rOmpF and OMVs as efficient subunit vaccines against Salmonella enterica serovar Enteritidis infections in poultry farms

Bacterial Extracellular Vesicles: Bridging Pathogen Biology and Therapeutic Innovation

The main role of bacterial extracellular vesicles (BEVs) has been associated with various processes such as intercellular communication and host-pathogen interactions. This comprehensive review explores the multifaceted functions of BEVs across different biological domains, emphasizing their multifaceted functions as contributors both to disease and as carriers of therapeutic possibilities. We examine the intricate interactions of BEVs within bacterial communities and between bacteria and hosts, their involvement in disease development through cargo delivery mechanisms, and their beneficial impact to microbial ecology. The review places a strong emphasis on BEVs' applications in biomedical sciences, where they are revolutionizing vaccine development, targeted drug delivery, and cancer therapy. By utilizing the inherent properties of BEVs for controlled drug release, targeted antigen delivery, and immune modulation, they offer a promising frontier in precision medicine. In addition, the diagnostic potential of BEVs is explored through their biomarker capabilities, providing valuable insights into disease states and treatment efficacy. Looking forward, this review underscores the challenges and opportunities in translating BEV research to clinical practice, promoting the use of standardized methods in BEV characterization and scaling up production. The diverse abilities of BEVs, ranging from contributing to pathogen virulence to driving therapeutic innovation, highlight their potential as a cornerstone in the future of biomedical advancements. STATEMENT OF SIGNIFICANCE: Bacterial extracellular vesicles (BEVs) are emerging as pivotal players in both pathogenesis and therapeutic innovation. This review explores their dual nature as agents of disease and as promising biomaterials in biomedical applications, and provides a comprehensive survey on their involvement in disease mechanisms and microbial ecology, and their potential in biomedical applications such as vaccine development, targeted drug delivery, cancer therapy and diagnosis. It highlights the complex interactions of BEVs within bacterial communities and between bacteria and hosts. This review also addresses current advancements, challenges, and opportunities in translating BEV research into clinical practice. The insights presented here position BEVs as a cornerstone in the future of biomedical advancements, advocating for standardized methods in BEV characterization and scalable production techniques.

Immune responses of chickens against recombinant Salmonella enterica serotype Heidelberg FimA and FimW fimbriae and FliD and FlgK flagellar proteins

Implementation of a vaccination program is one of the most effective means to control infectious diseases during food animal production. Salmonella, a Gram-negative bacterium, is a leading bacterial cause of human foodborne illnesses worldwide. The major source of this microorganism for human infection is from consumption of unsanitary poultry products. Although live attenuated vaccines are available, these vaccines suffer from problems including persistence and shedding of Salmonella in and from the vaccinated animals. To overcome these problems, the recombinant Salmonella enterica serotype Heidelberg FliD, FlgK, FimA and FimW subunit proteins that are surface-exposed were produced and tested for their immunogenicity in chickens in this study. As expected, there were no detrimental signs observed in chickens after vaccination during the six-week experimental period. These four proteins migrated in a single band to their respective positions. Analysis of immune responses to the proteins reveals that the immunoglobulin (Ig) G, IgM and IgA from most vaccinated chickens reacted strongly to the recombinant FliD and FlgK proteins, but not from unvaccinated chickens. On the other hand, IgG, IgM and IgA antibody responses to FimA and FimW from the vaccinated group were no difference from those from unvaccinated chickens, suggesting that the FimA and FimW proteins may be not good antigens, potentially due to their size, composition, and/or structural complexity. In addition, IgG could be induced by FliD and FlgK after a single vaccination. These antibody studies suggest that recombinant FliD and FlgK have potential as targets for vaccine development. Because of the importance of bacterial fimbriae in pathogenesis and for immunogenicity, a chimeric protein of the FimA and FimW proteins is needed.

Combatting Salmonella: a focus on antimicrobial resistance and the need for effective vaccination

BACKGROUND

Salmonella infections represent a major global public health concern due to their widespread zoonotic transmission, antimicrobial resistance, and associated morbidity and mortality. This review aimed to summarize the zoonotic nature of Salmonella, the challenges posed by antimicrobial resistance, the global burden of infections, and the need for effective vaccination strategies to mitigate the rising threat of Salmonella.

METHODS

A systematic review of literature was conducted using databases such as PubMed, Scopus, Web of Science, and Google Scholar. Relevant studies published in English were identified using keywords including Salmonella, vaccination, antimicrobial resistance, and public health. Articles focusing on epidemiology, vaccine development, and strategies to control Salmonella infections were included, while conference abstracts and non-peer-reviewed studies were excluded.

RESULTS

Salmonella infections result in approximately 95 million global cases annually, with an estimated 150,000 deaths. Regional variations were evident, with higher infection rates in low- and middle-income countries due to poor sanitation and food safety standards. Salmonella Enteritidis and S. Typhimurium were the most prevalent serovars associated with human infections. The review highlighted an alarming rise in multidrug-resistant (MDR) Salmonella strains, particularly due to the overuse of antibiotics in humans and livestock. Despite progress in vaccine development, challenges remain in achieving a universal vaccine that targets diverse Salmonella serovars. Live-attenuated, killed, recombinant, subunit, and conjugate vaccines are currently under development, but limitations such as efficacy, cost, and accessibility persist.

CONCLUSIONS

Salmonella infections continue to impose a significant burden on global health, exacerbated by rising antimicrobial resistance. There is an urgent need for a multifaceted approach, including improved sanitation, prudent antibiotic use, and the development of affordable, broad-spectrum vaccines. Strengthening surveillance systems and promoting collaborative global efforts are essential to effectively control and reduce the burden of Salmonella.

Pre-Harvest Non-Typhoidal Salmonella Control Strategies in Commercial Layer Chickens

Non-typhoidal Salmonella (NTS) infections in poultry, particularly in commercial-layer chickens, pose a critical risk to food safety and public health worldwide. NTS bacteria can remain undetected in poultry flocks, contaminating products and potentially leading to gastroenteritis in humans. This review examines pre-harvest control strategies for NTS in layer chickens, including biosecurity protocols, vaccinations, feed additives, genetic selection, and environmental management. These strategies have substantially reduced Salmonella colonization and product contamination rates in the commercial layer industry. By evaluating these strategies, this review highlights the importance of integrated control measures to limit NTS colonization, reduce antimicrobial resistance, and improve poultry health. This review aims to provide producers, researchers, and policymakers with insights into effective practices to minimize Salmonella contamination and enhance both animal and human health outcomes.

Immune Responses Elicited by Outer Membrane Vesicles of Gram-Negative Bacteria: Important Players in Vaccine Development

The attractiveness of OMVs derived from Gram-negative bacteria lies in the fact that they have two biomembranes sandwiching a peptidoglycan layer. It is well known that the envelope of OMVs consists of the outer bacterial membrane [OM] and not of the inner one [IM] of the source bacterium. This implies that all outer membranous molecules found in the OM act as antigens. However, under specific conditions, some of the inner membrane proteins can be exported into the outer membrane layer and perform as antigens. A key information was that the used purification procedures for OMVs, the induction methods to increase the production of OMVs as well as the specific mutant strains obtained via genetic engineering affect the composition of potential antigens on the surface and in the lumen of the OMVs. The available literature allowed us to list the major antigens that could be defined on OMVs. The functions of the antigens within the source bacterium are discussed for a better understanding of the various available hypotheses on the biogenesis of vesicle formation. Also, the impacts of OMV antigens on the immune system using animal models are assessed. Furthermore, information on the pathways of OMVs entering the host cell is presented. An example of a bacterial infection that causes epidemic diseases, namely via Neisseria meningitidis, is used to demonstrate that OMVs derived from this pathogen elicit protective immune responses when administered as a vaccine. Furthermore, information on OMV vaccines under development is presented. The assembled knowledge allowed us to formulate a number of reasons why OMVs are attractive as vaccine platforms, as their undesirable side effects remain small, and to provide an outlook on the potential use of OMVs as a vaccine platform.

Progress in the application of Salmonella vaccines in poultry: A mini review

Salmonella is a critical group of zoonotic pathogens that are widely spread in poultry, causing avian salmonellosis. This disease usually leads to significant reductions in poultry performance, including reduced egg production in laying hens, decreased hatchability in chicks, and retarded growth in broilers. As a result, worldwide poultry industry suffers serious economic losses. Vaccination serves as an essential strategy for preventing Salmonella infection in poultry, effectively reducing susceptibility and alleviating disease symptoms, while also minimizing fecal shedding and environmental contamination. This subsequently diminishes public health risks. Various Salmonella vaccines can induce humoral and cellular immune responses to different extents. Therefore, a thorough understanding of the immune defense mechanisms, especially adaptive immune responses in poultry infected with Salmonella, is crucial for the development of Salmonella vaccines. This review summarizes the progress in the application of Salmonella vaccines in poultry, including adaptive immune responses induced by Salmonella and vaccines targeting the predominant circulating serotypes in poultry. It also provides an insight into the future of poultry-origin Salmonella vaccines.

A review on the development of bacterial multi-epitope recombinant protein vaccines via reverse vaccinology

Bacterial vaccines play a crucial role in combating bacterial infectious diseases. Apart from the prevention of disease, bacterial vaccines also help to reduce the mortality rates in infected populations. Advancements in vaccine development technologies have addressed the constraints of traditional vaccine design, providing novel approaches for the development of next-generation vaccines. Advancements in reverse vaccinology, bioinformatics, and comparative proteomics have opened horizons in vaccine development. Specifically, the use of protein structural data in crafting multi-epitope vaccines (MEVs) to target pathogens has become an important research focus in vaccinology. In this review, we focused on describing the methodologies and tools for epitope vaccine development, along with recent progress in this field. Moreover, this article also discusses the challenges in epitope vaccine development, providing insights for the future development of bacterial multi-epitope genetically engineered vaccines.

Exploiting membrane vesicles derived from avian pathogenic Escherichia coli as a cross-protective subunit vaccine candidate against avian colibacillosis

Avian pathogenic Escherichia coli (APEC) is a notable pathogen that frequently leads to avian colibacillosis, posing a substantial risk to both the poultry industry and public health. The commercial vaccines against avian colibacillosis are primarily inactivated vaccines, but their effectiveness is limited to specific serotypes. Recent advances have highlighted bacterial membrane vesicles (MV) as a promising candidate in vaccine research. How to produce bacterial MVs vaccines on a large scale is a significant challenge for the industrialization of MVs. The msbB gene encodes an acyltransferase and has been implicated in altering the acylation pattern of lipid A, leading to a decrease in lipid A content in lipopolysaccharides (LPS). Here, we evaluated the immunoprotective efficacy of MVs derived from the LPS low-expressed APEC strain FY26ΔmsbB, which was an APEC mutant strain with a deletion of the msbB gene. The nitrogen cavitation technique was employed to extract APEC MVs, with results indicating a significant increase in MVs yield compared to that obtained under natural culture. The immunization effectiveness was assessed, revealing that FY26ΔmsbB MVs elicited an antibody response of laying hens and facilitated bacterial clearance. Protective efficacy studies demonstrated that immunization with FY26ΔmsbB MVs conferred the immune protection in chickens challenged with the wild-type APEC strain FY26. Notably, LPS low-carried MVs recovered from the mutant FY26ΔmsbB also displayed cross-protective capabilities, and effectively safeguarding against infections caused by O1, O7, O45, O78, and O101 serotypes virulent APEC strains. These findings suggest that MVs generated from the LPS low-expressed APEC strain FY26ΔmsbB represent a novel and empirically validated subunit vaccine for the prevention and control of infections by various APEC serotypes.

Combatting extensively drug-resistant Salmonella: a global perspective on outbreaks, impacts, and control strategies

Antibiotic resistance in typhoid fever poses a critical public health problem due to the emergence of extensively drug-resistant (XDR) Salmonella, resulting in prolonged illness and treatment failure. Salmonella enterica serovar Typhi is the most predominant among all serotypes and can acquire resistance. The emergence of XDR Salmonella in various regions globally, particularly Pakistan, presents a concerning trend. However, limited data availability impedes a comprehensive understanding of the outbreaks and hinders the development of real-time solutions. Here, we have provided an updated overview of the current outbreaks of XDR Salmonella in epidemic and endemic regions. Treatments of XDR Salmonella infections are challenging, as there are records of treatment failure in humans and animals. However, intensive prevention techniques can be implemented pending the advent of novel antibiotics. Emphasis on antimicrobial stewardship and frequent surveillance of the pathogen should be made to keep track of potential outbreaks in both human and animal populations. Although progress is being made to combat XDR Salmonella within some regions, a unified and efficient effort on an international scale is required to curtail the XDR outbreak before it escalates and leads us back to the pre-antibiotic era.

Salmonella spp. in poultry production-A review of the role of interventions along the production continuum

Salmonella is a significant pathogen of human and animal health and poultry are one of the most common sources linked with foodborne illness worldwide. Global production of poultry meat and products has increased significantly over the last decade or more as a result of consumer demand and the changing demographics of the world's population, where poultry meat forms a greater part of the diet. In addition, the relatively fast growth rate of birds which is significantly higher than other meat species also plays a role in how poultry production has intensified. In an effort to meet the greater demand for poultry meat and products, modern poultry production and processing practices have changed and practices to target control and reduction of foodborne pathogens such as Salmonella have been implemented. These strategies are implemented along the continuum from parent and grandparent flocks to breeders, the farm and finished broilers to transport and processing and finally from retail to the consumer. This review focuses on common practices, interventions and strategies that have potential impact for the control of Salmonella along the poultry production continuum from farm to plate.

Prevention and Control of Human Salmonella enterica Infections: An Implication in Food Safety

Salmonella is a foodborne zoonotic pathogen causing diarrhoeal disease to humans after consuming contaminated water, animal, and plant products. The bacterium is the third leading cause of human death among diarrhoeal diseases worldwide. Therefore, human salmonellosis is of public health concern demanding integrated interventions against the causative agent, Salmonella enterica. The prevention of salmonellosis in humans is intricate due to several factors, including an immune-stable individual infected with S. enterica continuing to shed live bacteria without showing any clinical signs. Similarly, the asymptomatic Salmonella animals are the source of salmonellosis in humans after consuming contaminated food products. Furthermore, the contaminated products of plant and animal origin are a menace in food industries due to Salmonella biofilms, which enhance colonization, persistence, and survival of bacteria on equipment. The contaminated food products resulting from bacteria on equipment offset the economic competition of food industries and partner institutions in international business. The most worldwide prevalent broad-range Salmonella serovars affecting humans are Salmonella Typhimurium and Salmonella Enteritidis, and poultry products, among others, are the primary source of infection. The broader range of Salmonella serovars creates concern over multiple strategies for preventing and controlling Salmonella contamination in foods to enhance food safety for humans. Among the strategies for preventing and controlling Salmonella spread in animal and plant products include biosecurity measures, isolation and quarantine, epidemiological surveillance, farming systems, herbs and spices, and vaccination. Other measures are the application of phages, probiotics, prebiotics, and nanoparticles reduced and capped with antimicrobial agents. Therefore, Salmonella-free products, such as beef, pork, poultry meat, eggs, milk, and plant foods, such as vegetables and fruits, will prevent humans from Salmonella infection. This review explains Salmonella infection in humans caused by consuming contaminated foods and the interventions against Salmonella contamination in foods to enhance food safety and quality for humans.

Oral administration of recombinant Bacillus subtilis expressing a multi-epitope protein induces strong immune responses against Salmonella Enteritidis

The S. Enteritidis causes serious economic losses to the poultry industry every year. Vaccines that induce a mucosal immune response may be successful against an S. Enteritidis infection because mucosa plays an important role in preventing S. Enteritidis from entering the body. In order to develop novel and potent oral vaccines based on Bacillus subtilis (B. subtilis) to control the spread of S. Enteritidis in the poultry industry, we constructed a B. subtilis that can secrete a multi-epitope protein (OmpC-FliC-SopF-SseB-IL-18). Oral immunization of chickens was performed, and serum antibodies, mucosal antibodies, specific cellular immunity and serum cytokines were detected. Immunizing chicks with S. Enteritidis was evaluated. The results showed high levels of specific IgG in addition to high levels of specific secretory immunoglobulin A (sIgA) in chickens who received oral administrations of recombinant B. subtilis. Additionally, recombinant B. subtilis may significantly increase the levels of IL-2 and T cell-mediated immunity. Recombinant B. subtilis effectively protected chickens against S. Enteritidis and reduced pathological damage to the spleen and jejunum. Our study's outcomes indicate that the expression of the multi-epitope protein OmpC-FliC-SopF-SseB-IL-18 by B. subtilis could generate a mucosal vaccine candidate for animals to defend against S. Enteritidis in the future.

Bacterial membrane vesicles for vaccine applications

Vaccines have been highly successful in the management of many diseases. However, there are still numerous illnesses, both infectious and noncommunicable, for which there are no clinically approved vaccine formulations. While there are unique difficulties that must be overcome in the case of each specific disease, there are also a number of common challenges that have to be addressed for effective vaccine development. In recent years, bacterial membrane vesicles (BMVs) have received increased attention as a potent and versatile vaccine platform. BMVs are inherently immunostimulatory and are able to activate both innate and adaptive immune responses. Additionally, BMVs can be readily taken up and processed by immune cells due to their nanoscale size. Finally, BMVs can be modified in a variety of ways, including by genetic engineering, cargo loading, and nanoparticle coating, in order to create multifunctional platforms that can be leveraged against different diseases. Here, an overview of the interactions between BMVs and immune cells is provided, followed by discussion on the applications of BMV vaccine nanotechnology against bacterial infections, viral infections, and cancers.

The Outer Membrane Proteins and Their Synergy Triggered the Protective Effects against Pathogenic Escherichia coli

Colibacillosis caused by pathogenic Escherichia coli (E. coli) is one of the most serious infectious diseases, causing an extensive burden on animal husbandry and the human healthcare system. Vaccination is one of the ideal ways to prevent E. coli infection. In this work, recombinant outer membrane protein A (rOmpA), outer membrane protein C (rOmpC) and BamA (rBamA) from E. coli O78 (CVCC CAU0768) were expressed in a prokaryotic expression system with the concentration of 1–2 mg/mL after purification. Considerable immune responses could be triggered in mice that were immunized with these recombinant proteins, high antibody titers, high total IgG level and various antibody isotypes were detected in antisera after booster immunizations. Moreover, mice immunized with several recombinant proteins in combination showed a higher survival rate with the challenge of homologous strain E. coli O78 and a more significant cross-protection effect against heterologous strain E. coli O157:H7 (CICC 21530) in vivo than those of immunized alone. The antisera from immunized mice showed high affinity to multiple strains of Escherichia, Shigella and Salmonella in vitro, indicating that recombinant outer membrane proteins from E. coli O78 had the potential to be developed into universal antigenic substances against not only E. coli but also a variety of Gram-negative bacteria. rOmpA was considered as the most immunogenic protein in this work and the combination of different proteins could further enhance the immune response of immunized mice, which provided the reference for the construction of novel antigens with higher efficiency.

Salmonella Enteritidis Subunit Vaccine Candidate Based on SseB Protein Co-Delivered with Simvastatin as Adjuvant

Salmonella enterica serovar Enteritidis (S. Enteritidis) is an important zoonotic pathogen that can lead to diarrhea and systemic infections in humans and mortality in animals. This is a major public health issue worldwide. Safe and effective vaccines are urgently needed to control and prevent Salmonella infection. Subunit vaccines are safe and provide targeted protection against Salmonella spp. Here, we developed and evaluated an S. Enteritidis subunit vaccine candidate, the rHis-SseB adjuvant with simvastatin. We amplified the SseB gene from S. Enteritidis C50041 genomic DNA and expressed the recombinant proteins rHis-SseB and rGST-SseB using the Escherichia coli system. Western blotting confirmed the immunoreactivity of recombinant proteins rHis-SseB and rGST-SseB with antisera against Salmonella Enteritidis C50041. In a mouse model of intramuscular vaccination, co-immunization with rHis-SseB and simvastatin significantly enhanced both the SseB-specific antibody titer in serum (humoral immune response) and splenic lymphocyte proliferation (cellular immune response). Co-immunization with rHis-SseB and simvastatin provided 60% protection against subsequent challenge with the S. Enteritidis C50041 strain and decreased bacterial colonization in the liver and spleen. These findings provide a basis for the development of an S. Enteritidis subunit vaccine.

The Outer Membrane Vesicles of Salmonella enterica Serovar Typhimurium Activate Chicken Immune Cells through Lipopolysaccharides and Membrane Proteins

Salmonella is a common pathogen which can secrete outer membrane vesicles (OMVs). However, the effect of OMVs from Salmonella enterica Serovar Typhimurium (S. Typhimurium) of poultry origin on cells of the chicken innate immune system is not well known. In this study, S. Typhimurium OMVs were first isolated from three different poultry strains of Salmonella, Salmonella CVCC542, SALA, and SALB. In order to investigate the effect of OMVs on the maturation of monocytes into macrophages, both bone marrow-derived (BMD) monocytes and macrophage cell line HD11 cells were used. OMVs promoted the formation of monocyte dendrites in both types of cells, enabled BMD cells to become larger, and stimulated expression of LPS-induced TNF-αfactor (LITAF), IL-6, and inducible nitric oxide synthase (iNOS) genes in HD11 cells. These results demonstrated the capability of OMVs to promote the development of chicken monocytes into macrophages and the maturation of macrophages. In order to study the effect of OMVs on the phagocytosis of macrophages, chicken spleen-derived monocytes and HD11 cells were used. Phagocytosis of FITC-Salmonella and FITC-dextran by these two types of cells was enhanced after stimulation with OMVs. To determine which components in OMVs were responsible for the above observed results, OMVs were treated with proteinase K(PK) or polymyxin B (PMB). Both treatments reduced the phagocytosis of FITC-Salmonella by HD11 cells and chicken spleen mononuclear cells and reduced the secretion of IL-1β, LITAF, and IL-6 cytokines. These results demonstrated that Salmonella OMVs activated chicken macrophages and spleen mononuclear cells and the activation was achieved mainly through lipopolysaccharides and membrane proteins.

Efficacy of Multivalent, Cochleate-Based Vaccine against Salmonella Infantis, S. Enteritidis and S. Typhimurium in Laying Hens

Salmonella enterica is an important foodborne pathogen. Commercial poultry are the main reservoirs of Salmonella enterica, leading to the contamination of food and outbreaks in humans. The vaccination of chickens is one of the most important strategies to reduce the number of Salmonella in poultry farms. Unfortunately, commercial vaccines have not been fully effective in controlling the spread and do not contain all the Salmonella serovars that circulate on farms. In this study, we evaluate a new, cochleate-based, trivalent injectable vaccine against S. Enteritidis, S. Typhimurium and S. Infantis, describing the vaccine security, capacity to induce specific anti-Salmonella serovar IgY and the gene expression of immune markers related to CD4 and CD8 T-cell-mediated immunity. Efficacy was evaluated through oral challenges performed separately for each Salmonella serotype. The efficacy and safety of the trivalent vaccine was proven under controlled conditions. The vaccine has no local or systemic reactions or adverse effects on poultry performance related to the vaccine. The vaccine provided significantly increased serum IgY titer levels, significantly reduced Salmonella CFU/g present in the cecum and an increased CD4+/CD8+ ratio in vaccinated animals when challenged with S. Infantis, S. Enteritidis and S. Typhimurium. These results indicate that this new trivalent vaccine does not generate adverse effects in poultry and produces an increase in neutralizing antibodies against the three Salmonella serovars.

Non-Antibiotics Strategies to Control Salmonella Infection in Poultry

Salmonella spp. is a facultative intracellular pathogen causing localized or systemic infections, involving economic and public health significance, and remains the leading pathogen of food safety concern worldwide, with poultry being the primary transmission vector. Antibiotics have been the main strategy for Salmonella control for many years, which has allowed producers to improve the growth and health of food-producing animals. However, the utilization of antibiotics has been reconsidered since bacterial pathogens have established and shared a variety of antibiotic resistance mechanisms that can quickly increase within microbial communities. The use of alternatives to antibiotics has been recommended and successfully applied in many countries, leading to the core aim of this review, focused on (1) describing the importance of Salmonella infection in poultry and the effects associated with the use of antibiotics for disease control; (2) discussing the use of feeding-based (prebiotics, probiotics, bacterial subproducts, phytobiotics) and non-feeding-based (bacteriophages, in ovo injection, vaccines) strategies in poultry production for Salmonella control; and (3) exploring the use of complementary strategies, highlighting those based on -omics tools, to assess the effects of using the available antibiotic-free alternatives and their role in lowering dependency on the existing antimicrobial substances to manage bacterial infections in poultry effectively.

Immunization of chickens with Salmonella gallinarium ghosts expressing Salmonella Enteritidis NFliC-FimAC and CD40LC fusion antigen enhances cell-mediated immune responses and protects against wild-type challenges with both species

This study describes the construction and immunological characterization of a novel Salmonella gallinarium ghost vaccine to protect against S. gallinarium (SG) and S. Enteritidis (SE) serotypes. The SG ghost was designed to express N-terminus FliC (D0-D1 domain) and FimA retrieved from the SE genome, and the receptor-binding domain (RBD) of CD40L from the chicken as a single fusion construct. The construct was built in pJHL184, a phage lysis gene E-mediated ghost plasmid and the expression was confirmed by western blot resulting in an 85-kDa band. Chicken immunization was conducted by intramuscular route with SG ghost FliC-FimA-CD40L, vector control, or PBS alone in a prime-boost schedule. Antibody responses, cell-mediated immune responses (CMI), and cytokine induction was assessed in chicken demonstrating significantly high levels of IgY, CMI, cytokine responses in ghost immunized group delivering partial protection against SG wild type challenge and near complete protection against SE challenge wild type challenge.

Secretory System Components as Potential Prophylactic Targets for Bacterial Pathogens

Bacterial secretory systems are essential for virulence in human pathogens. The systems have become a target of alternative antibacterial strategies based on small molecules and antibodies. Strategies to use components of the systems to design prophylactics have been less publicized despite vaccines being the preferred solution to dealing with bacterial infections. In the current review, strategies to design vaccines against selected pathogens are presented and connected to the biology of the system. The examples are given for Y. pestis, S. enterica, B. anthracis, S. flexneri, and other human pathogens, and discussed in terms of effectiveness and long-term protection.

Outer membrane vesicles as versatile tools for therapeutic approaches

Budding of the bacterial surface results in the formation and secretion of outer membrane vesicles, which is a conserved phenomenon observed in Gram-negative bacteria. Recent studies highlight that these sphere-shaped facsimiles of the donor bacterium's surface with enclosed periplasmic content may serve multiple purposes for their host bacterium. These include inter- and intraspecies cell-cell communication, effector delivery to target cells and bacterial adaptation strategies. This review provides a concise overview of potential medical applications to exploit outer membrane vesicles for therapeutic approaches. Due to the fact that outer membrane vesicles resemble the surface of their donor cells, they represent interesting nonliving candidates for vaccine development. Furthermore, bacterial donor species can be genetically engineered to display various proteins and glycans of interest on the outer membrane vesicle surface or in their lumen. Outer membrane vesicles also possess valuable bioreactor features as they have the natural capacity to protect, stabilize and enhance the activity of luminal enzymes. Along these features, outer membrane vesicles not only might be suitable for biotechnological applications but may also enable cell-specific delivery of designed therapeutics as they are efficiently internalized by nonprofessional phagocytes. Finally, outer membrane vesicles are potent modulators of our immune system with pro- and anti-inflammatory properties. A deeper understanding of immunoregulatory effects provoked by different outer membrane vesicles is the basis for their possible future applications ranging from inflammation and immune response modulation to anticancer therapy.

Outer Membrane Vesicle Induction and Isolation for Vaccine Development

Gram-negative bacteria release vesicular structures from their outer membrane, so called outer membrane vesicles (OMVs). OMVs have a variety of functions such as waste disposal, communication, and antigen or toxin delivery. These vesicles are the promising structures for vaccine development since OMVs carry many surface antigens that are identical to the bacterial surface. However, isolation is often difficult and results in low yields. Several methods to enhance OMV yield exist, but these do affect the resulting OMVs. In this review, our current knowledge about OMVs will be presented. Different methods to induce OMVs will be reviewed and their advantages and disadvantages will be discussed. The effects of the induction and isolation methods used in several immunological studies on OMVs will be compared. Finally, the challenges for OMV-based vaccine development will be examined and one example of a successful OMV-based vaccine will be presented.