individual cells display variable degrees of gene expression and survival under selective conditions and stresses, and can exhibit differing propensities to host interactions. Last few years, numerous cell sorting approaches have been employed for resolving the properties of bacterial subpopulations. This review provides an overview of applications of cell sorting to analyze Salmonella lineage-specific traits, including bacterial evolution studies, gene expression analysis, response to diverse cellular stresses and characterization of diverse bacterial phenotypic variants.

Proteomic and Antibody Profiles Reveal Antigenic Composition and Signatures of Bacterial Ghost Vaccine of Brucella abortus A19

Brucellosis is an important zoonotic disease that causes great economic losses. Vaccine immunisation is the main strategy for the prevention and control of brucellosis. Although live attenuated vaccines play important roles in the prevention of this disease, they also have several limitations, such as residual virulence and difficulty in the differentiation of immunisation and infection. We developed and evaluated a new bacterial ghost vaccine of Brucella abortus A19 by a new double inactivation method. The results showed that the bacterial ghost vaccine of Brucella represents a more safe and efficient vaccine for brucellosis. We further characterised the antigenic components and signatures of the vaccine candidate A19BG. Here, we utilised a mass spectrometry-based label-free relative quantitative proteomics approach to investigate the global proteomics changes in A19BGs compared to its parental A19. The proteomic analysis identified 2014 proteins, 1116 of which were differentially expressed compared with those in A19. The common immunological proteins of OMPs (Bcsp31, Omp25, Omp10, Omp19, Omp28, and Omp2a), HSPs (DnaK, GroS, and GroL), and SodC were enriched in the proteome of A19BG. By protein micro array-based antibody profiling, significant differences were observed between A19BG and A19 immune response, and a number of signature immunogenic proteins were identified. Two of these proteins, the BMEII0032 and BMEI0892 proteins were significantly different (P < 0.01) in distinguishing between A19 and A19BG immune sera and were identified as differential diagnostic antigens for the A19BG vaccine candidate. In conclusion, using comparative proteomics and antibody profiling, protein components and signature antigens were identified for the ghost vaccine candidate A19BG, which are valuable for further developing the vaccine and its monitoring assays.

Bacteria from Infectious Particles to Cell Based Anticancer Targeted Drug Delivery Systems

Bacterial ghosts (BGs) are empty cell envelopes of nonliving evacuated bacterial cells. They are free from their cytoplasmic contents; however, they sustain their cellular 3D morphology and antigenic structures, counting on bioadhesive properties. Lately, they have been tested as an advanced drug delivery system (DDS) for different materials like DNA, peptides, or drugs, either single components or combinations. Different studies have revealed that, BG DDS were paid the greatest attention in recent years. The current review explores the impact of BGs on the field of drug delivery and drug targeting. BGs have a varied area of applications, including vaccine and tumor therapy. Moreover, the use of BGs, their synthesis, their uniqueness as a delivery system and application principles in cancer are discussed. Furthermore, the safety issues of BGs and stability aspects of using ghost bacteria as delivery systems are discussed. Future perspective efforts that must be followed for this important system to continue to grow are important and promising.

Actinobacillus pleuropneumoniae: The molecular determinants of virulence and pathogenesis

Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia, is responsible for high economic losses in swine herds across the globe. Pleuropneumonia is characterized by severe respiratory distress and high mortality. The knowledge about the interaction between bacterium and host within the porcine respiratory tract has improved significantly in recent years. A. pleuropneumoniae expresses multiple virulence factors, which are required for colonization, immune clearance, and tissue damage. Although vaccines are used to protect swine herds against A. pleuropneumoniae infection, they do not offer complete coverage, and often only protect against the serovar, or serovars, used to prepare the vaccine. This review will summarize the role of individual A. pleuropneumoniae virulence factors that are required during key stages of pathogenesis and disease progression, and highlight progress made toward developing effective and broadly protective vaccines against an organism of great importance to global agriculture and food production.

Subcutaneous Immunization of Dogs With Bordetella bronchiseptica Bacterial Ghost Vaccine

The Bordetella species are Gram-negative bacterial pathogens that colonizes mammalian respiratory tract causing respiratory diseases in humans and animals. B. bronchiseptica causes clinical conditions in many mammals including immunocompromised humans. Using the dog model of respiratory infection, it has been shown in this study that a newly developed B. bronchiseptica Bacterial Ghost (BbBG) vaccine exhibited significant protection in the face of a severe pathogenic bacterial challenge in seronegative dogs. The protein E-specific lysis mechanism was used to produce BbBGs. Bacterial Ghosts (BGs) are the empty cell envelope of Gram-negative bacterium. They are genetically processed to form a microscopic hole in their membrane, through which all the cytoplasmic contents are expelled leaving behind intact empty bacterial shells. Due to the intact surface structures of BGs, they offer the safety of inactivated but efficacy of live attenuated vaccines. In this study, seronegative dogs were vaccinated subcutaneously (s/c) with two different doses of a newly developed BbBG vaccine [lower 10^∧5 (BbBG – 5) and higher 10^∧7 (BbBG – 7)] on day 0 and 21. The animals were challenged (by aerosol) with virulent live B. bronchiseptica strains 41 days after first vaccination. The dogs vaccinated s/c with BbBG – 7 vaccine had significantly lower spontaneous coughing scores (P = 0.0001) than dogs in negative control group. Furthermore, the tested BbBG – 7 vaccine was equivalent to the positive control vaccine Bronchicine CAe in terms of safety and efficacy. For the first time, we report the successful use of liquid formulated BGs vaccines in animal studies. Earlier reported studies using BGs vaccines were performed with resuspended freeze-dried BGs preparations.

Cytoplasmic glycoengineering of Apx toxin fragments in the development of Actinobacillus pleuropneumoniae glycoconjugate vaccines

BACKGROUND

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia and represents a major burden to the livestock industry. Virulence can largely be attributed to the secretion of a series of haemolytic toxins, which are highly immunogenic. A. pleuropneumoniae also encodes a cytoplasmic N-glycosylation system, which involves the modification of high molecular weight adhesins with glucose residues. Central to this process is the soluble N-glycosyl transferase, ngt, which is encoded in an operon with a subsequent glycosyl transferase, agt. Plasmid-borne recombinant expression of these genes in E. coli results in the production of a glucose polymer on peptides containing the appropriate acceptor sequon, NX(S/T). However to date, there is little evidence to suggest that such a glucose polymer is formed on its target peptides in A. pleuropneumoniae. Both the toxins and glycosylation system represent potential targets for the basis of a vaccine against A. pleuropneumoniae infection.

RESULTS

In this study, we developed cytoplasmic glycoengineering to construct glycoconjugate vaccine candidates composed of soluble toxin fragments modified by glucose. We transferred ngt and agt to the chromosome of Escherichia coli in order to generate a native-like operon for glycoengineering. A single chromosomal copy of ngt and agt resulted in the glucosylation of toxin fragments by a short glycan, rather than a polymer.

CONCLUSIONS

A vaccine candidate that combines toxin fragment with a conserved glycan offers a novel approach to generating epitopes important for both colonisation and disease progression.

In vivo testing of novel vaccine prototypes against Actinobacillus pleuropneumoniae

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is a Gram-negative bacterium that represents the main cause of porcine pleuropneumonia in pigs, causing significant economic losses to the livestock industry worldwide. A. pleuropneumoniae, as the majority of Gram-negative bacteria, excrete vesicles from its outer membrane (OM), accordingly defined as outer membrane vesicles (OMVs). Thanks to their antigenic similarity to the OM, OMVs have emerged as a promising tool in vaccinology. In this study we describe the in vivo testing of several vaccine prototypes for the prevention of infection by all known A. pleuropneumoniae serotypes. Previously identified vaccine candidates, the recombinant proteins ApfA and VacJ, administered individually or in various combinations with the OMVs, were employed as vaccination strategies. Our data show that the addition of the OMVs in the vaccine formulations significantly increased the specific IgG titer against both ApfA and VacJ in the immunized animals, confirming the previously postulated potential of the OMVs as adjuvant. Unfortunately, the antibody response raised did not translate into an effective protection against A. pleuropneumoniae infection, as none of the immunized groups following challenge showed a significantly lower degree of lesions than the controls. Interestingly, quite the opposite was true, as the animals with the highest IgG titers were also the ones bearing the most extensive lesions in their lungs. These results shed new light on A. pleuropneumoniae pathogenicity, suggesting that antibody-mediated cytotoxicity from the host immune response may play a central role in the development of the lesions typically associated with A. pleuropneumoniae infections.

Identification and characterization of serovar-independent immunogens in Actinobacillus pleuropneumoniae

Despite numerous actions to prevent disease, Actinobacillus pleuropneumoniae (A. pleuropneumoniae) remains a major cause of porcine pleuropneumonia, resulting in economic losses to the swine industry worldwide. In this paper, we describe the utilization of a reverse vaccinology approach for the selection and in vitro testing of serovar-independent A. pleuropneumoniae immunogens. Potential immunogens were identified in the complete genomes of three A. pleuropneumoniae strains belonging to different serovars using the following parameters: predicted outer-membrane subcellular localization; ≤ 1 trans-membrane helices; presence of a signal peptide in the protein sequence; presence in all known A. pleuropneumoniae genomes; homology with other well characterized factors with relevant data regarding immunogenicity/protective potential. Using this approach, we selected the proteins ApfA and VacJ to be expressed and further characterized, both in silico and in vitro. Additionally, we analysed outer membrane vesicles (OMVs) of A. pleuropneumoniae MIDG2331 as potential immunogens, and compared deletions in degS and nlpI for increasing yields of OMVs compared to the parental strain. Our results indicated that ApfA and VacJ are highly conserved proteins, naturally expressed during infection by all A. pleuropneumoniae serovars tested. Furthermore, OMVs, ApfA and VacJ were shown to possess a high immunogenic potential in vitro. These findings favour the immunogen selection protocol used, and suggest that OMVs, along with ApfA and VacJ, could represent effective immunogens for the prevention of A. pleuropneumoniae infections in a serovar-independent manner. This hypothesis is nonetheless predictive in nature, and in vivo testing in a relevant animal model will be necessary to verify its validity.

Bacterial ghosts as adjuvants: mechanisms and potential

Bacterial ghosts (BG) are empty cell envelopes derived from Gram-negative bacteria. They contain many innate immunostimulatory agonists, and are potent activators of a broad range of cell types involved in innate and adaptive immunity. Several considerable studies have demonstrated the effectiveness of BG as adjuvants as well as their ability to induce proinflammatory cytokine production by a range of immune and non-immune cell types. These proinflammatory cytokines trigger a generalized recruitment of T and B lymphocytes to lymph nodes that maximize the chances of encounter with their cognate antigen, and subsequent elicitation of potent immune responses. The plasticity of BG has allowed for the generation of envelope-bound foreign antigens in immunologically active forms that have proven to be effective vaccines in animal models. Besides their adjuvant property, BG also effectively deliver DNA-encoded antigens to dendritic cells, thereby leading to high transfection efficiencies, which subsequently result in higher gene expressions and improved immunogenicity of DNA-based vaccines. In this review, we summarize our understanding of BG interactions with the host immune system, their exploitation as an adjuvant and a delivery system, and address important areas of future research interest.

Identification of novel Haemophilus parasuis serovar 5 vaccine candidates using an immunoproteomic approach

Haemophilus parasuis is the aetiological agent of Glässer's disease, which is responsible for cases of fibrinous polyserositis, polyarthritis and meningitis. No vaccine is known that provides cross-protection against all serovars. The identification of novel immunoprotective antigens would undoubtedly contribute to the development of efficient subunit vaccines. In the present study, an immunoproteomic approach was used to analyze secreted proteins of H. parasuis and six proteins with high immunogenicity were identified. Five of them were successfully expressed, and their immunogenicity and protective efficacy were assessed in a mouse challenge model. All five proteins elicited strong humoral antibody and cellular immune responses in mice. They all effectively reduced the growth of H. parasuis in mouse organs and conferred different levels of protection (40-80%) against challenge. IgG subtype analysis revealed that the five proteins induce a bias toward a Th1-type immune response, and a significant increase was observed in the cytokine levels of IL-2, IFN-γ and Th2-specific IL-4 in the culture supernatants of splenocytes isolated from immunized mice. The results suggest that both Th1 and Th2 responses are involved in mediating protection. These data suggest that the five proteins could be potential subunit vaccine candidates for use to prevent H. parasuis infection.

BIOLOGICAL SIGNIFICANCE

Haemophilus parasuis can cause huge financial loss in the swine industry worldwide. There are still no vaccines which can provide cross-protection against all serovars. To address this need, we applied an immunoproteomic approach involving 2-DE, MALDI-TOF/TOF MS and Western-blot to identify the secreted proteins which may be able to provide immunoprotection to this disease. We identified six immunogenic proteins, and the immunogenicity and protective efficacy were validated. This result provides a foundation for developing novel subunit vaccines against Haemophilus parasuis.

Immunoprotective Efficacy of Six In vivo-Induced Antigens against Actinobacillus pleuropneumoniae as Potential Vaccine Candidates in Murine Model

Six in vivo-induced (IVI) antigens—RnhB, GalU, GalT, Apl_1061, Apl_1166, and HflX were selected for a vaccine trial in a mouse model. The results showed that the IgG levels in each immune group was significantly higher than that of the negative control (P < 0.001). Except rRnhB group, proliferation of splenocytes was observed in all immunized groups and a relatively higher proliferation activity was observed in rGalU and rGalT groups (P < 0.05). In the rGalT vaccinated group, the proportion of CD4+ T cells in spleen was significant higher than that of negative control (P < 0.05). Moreover, proportions of CD4+ T cells in other vaccinated groups were all up-regulated to varying degrees. Up-regulation of both Th1 (IFN-γ, IL-2) and Th2 (IL-4) cytokines were detected. A survival rate of 87.5, 62.5, and 62.5% were obtained among rGalT, rAPL_1166, and rHflX group, respectively while the remaining three groups was only 25%. Histopathological analyses of lungs indicated that surviving animals from the vaccinated groups showed relatively normal pulmonary structure alveoli. These findings confirm that IVI antigens used as vaccine candidates provide partial protection against Actinobacillus pleuropneumoniae infection in a mouse model, which could be used as potential vaccine candidates in piglets.

Delivery of a Chlamydial Adhesin N-PmpC Subunit Vaccine to the Ocular Mucosa Using Particulate Carriers

Trachoma, caused by the intracellular bacterium Chlamydia trachomatis (Ct), remains the world's leading preventable infectious cause of blindness. Recent attempts to develop effective vaccines rely on modified chlamydial antigen delivery platforms. As the mechanisms engaged in the pathology of the disease are not fully understood, designing a subunit vaccine specific to chlamydial antigens could improve safety for human use. We propose the delivery of chlamydia-specific antigens to the ocular mucosa using particulate carriers, bacterial ghosts (BGs). We therefore characterized humoral and cellular immune responses after conjunctival and subcutaneous immunization with a N-terminal portion (amino acid 1-893) of the chlamydial polymorphic membrane protein C (PmpC) of Ct serovar B, expressed in probiotic Escherichia coli Nissle 1917 bacterial ghosts (EcN BGs) in BALB/c mice. Three immunizations were performed at two-week intervals, and the immune responses were evaluated two weeks after the final immunization in mice. In a guinea pig model of ocular infection animals were immunized in the same manner as the mice, and protection against challenge was assessed two weeks after the last immunization. N-PmpC was successfully expressed within BGs and delivery to the ocular mucosa was well tolerated without signs of inflammation. N-PmpC-specific mucosal IgA levels in tears yielded significantly increased levels in the group immunized via the conjunctiva compared with the subcutaneously immunized mice. Immunization with N-PmpC EcN BGs via both immunization routes prompted the establishment of an N-PmpC-specific IFNγ immune response. Immunization via the conjunctiva resulted in a decrease in intensity of the transitional inflammatory reaction in conjunctiva of challenged guinea pigs compared with subcutaneously and non-immunized animals. The delivery of the chlamydial subunit vaccine to the ocular mucosa using a particulate carrier, such as BGs, induced both humoral and cellular immune responses. Further investigations are needed to improve the immunization scheme and dosage.

Characterization of a Salmonella Typhimurium ghost carrying an adjuvant protein as a vaccine candidate for the protection of chickens against virulent challenge

In this study we describe the generation of a safe, immunogenic, genetically inactivated Salmonella Typhimurium ghost vaccine candidate carrying the Escherichia coli heat-labile enterotoxin B subunit (LTB) protein as an adjuvant molecule. An asd(+) p15A ori(-) plasmid pJHL187-LTB harbouring the E lysis gene cassette and a foreign antigen delivery cassette containing the eltB gene was used to transform a Δasd Salmonella Typhimurium (JOL1311) strain to construct the ghost strain, JOL1499. Incubation of mid-logarithmic phase JOL1499 cultures at 42°C resulted in co-expression of the eltB and E lysis genes, leading to the generation of Salmonella Typhimurium ghost cells carrying the LTB protein (Salmonella Typhimurium-LTB ghost). The production of LTB in Salmonella Typhimurium-LTB ghost preparations was confirmed by western blot analysis, and functional activity of the LTB protein to bind with GM1 receptors was determined by means of GM1 enzyme-linked immunosorbent assay. Efficacy of the Salmonella Typhimurium-LTB ghost as a vaccine candidate was evaluated in a chicken model using 56 chickens at 5 weeks old, which were divided into four groups (n = 14): group A was designated the non-vaccinated control group, whereas the birds in groups B, C, and D were immunized intramuscularly with 10(9), 10(8), and 10(7) ghost cells, respectively. Compared with the non-immunized chickens (group A), immunized chickens (groups B, C and D) exhibited increased titres of plasma IgG and intestinal secretory IgA antibodies. After oral challenge with 10(9) colony-forming units of a virulent Salmonella Typhimurium strain, the vaccinated group B birds showed a decrease in internal organ colonization with the challenge strain.

Acute and subacute response of iron, zinc, copper and selenium in pigs experimentally infected with Actinobacillus pleuropneumoniae

Experimental bacterial lung infection affects trace elements in blood and liver tissue.

Development of a biosafety enhanced and immunogenic Salmonella enteritidis ghost using an antibiotic resistance gene free plasmid carrying a bacteriophage lysis system

In the development of genetically inactivated bacterial vaccines, plasmid retention often requires the antibiotic resistance gene markers, the presence of which can cause the potential biosafety hazards such as the horizontal spread of resistance genes. The new lysis plasmid was constructed by utilizing the approach of balanced-lethal systems based on auxotrophic gene Aspartate semialdehyde dehydrogenase (asd). The PhiX174 lysis gene E and λPR37-cI857 temperature-sensitive regulatory system was cloned in the asd gene positive plasmid and this novel approach allowed the production of antibiotic resistance marker free Salmonella Enteritidis (S. Enteritidis) ghost. The immunogenic potential of the biosafety enhanced antibiotic resistance gene free S. Enteritidis ghost was evaluated in chickens by employing the prime-boost vaccination strategy using a combination of oral and intramuscular routes. A total of 75 two-week-old chickens were equally divided into five groups: group A (non-immunized control), group B (intramuscularly primed and boosted), group C (primed intramuscularly and boosted orally), group D (primed and boosted orally), and group E (primed orally and boosted intramuscularly). Chickens from all immunized groups demonstrated significant increases in plasma IgG, intestinal secretory IgA levels, and antigen-specific lymphocyte proliferative response. After a virulent S. Enteritidis challenge, all immunized groups showed fewer gross lesions and decreased bacterial recovery from organs in comparison with the non-immunized control group. Among the immunized chickens, groups B and D chickens showed optimized protection, indicating that the prime-booster immunization with the ghost via intramuscular or oral route is efficient. Taken together, our results demonstrate that an antibiotic resistance gene free lysis plasmid was successfully constructed and utilized for production of safety enhanced S. Enteritidis ghost, which can be used as a safe and effective vaccine against virulent S. Enteritidis infections.

Protection of piglets by a Haemophilus parasuis ghost vaccine against homologous challenge

Commercial bacterins for Glässer's disease are widely used for the prevention of this disease caused by Haemophilus parasuis; however, the protective efficacy varies depending on the strain and serovar. Bacterial ghosts (BGs) are empty bacterial envelopes that, unlike classic bacterins, suffer no denaturing steps during their production. These properties may lead to superior protection. In this study, a BG vaccine generated from the Haemophilus parasuis serovar 5 reference strain Nagasaki was prepared and used to inoculate piglets. The efficacy of the BG vaccine was evaluated by clinical, bacteriological, serological, and postmortem examinations. Inactivated bacterin (IB) and a placebo control (PC) were compared with the BG vaccine in this study. The results showed that the piglets inoculated with the BG vaccine developed higher antibody activity and higher gamma interferon and interleukin 4 levels than those vaccinated with IB or those in the PC group after primary and secondary exposure to the antigens and challenge. CD4(+) T lymphocyte levels were observed to increase following secondary immunization more in the BG-vaccinated group than in the IB (P < 0.05) and PC (P < 0.05) groups. CD8(+) T lymphocyte levels increased dramatically in all three groups after challenge, and the differences between groups were all significant (P < 0.05). There were fewer tissue lesions and lower bacterial loads in the tissue homogenates in the BG group after challenge. The results suggest that higher CD4(+) T lymphocyte levels and both CD4(+) major histocompatibility complex class II-restricted Th1-type and Th2-type immune responses in the BG group are relevant for protection.

Construction and immunogenicity of a ∆apxIC/ompP2 mutant of Actinobacillus pleuropneumoniae and Haemophilus parasuis

The apxIC genes of the Actinobacillus pleuropneumoniae serovar 5 (SC-1), encoding the ApxIactivating proteins, was deleted by a method involving sucrose counter-selection. In this study, a mutant strain of A. pleuropneumoniae (SC-1) was constructed and named DapxIC/ ompP2. The mutant strain contained foreign DNA in the deletion site of ompP2 gene of Haemophilus parasuis. It showed no haemolytic activity and lower virulence of cytotoxicity in mice compared with the parent strain, and its safety and immunogenicity were also evaluated in mice. The LD50 data shown that the mutant strain was attenuated 30-fold, compared with the parent strain (LD50 of the mutant strain and parent strain in mice were determined to be 1.0 × 107 CFU and 3.5 × 105 CFU respectively). The mutant strain that was attenuated could secrete inactivated ApxIA RTX toxins with complete antigenicity and could be used as a candidate live vaccine strain against infections of A. pleuropneumoniae and H. parasuis.

Escherichia coli ghosts or live E. coli expressing the ferri-siderophore receptors FepA, FhuE, IroN and IutA do not protect broiler chickens against avian pathogenic E. coli (APEC)

The aim of this study was to investigate if immunization with the ferri-siderophore receptors FepA, FhuE, IroN and IutA could protect chickens against avian pathogenic Escherichia coli (APEC) infection. The antigens were administered as recombinant proteins in the outer membrane (OM) of E. coli strain BL21 Star DE3. In a first immunization experiment, live E. coli expressing all 4 recombinant ferri-siderophore receptors (BL21(L)) were given intranasally. In a second immunization experiment, a mixture of E. coli ghosts containing recombinant FepA and IutA and ghosts containing recombinant FhuE and IroN was evaluated. For both experiments non-recombinant counterparts of the tentative vaccines were administered as placebo. At the time of challenge, the IgG antibody response for BL21(L) and a mixture of E. coli ghosts containing recombinant FepA and IutA and ghosts containing recombinant FhuE and IroN was significantly higher in all immunized groups as compared to the negative control groups (LB or PBS) confirming successful immunization. Although neither of the tentative vaccines could prevent lesions and mortality upon APEC infection, immunization with bacterial ghosts resulted in a decrease in mortality from 50% (PBS) to 31% (non-recombinant ghosts) or 20% (recombinant ghosts) and these differences were not found to be significant.

Characterization of a novel inactivated Salmonella enterica serovar Enteritidis vaccine candidate generated using a modified cI857/λ PR/gene E expression system

A new strategy to develop an effective vaccine is essential to control food-borne Salmonella enterica serovar Enteritidis infections. Bacterial ghosts (BGs), which are nonliving, Gram-negative bacterial cell envelopes, are generated by expulsion of the cytoplasmic contents from bacterial cells through controlled expression using the modified cI857/λ P(R)/gene E expression system. In the present study, the pJHL99 lysis plasmid carrying the mutated lambda pR37-cI857 repressor and PhiX174 lysis gene E was constructed and transformed in S. Enteritidis to produce a BG. Temperature induction of the lysis gene cassette at 42°C revealed quantitative killing of S. Enteritidis. The S. Enteritidis ghost was characterized using scanning and transmission electron microscopy to visualize the transmembrane tunnel structure and loss of cytoplasmic materials, respectively. The efficacy of the BG as a vaccine candidate was evaluated in a chicken model using 60 10-day-old chickens, which were divided into four groups (n = 15), A, B, C, and D. Group A was designated as the nonimmunized control group, whereas the birds in groups B, C, and D were immunized via the intramuscular, subcutaneous, and oral routes, respectively. The chickens from all immunized groups showed significant increases in plasma IgG and intestinal secretory IgA levels. The lymphocyte proliferation response and CD3(+) CD4(+) and CD3(+) CD8(+) T cell subpopulations were also significantly increased in all immunized groups. The data indicate that both humoral and cell-mediated immune responses are robustly stimulated. Based on an examination of the protection efficacy measured by observations of gross lesions in the organs and bacterial recovery, the candidate vaccine can provide efficient protection against virulent challenge.

The Bacterial Ghost platform system: production and applications

The Bacterial Ghost (BG) platform technology is an innovative system for vaccine, drug or active substance delivery and for technical applications in white biotechnology. BGs are cell envelopes derived from Gram-negative bacteria. BGs are devoid of all cytoplasmic content but have a preserved cellular morphology including all cell surface structures. Using BGs as delivery vehicles for subunit or DNA-vaccines the particle structure and surface properties of BGs are targeting the carrier itself to primary antigen-presenting cells. Furthermore, BGs exhibit intrinsic adjuvant properties and trigger an enhanced humoral and cellular immune response to the target antigen. Multiple antigens of the native BG envelope and recombinant protein or DNA antigens can be combined in a single type of BG. Antigens can be presented on the inner or outer membrane of the BG as well as in the periplasm that is sealed during BG formation. Drugs or supplements can also be loaded to the internal lumen or periplasmic space of the carrier. BGs are produced by batch fermentation with subsequent product recovery and purification via tangential flow filtration. For safety reasons all residual bacterial DNA is inactivated during the BG production process by the use of staphylococcal nuclease A and/or the treatment with β-propiolactone. After purification BGs can be stored long-term at ambient room temperature as lyophilized product. The production cycle from the inoculation of the pre-culture to the purified BG concentrate ready for lyophilization does not take longer than a day and thus meets modern criteria of rapid vaccine production rather than keeping large stocks of vaccines. The broad spectrum of possible applications in combination with the comparably low production costs make the BG platform technology a safe and sophisticated product for the targeted delivery of vaccines and active agents as well as carrier of immobilized enzymes for applications in white biotechnology.

Rectal single dose immunization of mice with Escherichia coli O157:H7 bacterial ghosts induces efficient humoral and cellular immune responses and protects against the lethal heterologous challenge

Bacterial ghosts (BGs) have been applied through oral, aerogenic, intraocular or intranasal routes for mucosal immunization using a wide range of experimental animals. All these applications required a booster after primary immunization to achieve protective immunity against the lethal challenge. Here we report for the first time that a single rectal dose of BGs produced from enterohaemorrhagic Escherichia coli (EHEC) O157:H7 fully protects mice against a 50% lethal challenge with a heterologous EHEC strain given at day 55. BGs from EHEC O157:H7 were prepared by a combination of protein E‐mediated cell lysis and expression of staphylococcal nuclease A guaranteeing the complete degradation of pathogen residual DNA. The lack of genetic material in the EHEC BGs vaccine abolished any potential hazard for horizontal gene transfer of plasmid encoded antibiotic resistance genes or pathogenic islands to the recipient's gut flora. Single rectal immunization using EHEC O157:H7 BGs without any addition of adjuvant significantly stimulated efficient humoral and cellular immune responses, and was equally protective as two immunizations, which indicates the possibility to develop a novel efficacious single dose mucosal EHEC O157:H7 BGs vaccine using a simplified immunization regimen.

Enhancement of bacteriolysis of shuffled phage PhiX174 gene E

Bacterial ghosts that are generated using the regulated PhiX174 lysis gene E offer a new avenue for the study of inactivated vaccines. Here, we constructed a library of mutant gene E using a gene-shuffling technique. After screening and recombination with the prokaryotic non-fusion expression vector pBV220, two lysis plasmids were selected. Among which, a novel mutant E gene (named mE), consisting of a 74-bp non-encoding sequence at 5'-end and a 201-bp gene ΔE, significantly increased the lysis effect on prokaryotic Escherichia coli and Salmonella enteritidis. Moreover, lysis efficiency, as measured by the OD₆₀₀ value, reached 1.0 (10⁹ CFU), avoiding the bottleneck problem observed with other bacterial lysis procedures, which results in a low concentration of bacteria in suspension, and consequent low production of bacterial ghosts. Our results may provide a promising avenue for the development of bacterial ghost vaccines.

Bacterial Ghosts as antigen and drug delivery system for ocular surface diseases: Effective internalization of Bacterial Ghosts by human conjunctival epithelial cells

The purpose of the presented investigation was to examine the efficiency of the novel carrier system Bacterial Ghosts (BGs), which are empty bacterial cell envelopes of Gram-negative bacteria to target human conjunctival epithelial cells, as well as to test the endocytic capacity of conjunctival cells after co-incubation with BGs generated from different bacterial species, and to foreclose potential cytotoxic effects caused by BGs. The efficiency of conjunctival cells to internalize BGs was investigated using the Chang conjunctival epithelial cell line and primary human conjunctiva-derived epithelial cells (HCDECs) as in vitro model. A high capacity of HCDECs to functionally internalize BGs was detected with the level of internalization depending on the type of species used for BGs generation. Detailed analysis showed no cytotoxic effect of BGs on HCDECs independently of the used bacterial species. Moreover, co-incubation with BGs did not enhance expression of both MHC class I and class II molecules by HCDECs, but increased expression of ICAM-1. The high rates of BG's internalization by HCDECs with no BG-mediated cytotoxic impact designate this carrier system to be a promising candidate for an ocular surface drug delivery system. BGs could be useful for future therapeutic ocular surface applications and eye-specific disease vaccine development including DNA transfer.

Salmonella enteritidis ghost vaccine induces effective protection against lethal challenge in specific-pathogen-free chicks

Bacterial ghosts (BGs) are empty bacterial envelopes generated by expulsion of the bacterial genome and cytoplasmic contents from bacterial cells, and the process is mediated by lysis protein E encoded on bacteriophage PhiX174. BGs represent a new approach in vaccine development and have been applied to a variety of gram-negative bacterial vaccine candidates. In this study, a BG vaccine generated from Salmonella enteritidis (S. enteritidis) strain DH091 was prepared using the highly efficient plasmid, pBV-mE. The efficacy of the BG vaccine was tested using 75 chicks (Gallus gallus) kept under specific pathogen-free (SPF) conditions. A comprehensive evaluation of the immune response, including humoral and cellular immune responses, interferon-γ (IFN-γ) and interleukin-4 (IL-4) production, and histopathology of various tissues, was performed in BG-vaccinated animals subsequently challenged with S. enteritidis. The results were compared with animals that were immunized with the inactivated vaccine. S. enteritidis ghosts not only promoted the generation of high titer antibodies and IFN-γ and IL-4 production but also stimulated a significant increase in CD8(+) and CD4(+) T lymphocytes. In particular, the dramatic increase in CD8(+) T cells indicated that the vaccine was able to induce clearance of intracellular Salmonella. The protective effects of BG vaccination in SPF chicks against 5×10(9) colony forming units of S. enteritidis were a result of the induction of a more effective immune response than that observed with the inactivated vaccine. These findings demonstrate the potential of S. enteritidis ghosts to be used as effective vaccines.

Field experience with two different vaccination strategies aiming to control infections with Actinobacillus pleuropneumoniae in a fattening pig herd

Background

The prevalence of pleurisies recorded at slaughter is increasing in Sweden, and acute outbreaks of actinobacillosis that require antimicrobial treatments have become more frequent. As an increased use of antimicrobials may result in the development of antimicrobial resistance it is essential to develop alternative measures to control the disease. Vaccinations present an appealing alternative to antimicrobial treatments. The aim of this work was to evaluate the potential of two different vaccination strategies in a specialized fattening herd affected by actinobacillosis.

Methods

The study was conducted in a specialized fattening herd employing age segregated rearing in eight units. The herd suffered from infections caused by Actinobacillus pleuropneumoniae serotype 2, confirmed by necropsy and serology. The study included 54 batches of pigs grouped into five periods. Batches of pigs of the second period were vaccinated against actinobacillosis twice, and pigs in the fourth period were vaccinated three times. Batches of pigs of the first, third and fifth period were not vaccinated. Concentrations of serum antibodies to A. pleuropneumoniae and serum amyloid A (SAA) were analysed and production data were recorded.

Results

Despite vaccinating, medical treatments were required to reduce the impact of the disease. The mean incidence of individual treatments for respiratory diseases during the rearing period ranged from 0 to 4.7 ± 1.8%, and was greatest during the triple vaccination period (period IV; p < 0.05 when compared to other groups). A large proportion of the vaccinated pigs seroconverted to A. pleuropneumoniae serotype 2 in the absence of a SAA-response. The prevalence of pleuritis decreased from 25.4 ± 6.5% in the first period to 5.0 ± 3.7% in the fifth period (p < 0.001).

Conclusions

The vaccine did not effectively prevent clinical expression of A. pleuropneumoniae infections, but seroconversion to A. pleuropneumoniae in the absence of a SAA-response in a large number pigs indicated that the vaccine had activated the immune system. Further, the prevalence of pleuritis decreased with time. This indicates that vaccinations together with intensified medical treatments of affected pigs could be useful in reducing the impact of A. pleuropneumoniae serotype 2 infections.

Selection of serotype-specific vaccine candidate genes in Actinobacillus pleuropneumoniae and heterologous immunization with Propionibacterium acnes

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) is a highly contagious lethal causative agent of swine pleuropneumoniae. Vaccines for this disease are usually serotype specific. In order to identify immunogenic genes specific to serotypes, two differentially expressed gene cDNA libraries of A. pleuropneumoniae CCVC259 (serotype 1) and CCVC263 (serotype 5) had been constructed by using a cDNA representational difference analysis (cDNA-RDA). From the libraries, six potential vaccine candidate genes expressed only in serotype 1 and 13 genes in serotype 5 were identified by antibody screening after gene expression in vitro with a ribosome display system. Eight sequences out of these exhibited 77-100% identity to the corresponding genes in Propionibacterium acnes. The antisera raised against A. pleuropneumoniae serotypes 1 and 5 were reactive with P. acnes at a titer of 1:6400 and vice versa (ELISA titer, 1:3200). Mice immunized with P. acnes were protected against 10 x LD50 challenge with A. pleuropneumoniae serotypes 1 and 5, and the survival rates were 90% and 95%, respectively. Pigs vaccinated with the P. acnes strain could develop high level antibody cross-reacted with A. pleuropneumoniae and obtain noticeable protection from A. pleuropneumoniae infection. These data demonstrate that there were common antigens between A. pleuropneumoniae and P. acnes, and the cross protectivity highlights the possibility of using P. acnes vaccines for preventing infection by A. pleuropneumoniae.

Actinobacillus pleuropneumoniaevaccines: from bacterins to new insights into vaccination strategies

With the growing emergence of antibiotic resistance and rising consumer demands concerning food safety, vaccination to prevent bacterial infections is of increasing relevance.Actinobacillus pleuropneumoniaeis the etiological agent of porcine pleuropneumonia, a respiratory disease leading to severe economic losses in the swine industry. Despite all the research and trials that were performed withA. pleuropneumoniaevaccination in the past, a safe vaccine that offers complete protection against all serotypes has yet not reached the market. However, recent advances made in the identification of new potential vaccine candidates and in the targeting of specific immune responses, give encouraging vaccination perspectives. Here, we review past and current knowledge onA. pleuropneumoniaevaccines as well as the newly available genomic tools and vaccination strategies that could be useful in the design of an efficient vaccine againstA. pleuropneumoniaeinfection.

Helicobacter pylori vaccine development: optimisation of strategies and importance of challenging strain and animal model

Gastric infection with the gram-negative bacterial pathogen Helicobacter pylori is widespread (approximately 50% of the human population is affected) and is associated with the induction of specific gastroduodenal disease. Although extensive studies in the H. pylori mouse model have demonstrated the feasibility of both therapeutic and prophylactic immunisations, the mechanism of vaccine-induced protection is still poorly understood. We report here on novel strategies to optimise the generation of H. pylori ghosts as vaccine candidates and highlight the need to concentrate on alternative animal models and the use of fully virulent H. pylori type I strains for vaccination. An effective vaccine strategy against H. pylori has the potential to significantly improve population health worldwide.

Bacterial ghosts as adjuvant particles

The development of more advanced and effective vaccines is of great interest in modern medicine. These new-generation vaccines, based on recombinant proteins or DNA, are often less reactogenic and immunogenic than traditional vaccines. Thus, there is an urgent need for the development of new and improved adjuvants. Besides many other immunostimulatory components, the bacterial ghost (BG) system is currently under investigation as a potent vaccine delivery system with intrinsic adjuvant properties. BGs are nonliving cell envelope preparations from Gram-negative cells, devoid of cytoplasmic contents, while their cellular morphology and native surface antigenic structures remain preserved. Owing to the particulate nature of BGs and the fact that they contain many well known immune-stimulating compounds, BGs have the potential to enhance immune responses against ghost-delivered target antigens.

Antigen discovery and delivery of subunit vaccines by nonliving bacterial ghost vectors

The bacterial ghost (BG) platform system is a novel vaccine delivery system endowed with intrinsic adjuvant properties. BGs are nonliving Gram-negative bacterial cell envelopes which are devoid of their cytoplasmic contents, yet maintain their cellular morphology and antigenic structures, including bioadhesive properties. The main advantages of BGs as carriers of subunit vaccines include their ability to stimulate a high immune response and to target the carrier itself to primary antigen-presenting cells. The intrinsic adjuvant properties of BGs enhance the immune response to target antigens, including T-cell activation and mucosal immunity. Since native and foreign antigens can be carried in the envelope complex of BGs, combination vaccines with multiple antigens of diverse origin can be presented to the immune system simultaneously. Beside the capacity of BGs to function as carriers of protein antigens, they also have a high loading capacity for DNA. Thus, loading BGs with recombinant DNA takes advantage of the excellent bioavailability for DNA-based vaccines and the high expression rates of the DNA-encoded antigens in target cell types such as macrophages and dendritic cells. There are many spaces within BGs including the inner and outer membranes, the periplasmic space and the internal lumen which can carry antigens, DNA or mediators of the immune response. All can be used for subunit antigen to design new vaccine candidates with particle presentation technology. In addition, the fact that BGs can also carry piggyback large-size foreign antigen particles, increases the technologic usefulness of BGs as combination vaccines against viral and bacterial pathogens. Furthermore, the BG antigen carriers can be stored as freeze-dried preparations at room temperature for extended periods without loss of efficacy. The potency, safety and relatively low production cost of BGs offer a significant technical advantage over currently utilized vaccine technologies.

Bacterial ghosts as a novel advanced targeting system for drug and DNA delivery

Although there are powerful drugs against infectious diseases and cancer on the market, delivery systems are needed to decrease serious toxic and noncurative side effects. In order to enhance compliance, several delivery systems such as polymeric micro- and nanoparticles, liposomal systems and erythrocyte ghosts have been developed. Bacterial ghosts representing novel advanced delivery and targeting vehicles suitable for the delivery of hydrophobic or water-soluble drugs, are the main focus of this review. They are useful nonliving carriers, as they can carry different active substances in more than one cellular location separately and simultaneously. Bacterial ghosts combine excellent natural or engineered adhesion properties with versatile carrier functions for drugs, proteins and DNA plasmids or DNA minicircles. The simplicity of both bacterial ghost production and packaging of drugs and/or DNA makes them particularly suitable for the use as a delivery system. Further advantages of bacterial ghost delivery vehicles include high bioavailability and a long shelf life without the need of cold-chain storage due to the possibility to freeze-dry the material.

Bacterial ghosts as antigen delivery vehicles

The bacterial ghost system is a novel vaccine delivery system unusual in that it combines excellent natural intrinsic adjuvant properties with versatile carrier functions for foreign antigens. The efficient tropism of bacterial ghosts (BG) for antigen presenting cells promotes the generation of both cellular and humoral responses to heterologous antigens and carrier envelope structures. The simplicity of both BG production and packaging of (multiple) target antigens makes them particularly suitable for use as combination vaccines. Further advantages of BG vaccines include a long shelf-life without the need of cold-chain storage due to their freeze-dried status, they are safe as they do not involve host DNA or live organisms, they exhibit improved potency with regard to target antigens compared to conventional approaches, they are versatile with regards to DNA or protein antigen choice and size, and as a delivery system they offer high bioavailability.

Systemic and Local Antibody Responses after Experimental Infection with Actinobacillus pleuropneumoniae in Piglets with Passive or Active Immunity

The objectives of the present study was to describe different dynamics of humoral immune responses to experimental infection in piglets of different stages of infection and immunity. Two groups of piglets originating from non-immune (group 1) and immune (group 2) sows at the age of 3 weeks were subdivided as follows: a half of each group of piglets was exposed to a low-dose infection with Actinobacillus pleuropneumoniae (APP) strain 9. At the age of 8 weeks, all four groups of piglets were challenged with a high infection dose of APP of the same strain. Isotype characterization of the specific antibodies in sera and in bronchoalveolar lavage fluids (BALF) to a lipopolysaccharide was carried out, besides monitoring clinical signs and post-mortem examinations. A typical primary immune response was observed in specific antibody-free piglets infected with a challenge infection. Colostrum-derived immunoglobulin-G (IgG) antibodies persisted in sera and BALF of piglets up to the age of 8 weeks. However, they did not prevent induction of specific-primary antibody response, either in 8 or 4 weeks of age, when levels of specific colostrum-derived antibodies were still high. It was demonstrated by the increase of specific IgM antibodies in sera. The infection induced an increase in the levels of IgA antibodies in BALF regardless the severity of infection and presence of specific colostrum-derived antibodies. The specific antibodies of IgG isotype increased only in BALF from piglets without colostrum-derived antibodies.

Escherichia coli ghost production by expression of lysis gene E and Staphylococcal nuclease

The production of bacterial ghosts from Escherichia coli is accomplished by the controlled expression of phage phiX174 lysis gene E and, in contrast to other gram-negative bacterial species, is accompanied by the rare detection of nonlysed, reproductive cells within the ghost preparation. To overcome this problem, the expression of a secondary killing gene was suggested to give rise to the complete genetic inactivation of the bacterial samples. The expression of staphylococcal nuclease A in E. coli resulted in intracellular accumulation of the protein and degradation of the host DNA into fragments shorter than 100 bp. Two expression systems for the nuclease are presented and were combined with the protein E-mediated lysis system. Under optimized conditions for the coexpression of gene E and the staphylococcal nuclease, the concentration of viable cells fell below the lower limit of detection, whereas the rates of ghost formation were not affected. With regard to the absence of reproductive cells from the ghost fractions, the reduction of viability could be determined as being at least 7 to 8 orders of magnitude. The lysis process was characterized by electrophoretic analysis and absolute quantification of the genetic material within the cells and the culture supernatant via real-time PCR. The ongoing degradation of the bacterial nucleic acids resulted in a continuous quantitative clearance of the genetic material associated with the lysing cells until the concentrations fell below the detection limits of either assay. No functional, released genetic units (genes) were detected within the supernatant during the lysis process, including nuclease expression.

Pasteurella multocida- and Pasteurella haemolytica-ghosts: new vaccine candidates

Pasteurella multocida is an important animal pathogen. Bacterial ghosts produced by the expression of phage PhiX174 lysis gene E are empty cells devoid of cytoplasmic and genomic material. Lysis of P. multocida 7A and P. haemolytica A1 carrying Pasteurella-specific lysis vectors (pSR2 and pSON2) occurred 140 min after induction of gene E expression induced by temperature upshift. The E-mediated cell lysis and killing activity was the same in both Pasteurella species and no viable cells could be detected after lysis of P. multocida and P. haemolytica. Pasteurella ghosts were used for immunization of rabbits and mice. Rabbits immunized subcutaneously with either P. multocida- or P. haemolytica-ghosts developed antibodies reacting with the immunizating strain, as well as with other Pasteurella strains. The number of proteins in whole cell protein extracts recognized by the sera constantly increased during the observation period of 51 days. In addition, dose-dependent protection against homologous challenge was observed in mice immunized with P. multocida-ghosts. Animals which received 1.15 x 10(8) ghosts and a challenge dose of up to 60 cfu (LD90), showed 100% protection. According to these results, we suggest ghosts of P. multocida and P. haemolytica as new vaccine candidates.

Construction of recombinant S-layer proteins (rSbsA) and their expression in bacterial ghosts--a delivery system for the nontypeable Haemophilus influenzae antigen Omp26

This study has investigated the feasibility of a combination of recombinant surface layer (S-layer) proteins and empty bacterial cell envelopes (ghosts) to deliver candidate antigens for a vaccine against nontypeable Haemophilus influenzae (NTHi) infections. The S-layer gene sbsA from Bacillus stearothermophilus PV72 was used for the construction of fusion proteins. Fusion of maltose binding protein (MBP) to the N-terminus of SbsA allowed expression of the S-layer in the periplasm of Escherichia coli. The outer membrane protein (Omp) 26 of NTHi was inserted into the N-terminal and C-terminal regions of SbsA. The presence of the fused antigen Omp26 was demonstrated by Western blot experiments using anti-Omp26 antisera. Electron microscopy showed that the recombinant SbsA maintained the ability to self-assemble into sheet-like and cylindrical structures. Recombinant E. coli cell envelopes (ghosts) were produced by the expression of SbsA/Omp26 fusion proteins prior to gene E-mediated lysis. Intraperitoneal immunization with these recombinant bacterial ghosts induced an Omp26-specific antibody response in BALB/c mice. These results demonstrate that the NTHi antigen, Omp26, was expressed in the S-layer self-assembly product and this construct was immunogenic for Omp26 when administered to mice in bacterial cell envelopes.

Online monitoring of Escherichia coli ghost production

Controlled expression of cloned phi X174 gene E in gram-negative bacteria results in lysis of the bacteria by the formation of a transmembrane tunnel structure built through the cell envelope complex. Production of bacterial ghosts is routinely monitored by classical microbiological procedures. These include determination of the turbidity of the culture and the total number of cells and the number of reproductive cells present during the time course of growth and lysis. Although conceptually simple, these methods are labor intensive and time consuming, providing a complete set of results after the determination of viable cell counts. To avoid culturing methods for bacterial growth, an alternative flow cytometric procedure is presented for the quantification of ghosts and polarized, as well as depolarized, nonlysed cells within a culture. For this method, which is based on the discriminatory power of the membrane potential-sensitive dye bis-(1,3-dibutylbarbituric acid) trimethine oxonol, a staining protocol was developed and optimized for the maximum discrepancy in fluorescence between bacterial ghosts and viable cells. The total quantitative analysis procedure takes less than 2 min. The results derived from classical or cytometric analyses correlate with respect to the total cell numbers and the viability of the culture.

Generation of Helicobacter pylori ghosts by PhiX protein E-mediated inactivation and their evaluation as vaccine candidates

Bacterial ghosts are empty cell envelopes, which may be generated by the controlled expression of the PhiX174 lysis gene E in gram-negative bacteria to obtain vaccine candidates. We describe here the application of this technology to Helicobacter pylori. The lysis gene cassette was cloned into an Escherichia coli-Helicobacter pylori shuttle vector and introduced into an H. pylori recipient strain by bacterial conjugation. Temperature induction of the lysis gene cassette revealed a quantitative killing of the H. pylori culture without induction of lysis-resistant bacteria. Biochemical and transmission electron microscopic studies identified structurally intact H. pylori. Prophylactic oral vaccination experiments using these H. pylori ghosts in the BALB/c mouse model showed a significant reduction of the bacterial load in the ghost group, as measured by a quantitative bacterial reisolation procedure. Ten of 10 and 5 of 10 mice were protected, respectively, without the use of a mucosal adjuvant. Coadministration of ghosts with cholera toxin as mucosal adjuvant resulted in a complete protection of 10 of 10 and 8 of 8 mice against H. pylori challenge, with three animals showing a sterile immunity.

Construction of an Actinobacillus pleuropneumoniae serotype 2 prototype live negative-marker vaccine

Deletions were introduced into the ureC and apxIIA genes of an Actinobacillus pleuropneumoniae serotype 2 strain by homologous recombination and counterselection. The double-mutant contains no foreign DNA, is highly attenuated, protects pigs from homologous challenge upon a single aerosol application, and facilitates the serological discrimination of immunized and infected herds.