Salmonella: immune responses and vaccines

Genetic engineering of Salmonella spp. for novel vaccine strategies and therapeutics

Salmonella enterica is a diverse species that infects both humans and animals. S. enterica subspecies enterica consists of more than 1,500 serovars. Unlike typhoidal Salmonella serovars which are human host-restricted, non-typhoidal Salmonella (NTS) serovars are associated with foodborne illnesses worldwide and are transmitted via the food chain. Additionally, NTS serovars can cause disease in livestock animals causing significant economic losses. Salmonella is a well-studied model organism that is easy to manipulate and evaluate in animal models of infection. Advances in genetic engineering approaches in recent years have led to the development of Salmonella vaccines for both humans and animals. In this review, we focus on current progress of recombinant live-attenuated Salmonella vaccines, their use as a source of antigens for parenteral vaccines, their use as live-vector vaccines to deliver foreign antigens, and their use as therapeutic cancer vaccines in humans. We also describe development of live-attenuated Salmonella vaccines and live-vector vaccines for use in animals.

Assessment of environmental safety and protective efficacy of O-antigen deficient DIVA capable Salmonella Enteritidis against chicken salmonellosis

In this study, we incorporated deletion of the O-antigen ligase gene to an attenuated Salmonella Enteritidis (SE) strain, JOL919 (SE PS; Δlon ΔcpxR), using the Lambda-Red recombination method and evaluated the safety and immunological aspects of the novel genotype, JOL2381 (SE VS: Δlon, ΔcpxR, ΔrfaL). Assessment of fecal shedding and organ persistence following administration via oral and IM routes revealed that the SE VS was safer than its parent strain, SE PS. Immunological assays confirmed that immunization via the oral route with SE PS was superior to the SE VS. However, chickens immunized with SE PS and SE VS strains via the IM route showed higher humoral and cell-mediated immune responses. Compared to PBS control, the IM route of immunization with SE VS resulted in a higher IgY antibody titer and expansion of CD4+ and CD8+ T-cell populations, which resulted in the clearance of Salmonella from the liver and splenic tissues. Furthermore, deletion of the O-antigen ligase gene caused lower production of LPS-specific antibodies in the host, promoting DIVA functionality and making it a plausible candidate for field utilization. Due to significant protection, high attenuation, and environmental safety concerns, the present SE VS strain is an ideal choice to prevent chicken salmonellosis and ensure public health.

Evaluation of Three Candidate Live-Attenuated Salmonella enterica Serovar Typhimurium Vaccines to Prevent Non-Typhoidal Salmonella Infection in an Infant Mouse Model

Nontyphoidal Salmonella enterica (NTS) is a leading cause of foodborne illness worldwide, including in the United States, where infants show the highest incidence amongst all age groups. S. enterica serovar Typhimurium is one of the most frequently isolated serovars from NTS infections. We have developed several candidate live-attenuated S. Typhimurium vaccines to prevent NTS infection. The goal of the current study was to assess three live S. Typhimurium vaccine strains (CVD 1921, CVD 1921 ∆htrA and CVD 1926, which have two, three and four gene deletions, respectively) with various levels of reactogenicity and immunogenicity in infant BALB/c mice to predict how they would perform following peroral immunization of infants. We first tested intranasal immunization of 14-day-old mice with three doses delivered at 1-week intervals and evaluated antibody responses and protection against lethal infection with wild-type S. Typhimurium. The vaccines were administered to 14-day-old mice via the peroral route at 1- or 2-week intervals and to 28-day-old mice at 2-week intervals. The three vaccine strains were immunogenic following intranasal immunization of infant mice with vaccine efficacies of 80% (CVD 1921), 63% (CVD 1921 ∆htrA) and 31% (CVD 1926). In contrast, peroral immunization of 14-day-old mice yielded much poorer protection against lethal infection and only immunization of 28-day-old mice at 2-week intervals showed similar protective capacity as intranasal administration (CVD 1921: 83%, CVD 1921 ∆htrA: 43% and CVD 1926: 58%). CVD 1921 was consistently more protective than both CVD 1921 ∆htrA and CVD 1926, regardless of the route of vaccination, immunization schedule and age of mice. Anti-LPS serum IgG responses were similar between the three strains and did not correlate with protection. Due to previously observed reactogenicity of CVD 1921, CVD 1921 ∆htrA and CVD 1926 are our preferred vaccines, but these data show that further improvements would need to be made to achieve suitable protection in young infants when using peroral immunization.

The impact of lipid A modification on biofilm and related pathophysiological phenotypes, endotoxicity, immunogenicity, and protection of Salmonella Typhimurium

This study presents the engineering of a less endotoxic Salmonella Typhimurium strain by manipulating the lipid-A structure of the lipopolysaccharide (LPS) component. Salmonella lipid A was dephosphorylated by using lpxE from Francisella tularensis. The 1-phosphate group from lipid-A was removed selectively, resulting in a close analog of monophosphoryl lipid A. We observed a significant impact of ∆pagL on major virulence factors such as biofilm formation, motility, persistency, and immune evasion. In correlation with biofilm and motility retardation, adhesion and invasion were elevated but with reduced intracellular survival, a favorable phenotype prospect of a vaccine strain. Western blotting and silver staining confirmed the absence of the O-antigen and truncated lipid-A core in the detoxified Salmonella mutant. In vitro and in vivo studies demonstrated that the dephosphorylated Salmonella mutant mediated lower pro-inflammatory cytokine secretion than the wild-type strain. The vaccine strains were present in the spleen and liver for five days and were cleared from the organs by day seven. However, the wild-type strain persisted in the spleen, liver, and brain, leading to sepsis-induced death. Histological evaluations of tissue samples further confirmed the reduced endotoxic activity of the detoxified Salmonella mutant. The detoxification strategy did not compromise the level of protective immunity, as the vaccine strain could enhance humoral and cellular immune responses and protect against the wild-type challenge in immunized mice.

Large Mammals Have More Powerful Antibacterial Defenses Than Expected from Their Metabolic Rates

AbstractTerrestrial mammals span seven orders of magnitude in body size, ranging from the <2-g Etruscan pygmy shrew (Suncus etruscus) to the >3,900-kg African elephant (Loxodonta africana). Although body size profoundly affects the behavior, physiology, ecology, and evolution of species, how investment in functional immune defenses changes with body size across species is unknown. Here, we (1) developed a novel 12-point dilution curve approach to describe and compare antibacterial capacity against three bacterial species among >160 terrestrial species of mammals and (2) tested published predictions about the scaling of immune defenses. Our study focused on the safety factor hypothesis, which predicts that broad, early-acting immune defenses should scale hypermetrically with body mass. However, our three statistical approaches demonstrated that antibacterial activity in sera across mammals exhibits isometry; killing capacity did not change with body size across species. Intriguingly, this result indicates that the serum of a large mammal is less hospitable to bacteria than would be predicted by its metabolic rates. In other words, if metabolic rates underlie the rates of physiological reactions as postulated by the metabolic theory of ecology, large species should have disproportionately lower antibacterial capacity than small species, but they do not. These results have direct implications for effectively modeling the evolution of immune defenses and identifying potential reservoir hosts of pathogens.

The recombinant swinepox virus expressing sseB could provide piglets with strong protection against Salmonella typhimurium challenge

Salmonella spp. poses a great threat to the livestock, food safety and public health. A recombinant swinepox virus expressing a protective antigen sseB was constructed by homologous recombination to develop a vaccine against Salmonella infection. The rSPV-sseB was verified using PCR, Western blot and indirect immunofluorescence assay. The immune responses and protective efficacy of rSPV-sseB were assessed in piglets. Forty piglets were immunized with rSPV-sseB, inactive Salmonella vaccine, wild-type SPV (wtSPV), or PBS. The results showed that the level of the sseB-specific antibody of the rSPV-sseB-vaccinated piglets was significantly higher at all time points post-vaccination than those of the inactivated Salmonella vaccine (P < 0.05), wtSPV (P < 0.001) or mock treated piglets (P < 0.001). The IL-4 and IFN-γ in the rSPV-sseB group were significantly higher than the other three groups at all post-infection time points. rSPV-sseB provided piglets with strong protection against the challenge of S. typhimurium with lethal dose. These results suggest the possibility of using recombinant swinepox virus rSPV-sseB as a promising vaccine to prevent Salmonella infection.

Effect of Antibiotics on the Colonization of Live Attenuated Salmonella Enteritidis Vaccine in Chickens

Salmonellosis, caused by Salmonella Enteritidis, is a prevalent zoonosis that has serious consequences for human health and the development of the poultry sector. The Salmonella Enteritis live vaccine (Sm24/Rif12/Ssq strain) is used to prevent Salmonella Enteritidis around the world. However, in some parts of the world, poultry flocks are frequently raised under intensive conditions, with significant amounts of antimicrobials to prevent and treat disease and to promote growth. To investigate whether antibiotic use influences the colonization of orally administered Salmonella live vaccines, 240 1-day-old specific pathogen-free chicks were randomly divided into 24 groups of 10 animals for this study. The different groups were treated with different antibiotics, which included ceftiofur, amoxicillin, enrofloxacin, and lincomycin–spectinomycin. Each group was immunized 2, 3, 4, and 5 days after withdrawal, respectively. At 5 days after immunization, the blood, liver, and ceca with contents were collected for the isolation of the Salmonella live vaccine strain. The result showed that no Salmonella vaccine strain was isolated in the blood and liver of the chicks in those groups. The highest number of Salmonella vaccine strains was isolated in the cecum from chicks vaccinated 2 days after ceftiofur withdrawal, and no Salmonella vaccine strain was isolated from the cecum in chicks immunized 3 days after ceftiofur withdrawal. Among the chickens immunized 4 days after the withdrawal of amoxicillin, enrofloxacin, and lincomycin–spectinomycin, the number of Salmonella vaccine colonization in the cecum was the highest, which was higher than that of the chickens immunized at other withdrawal interval (2, 3, and 5 days) groups and was higher than that of the chickens without treatment (P < 0.05). This study provides a reference for the effective use of the Salmonella Enteritidis live vaccine and key antibiotics commonly utilized in the poultry industry.

Safety and protective efficacy of Salmonella Pullorum spiC and rfaH deletion rough mutant as a live attenuated DIVA vaccine candidate

Salmonella enterica serovar Pullorum (S. Pullorum) causes pullorum disease (PD), which is an acute systemic disease, in chickens, and leads to serious economic losses in many developing countries because of its high morbidity and mortality rate in young chicks. The live-attenuated vaccine is considered to be an effective measure to control the Salmonella infection. In addition, the DIVA (differentiation of infected and vaccinated animals) feature without the interference of serological monitoring of Salmonella infection is an important consideration in the development of the Salmonella vaccine. In this study, we evaluated the immunogenicity and protective efficacy of a S. Pullorum rough mutant S06004ΔspiCΔrfaH as a live attenuated DIVA vaccine candidate in chickens. The S06004ΔspiCΔrfaH exhibited a significant rough lipopolysaccharides (LPS) phenotype which was agglutinated with the acriflavine, not with the O₉ mono antibody. Compared to the wild-type, 50% lethal dose (LD₅₀) of the rough mutant increased 100-fold confirmed its attenuation. The mutant strain also showed a decreased bacterial colonization in the spleen and liver. The immunization with the mutant strain had no effect on the body weight and no tissue lesions were observed in the liver and spleen. The high level of the S. Pullorum-specific IgG titers in the serum indicated that significant humoral immune responses were induced in the immunization group. The cellular immune responses were also elicited from the analysis of lymphocyte proliferation and expression of cytokines in the spleen. In addition, the S06004ΔspiCΔrfaH immunized group exhibited a negative response for the serological test, while the wild-type S06004 infection group was strongly positive for the serological test showing a DIVA capability. The survival rates in the vaccinated chickens were 87% after intramuscular challenge with wild-type S. Pullorum, while the survival rates were 20% in the control groups. Overall, these results have demonstrated that the rough mutant S06004ΔspiCΔrfaH strain can be developed as an efficient live attenuated DIVA vaccine candidate to control the systemic S. Pullorum infection without the interference of salmonellosis monitoring program in poultry.

Evaluation of Safety and Protective Efficacy of a waaJ and spiC Double Deletion Korean Epidemic Strain of Salmonella enterica Serovar Gallinarum

With an aim to develop a highly attenuated and strongly immunogenic distinguishable vaccine candidate, a waaJ (a gene involved in the synthesis of lipopolysaccharide) and spiC (a virulence gene) double deletion Korean epidemic strain of S. enterica ser. Gallinarum (SG005) was constructed. Our results showed that the growth and biochemical characteristics were not altered by this double deletion. The double deletion strain contained dual markers. One was a bacteriological marker (rough phenotype) and the other was a serological marker helping distinguish infected chickens from vaccinated chickens. The double deletion strain showed good genetic stability and reduced resistance to environmental stresses in vitro; furthermore, it was extremely safe and highly avirulent in broilers. Single intramuscular or oral immunization of 7-day-old broilers with the double deletion strain could stimulate the body to produce antibody levels similar to the conventional vaccine strain SG9R. In addition, against a lethal wild-type challenge, it conferred effective protection that was comparable to that seen in the group vaccinated with SG9R. In conclusion, this double deletion strain may be an effective vaccine candidate for controlling S. enterica ser. Gallinarum infection in broilers.

Salmonella Pullorum lacking srfA is attenuated, immunogenic and protective in chickens

Sallmonella Pullorum is a host-restricted pathogen for poultry and causes severe economic importance in many developing countries. The development of novel vaccines for Salmonella Pullorum is necessary to eradicate the prevalence of the pathogen. In our study, a srfA deletion mutant (C79-13ΔsrfA) of Salmonella Pullorum was constructed, and then the biological characteristics and protective efficacy of the mutant were evaluated. The mutant C79-13ΔsrfA was much less virulent than its parental strain C79-13 in one-day-old HY-line white chickens, immunization with C79-13ΔsrfA (4 × 10⁷ CFU) through oral pathway induced highly specific humoral and cellular immune responses, the growth performance of vaccinated chickens was consistent with that of unvaccinated chickens. The survival percentages of vaccinated chickens reached 90% and 80%, after challenge with Salmonella Pullorum strain C79-13 and Salmonella Gallinarum strain SG9 at 10 days post-immunization (dpi), respectively. Collectively, our results indicate that C79-13ΔsrfA is a live attenuated vaccine candidate.

Comparative genome analysis of Salmonella enterica serovar Gallinarum biovars Pullorum and Gallinarum decodes strain specific genes

Salmonella enterica serovar Gallinarum biovar Pullorum (bvP) and biovar Gallinarum (bvG) are the etiological agents of pullorum disease (PD) and fowl typhoid (FT) respectively, which cause huge economic losses to poultry industry especially in developing countries including India. Vaccination and biosecurity measures are currently being employed to control and reduce the S. Gallinarum infections. High endemicity, poor implementation of hygiene and lack of effective vaccines pose challenges in prevention and control of disease in intensively maintained poultry flocks. Comparative genome analysis unravels similarities and dissimilarities thus facilitating identification of genomic features that aids in pathogenesis, niche adaptation and in tracing of evolutionary history. The present investigation was carried out to assess the genotypic differences amongst S.enterica serovar Gallinarum strains including Indian strain S. Gallinarum Sal40 VTCCBAA614. The comparative genome analysis revealed an open pan-genome consisting of 5091 coding sequence (CDS) with 3270 CDS belonging to core-genome, 1254 CDS to dispensable genome and strain specific genes i.e. singletons ranging from 3 to 102 amongst the analyzed strains. Moreover, the investigated strains exhibited diversity in genomic features such as virulence factors, genomic islands, prophage regions, toxin-antitoxin cassettes, and acquired antimicrobial resistance genes. Core genome identified in the study can give important leads in the direction of design of rapid and reliable diagnostics, and vaccine design for effective infection control as well as eradication. Additionally, the identified genetic differences among the S. enterica serovar Gallinarum strains could be used for bacterial typing, structure based inhibitor development by future experimental investigations on the data generated.

Assessment of microbiological correlates and immunostimulatory potential of electron beam inactivated metabolically active yet non culturable (MAyNC) Salmonella Typhimurium

This study investigates the microbiological and immunological basis underlying the efficacy of electron beam-inactivated immune modulators. The underlying hypothesis is that exposure to eBeam-based ionization reactions inactivate microorganisms without modifying their antigenic properties and thereby creating immune modulators. The immunological correlates of protection induced by such eBeam based Salmonella Typhimurium (EBST) immune modulators in dendritic cell (DC) (in vitro) and mice (in vivo) models were assessed. The EBST stimulated innate pro inflammatory response (TNFα) and maturation (MHC-II, CD40, CD80 and CD86) of DC. Immuno-stimulatory potential of EBST was on par with both a commercial Salmonella vaccine, and live Salmonella cells. The EBST cells did not multiply under permissive in vitro and in vivo conditions. However, EBST cells remained metabolically active. EBST immunized mice developed Salmonella-specific CD4+ T-cells that produced the Th1 cytokine IFNγ at a level similar to that induced by the live attenuated vaccine (AroA^- ST) formulation. The EBST retained stable immunogenic properties for several months at room temperature, 4°C, and -20°C as well as after lyophilization. Therefore, such eBeam-based immune modulators have potential as vaccine candidates since they offer the safety of a “killed” vaccine, while retaining the immunogenicity of an “attenuated” vaccine. The ability to store eBeam based immune modulators at room temperature without loss of potency is also noteworthy.

Advances in the development of Salmonella-based vaccine strategies for protection against Salmonellosis in humans

Salmonella spp. are important human pathogens globally causing millions of cases of typhoid fever and non‐typhoidal salmonellosis annually. There are only a few vaccines licensed for use in humans which all target Salmonella enterica serovar Typhi. Vaccine development is hampered by antigenic diversity between the thousands of serovars capable of causing infection in humans. However, a number of attenuated candidate vaccine strains are currently being developed. As facultative intracellular pathogens with multiple systems for transporting effector proteins to host cells, attenuated Salmonella strains can also serve as ideal tools for the delivery of foreign antigens to create multivalent live carrier vaccines for simultaneous immunization against several unrelated pathogens. Further, the ease with which Salmonella can be genetically modified and the extensive knowledge of the virulence mechanisms of this pathogen means that this bacterium has often served as a model organism to test new approaches. In this review we focus on (1) recent advances in live attenuated Salmonella vaccine development, (2) improvements in expression of foreign antigens in carrier vaccines and (3) adaptation of attenuated strains as sources of purified antigens and vesicles that can be used for subunit and conjugate vaccines or together with attenuated vaccine strains in heterologous prime‐boosting immunization strategies. These advances have led to the development of new vaccines against Salmonella which have or will soon be tested in clinical trials.

Salmonella Pullorum spiC mutant is a desirable LASV candidate with proper virulence, high immune protection and easy-to-use oral administration

Live attenuated Salmonellavaccine (LASV) is considered to be an effective contributory measure during the control of Salmonella infection. A Salmonella Pullorum spiC mutant was evaluated comprehensively as a LASV candidate (LASV-p) for broilers in terms of safety and immunogenicity. LASV-p was adminstered to 3-day broilers by intramuscular injection. The LD₅₀ increased 126 fold, and no tissue lesions were observed in the liver, spleen and cecum, in comparison with the control group inoculated with PBS and a passive group by wild-type Salmonella. Growth rates of all broilers were normal and not affected. LASV-p persisted in vivo until 21 days in liver, 28 days in spleen and 35 days in feces, and induced high levels of humoral IgG and mucosal IgA. Cellular immunity was also stimulated in the form of antigen-specific lymphocyte proliferation and higher counts of CD3⁺CD8⁺ T cells and increased expression of mRNA of Th1 cytokines, IFN-γ and IL-2, in the early stage, and Th2 cytokines, IL-4 and IL-10, in the later stages. LASV-p provided at least 90% immuneprotection against a wild-type Salmonella Pullorum and cross-protection in different degree against other Salmonella searovars. Oral vaccine could also offer high immune protection of 87.5%. These results indicated that LASV-p vaccine candidate had a high level of safety and immune protection and it might be developed as a novel easy-to-use oral vaccine to improve poultry health in the future.

Efficacy of a Salmonella enterica serovar Abortusovis (S. Abortusovis) inactivated vaccine in experimentally infected gestating ewes

Salmonella enterica serovar Abortusovis (S. Abortusovis) infection is one of the most important causes of infectious late-term abortion as well as birth of weak lambs in sheep in many countries throughout the world. Implementation of protocols based on the application of effective vaccines is one of the most effective approaches for controlling this disease, but variable efficacy has been reported, possibly related to factors associated with the host, the vaccine, the parameters used for determining efficacy and the challenge protocols. In this context, a new commercial inactivated vaccine (INMEVA; Laboratorios Hipra S.A., Spain) was evaluated in 20 control and 17 vaccinated gestating ewes, subcutaneously challenged at 90 days of gestation with 5 × 10⁶ colony-forming units (cfu) of a wild strain of S. Abortusovis. Incidence of reproductive failures, bacterial vaginal excretion (by real time PCR), and lamb survival were evaluated as indicators of the vaccine's level of protection. Moreover, humoral response (by ELISA test in serum samples) was studied. Vaccination was showed to be safe under the study conditions. Vaccine efficacy was demonstrated in two different ways: i) it significantly decreased the percentage of abortions [29.4% (5/17) in the vaccinated group compared to the control group (65%; 13/20)] and ii) there was a significant reduction of the overall vaginal excretion in the sampling period (3.05 log cfu/mL ± 0.84 in the vaccinated group vs. 5.68 ± 0.67 in the control group). Given these results, the vaccine evaluated can be considered as an effective alternative for controlling S. Abortusovis infection in ovine flocks.

Characterization and protective efficacy of a sptP mutant of Salmonella Paratyphi A

Background

Salmonella Paratyphi A causes paratyphoid A, a severe systemic disease of people and remains a major public health problem in many parts of the world. In the interest of researching the roles of sptP on Salmonella Paratyphi A and developing a live‐attenuated vaccine candidate, an sptP mutant of Salmonella Paratyphi A SPA017 (SPA017ΔsptP) was constructed, and then its characterization, immunogenicity, and protective ability were evaluated.

Results

The deletion of sptP had no effect on growth and biochemical properties. Adhesion and invasion assays showed that the lack of sptP did not affect the adhesion of Salmonella Paratyphi A, but the invasive ability of the mutant strain was significantly decreased, the half‐lethal dose (LD₅₀) of the mutant strain was 1.43 × 10⁴ times of the parent strain in intraperitoneally injected mice. Single intraperitoneal vaccination with SPA017ΔsptP (1 × 10⁵ CFU) in mice did not affect the body weight or elicit clinical symptoms relative to the control group, SPA017ΔsptP bacteria were isolated from livers and spleens of vaccinated mice at 14 days postvaccination. Notably, specific humoral and cellular immune responses were significantly induced. The protective assessment showed that the mutant strain could provide high‐level protection against subsequent challenge with the wild‐type SPA017 strain.

Conclusions

These results demonstrated that SptP plays an essential role in the pathogenicity of Salmonella Paratyphi A, and Salmonella Paratyphi A lacking sptP is immunogenic and protective in mice.

Antibodies and Protection in Systemic Salmonella Infections: Do We Still Have More Questions than Answers?

Salmonella causes grave systemic infections in humans and other animals and provides a paradigm for other diseases in which the bacteria have both intracellular and extracellular lifestyles. New generations of vaccines rely on the essential contribution of the antibody responses for their protection. The quality, antigen specificity, and functions associated with antibody responses to this pathogen have been elusive for a long time. Recent approaches that combine studies in humans and genetically manipulated experimental models and that exploit awareness of the location and within-host life cycle of the pathogen are shedding light on how humoral immunity to Salmonella operates. However, this area of research remains full of controversy and discrepancies. The overall scenario indicates that antibodies are essential for resistance against systemic Salmonella infections and can express the highest protective function when operating in conjunction with cell-mediated immunity. Antigen specificity, isotype profile, Fc-gamma receptor usage, and complement activation are all intertwined factors that still arcanely influence antibody-mediated protection to Salmonella.

Development of Salmonella Enteritidis vaccine candidate based on streptomycin independent suppressor and metabolic drift rifampicin resistance-attenuating markers

Salmonella is one of the most frequent food-borne pathogens and remains public health threat globally. The control of Salmonella in poultry, the main reservoir of non-typhoidal salmonellae, is a fundamental approach to ensure the safety of poultry products for human consumption. In the present study, a new live attenuated Salmonella enterica serovar Enteritidis vaccine candidate containing three attenuating markers based on streptomycin-independent (Sm-id) suppressor, and metabolic drift antibiotic resistance (MD- “res”) was developed. The streptomycin dependent (Smd) mutants were derived from Salmonella Enteritidis wild-type strain using streptomycin. Then the Sm-id mutants were derived from the isolated Smd mutants and designated “Smd→Sm-id”. A third MD- “res” marker was generated from Smd→Sm-id using rifampicin (Rif) and designated “Smd→Sm-id→Rif”. The colony sizes of these mutants were stable after more than 50 serial passages on blood agar; reversion to virulence can be almost excluded. The safety and efficacy of Smd→Sm-id and Smd→Sm-id→Rif were evaluated in one-day-old commercial layer chicks. Both mutants proved to be safe in terms of clinical signs, mortalities, lesion scores of visceral organs and rapid clearance when administered orally at a dose of 10⁸ colony forming unit (CFU), whereas birds inoculated with 10⁸ CFU Salmonella Enteritidis wild-type strain showed diarrhea, mortalities (3/40) and necrosis in liver and spleen. Chickens vaccinated with the developed mutants showed no seroconversion; however, wild-type strain induced a significant seroconversion at 3-week-postvaccination (wpv). The developed mutants protected chickens against challenge with 10⁸ CFU of Salmonella Enteritidis wild-type strain at 3-wpv. Vaccinated birds showed neither clinical signs nor mortalities during two-week post-challenge. In addition, the challenge strain could not be detected in pooled liver and spleen samples (0/5) at 7^th day post-inoculation (dpi). However, non-vaccinated challenged birds showed diarrhea and the challenge strain was re-isolated from pooled liver and spleen samples (3/5) at 7^th dpi. In conclusion, the developed mutants are safe and fully protected immunized chickens following heterologous challenge. It is obvious that the genetic characterization of these mutants and evaluation of different vaccination regimes are still in demand.

Evaluation of the Safety and Protection Efficacy of spiC and nmpC or rfaL Deletion Mutants of Salmonella Enteritidis as Live Vaccine Candidates for Poultry Non-Typhoidal Salmonellosis

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a host-ranged pathogen that can infect both animals and humans. Poultry and poultry products are the main carriers of S. Enteritidis, which can be transmitted to humans through the food chain. To eradicate the prevalence of S. Enteritidis in poultry farms, it is necessary to develop novel vaccines against the pathogen. In this study, we constructed two vaccine candidates, CZ14-1∆spiC∆nmpC and CZ14-1∆spiC∆rfaL, and evaluated their protective efficacy. Both mutant strains were much less virulent than the parental strain, as determined by the 50% lethal dose (LD50) for three-day-old specific-pathogen free (SPF) White Leghorns and Hyline White chickens. Immunization with the mutant candidates induced highly specific humoral immune responses and expression of cytokines IFN-γ, IL-1β, and IL-6. In addition, the mutant strains were found to be persistent for almost three weeks post-infection. The survival percentages of chickens immunized with CZ14-1∆spiC∆nmpC and CZ14-1∆spiC∆rfaL reached 80% and 75%, respectively, after challenge with the parental strain. Overall, these results demonstrate that the two mutant strains can be developed as live attenuated vaccines.

Imperfection of Commercial Inactivated Salmonella Vaccine Against Salmonella Infantis During Induced Molting in Chickens and Proposed Evaluation Method

In the present study, we evaluated the continuance and efficacy of inactivated vaccine against Salmonella Infantis (SI) in chickens raised on a commercial farm. Chickens (88-days-old) were inoculated with 1 or 0.5 doses of commercially available trivalent inactivated Salmonella vaccine; anti-SI antibody titer was examined continuously for 11 mo thereafter. Molting was induced 11 mo after vaccination, and SI was administered orally. SI colony-forming units (CFUs) were measured in cecal feces, cecal contents, liver, and spleen samples. Anti-SI antibodies in the 1 dose vaccination group could be detected in at least 90% of cases until the end of testing. SI discharge was significantly reduced in birds treated with either dose of vaccine. However, SI CFUs were elevated in the induced molting group, regardless of vaccination dose, particularly in the cecal feces, cecal contents, and spleen. Thus, the vaccine provided remarkable protection against SI infection under ordinary rearing methods but not during induced molting. To achieve sufficient SI protective efficacy, we recommend inoculation with 1 dose of vaccine. Moreover, the efficacy of inactivated Salmonella vaccine is recommended to be evaluated by challenging chickens with live Salmonella in addition to Salmonella antibody titration.

Characterization and protective efficacy of a Salmonella pathogenicity island 2 (SPI2) mutant of Salmonella Paratyphi A

Paratyphoid fever caused by Salmonella Paratyphi A is a serious public health problem in many countries. In order to and develop a live attenuated candidate vaccine of Salmonella Paratyphi A, a Salmonella pathogenicity island 2 (SPI2, approximate 40 kb) deletion mutant of Salmonella Paratyphi A was constructed by lambda Red recombination, then the biological characteristics and protective ability of the Salmonella Paratyphi A SPI2 mutant were evaluated. Our results showed that the growth and biochemical properties of the SPI2 mutant were consistent with that of its parent strain, and the mutant was stable with the loss of SPI2. The mice lethal test showed that the virulence of the SPI2 mutant was significantly decreased, it can colonize and persistent more than 14 days in the liver and spleen of mice. Vaccination with the SPI2 mutant in mice revealed no significant effect on body weight and clinical symptoms compared to control animals, and specific humoral and cellular immune responses were also significantly induced. Immunization of mice offered efficient protection against Salmonella Paratyphi A strain challenge at 14 days post vaccination based on mortality and clinical symptoms relative to control group. Overall, these findings suggested that SPI2 plays an important role in pathogenicity of Salmonella Paratyphi A, and the SPI2 mutant showed its potential to develop a live attenuated vaccine candidate.

Recent advances in delivery of veterinary DNA vaccines against avian pathogens

Veterinary vaccines need to have desired characteristics, such as being effective, inexpensive, easy to administer, suitable for mass vaccination and stable under field conditions. DNA vaccines have been proposed as potential solutions for poultry diseases since they are subunit vaccines with no risk of infection or reversion to virulence. DNA vaccines can be utilized for simultaneous immunizations against multiple pathogens and are relatively easy to design and inexpensive to manufacture and store. Administration of DNA vaccines has been shown to stimulate immune responses and provide protection from challenges in different animal models. Although DNA vaccines offer advantages, setbacks including the inability to induce strong immunity, and the fact that they are not currently applicable for mass vaccination impede the use of DNA vaccines in the poultry industry. The use of either biological or physical carriers has been proposed as a solution to overcome the current delivery limitations of DNA vaccines for veterinary applications. This review presents an overview of the recent development of carriers for delivery of veterinary DNA vaccines against avian pathogens.

Immunogenic potential and protective efficacy of a sptP deletion mutant of Salmonella Enteritidis as a live vaccine for chickens against a lethal challenge

Salmonella Enteritidis (SE) is a highly adapted pathogen causing severe economic losses in the poultry industry worldwide. Chickens infected by SE are a major source of human food poisoning. Vaccination is an effective approach to control SE infections. This study evaluated the immunogenicity and protective efficacy of a SE sptP deletion mutant (C50336ΔsptP) as a live attenuated vaccine (LAV) candidate in chickens. 14 day-old specific pathogen-free (SPF) chickens were intramuscularly immunized with various doses of C50336ΔsptP. Several groups of chickens were challenged with the virulent wild-type SE strain Z-11 via the same route at 14 days post vaccination. Compared to the control group, the groups vaccinated with 1 × 10⁶, 1 × 10⁷ and 1 × 10⁸ colony-forming units (CFU) of C50336ΔsptP exhibited no clinical symptoms after immunization. Only slight pathological changes occurred in the organs of the 1 × 10⁹ CFU vaccinated group. C50336ΔsptP bacteria were cleared from the organs of immunized chickens within 14 days after vaccination. Lymphocyte proliferation and serum cytokine analyses indicated that significant cellular immune responses were induced after the vaccination of C50336ΔsptP. Compared to the control group, specific IgG antibody levels increased significantly in vaccinated chickens, and the levels increased markedly after the challenge. The 1 × 10⁷, 1 × 10⁸, and 1 × 10⁹ CFU vaccinated chickens groups showed no clinical symptoms or pathological changes, and no death after the lethal challenge. Whereas severe clinical signs of disease and pathological changes were observed in the control group chickens after the challenge. These results suggest that a single dose of C50336ΔsptP could be an effective LAV candidate to against SE infection in chickens.

The impact of sseK2 deletion on Salmonella enterica serovar typhimurium virulence in vivo and in vitro

Background

Salmonella enterica is regarded as a major public health threat worldwide. Salmonella secretes the novel translocated effector protein K2 (SseK2), but it is unclear whether this protein plays a significant role in Salmonella enterica Typhimurium virulence.

Results

A ΔsseK2 mutant of S. Typhimurium exhibited similar growth curves, adhesion and invasive ability compared with wild-type (WT) bacteria. However, deletion of sseK2 rendered Salmonella deficient in biofilm formation and the early proliferative capacity of the ΔsseK2 mutant was significantly lower than that of the WT strain. In vivo, the LD₅₀ (median lethal dose) of the ΔsseK2 mutant strain was increased 1.62 × 10³-fold compared with the WT strain. In addition, vaccinating mice with the ΔsseK2 mutant protected them against challenge with a lethal dose of the WT strain. The ability of the ΔsseK2 mutant strain to induce systemic infection was highly attenuated compared with the WT strain, and the bacterial load in the animals’ internal organs was lower when they were infected with the ΔsseK2 mutant strain than when they were infected with the WT strain.

Conclusions

We conclude that sseK2 is a virulence-associated gene that plays a vital role in Salmonella virulence.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1543-2) contains supplementary material, which is available to authorized users.

Evaluation of different live Salmonella enteritidis vaccine schedules administered during layer hen rearing to reduce excretion, organ colonization, and egg contamination

Salmonellosis caused by Salmonella Enteritidis is a widespread zoonosis and poultry products are an important source of infection. This study was carried out to evaluate the protection of different vaccination schedules in layers using a live commercial attenuated Salmonella Enteritidis vaccine based on strain Sm24/Rif12/Ssq (AviPro® Salmonella Vac E, ELANCO) during rearing and egg production. Three hundred and fifty Salmonella-free chickens were distributed into 7 vaccinated groups and 1 unvaccinated group. Different vaccination schedules were performed combining either 1, 2, or 3 oral gavage doses. Chickens from Group A, B, and C were vaccinated once, either at the first day, at 7 or 16 wk old, respectively. Chickens from Group D were vaccinated twice-at the first day and 7 wk old. Chickens from Group E were vaccinated twice-at the first day and 16 wk old. Chickens from Group F were vaccinated twice-at 7 and 16 wk old. Chickens from Group G were vaccinated 3 times, following the manufacturer's recommendation: at the first day, 7 and 16 wk old. Chickens from Group H remained unvaccinated. Five challenge trials numbered 1 to 5 were carried out at 8, 12, 16, 29, and 55 wk old, respectively. After challenge, chickens were sampled by cloacal swabbing and, after euthanasia, livers, ovaries, spleens, and cecal contents were cultured to isolate S. Enteritidis. Additionally, eggs were collected after challenge and cultured to isolate S. Enteritidis on egg shells (Trials 4 and 5). Protection against experimental infection with a virulent nalidixic acid resistant S. Enteritidis strain K285/94, was evaluated by measuring reduction of excretion, colonization, invasion into organs, eggshell contamination, and egg production. The live S. Enteritidis vaccine protected the hens by reducing S. Enteritidis excretion, isolation from organs, and egg contamination. Higher protection throughout laying period was afforded after administration of three vaccine doses during rearing period.

Short communication: Oral and intranasal administration of a modified-live Salmonella Dublin vaccine in dairy calves: Clinical efficacy and serologic response

Our objectives were to evaluate the clinical efficacy of oral and intranasal administration of a commercial modified-live Salmonella Dublin vaccine in dairy calves and to determine the serologic response associated with these extralabel routes of administration. We conducted a randomized field trial with calves from a New York dairy farm following an outbreak of Salmonella Dublin. A total of 399 Holstein calves were allocated by pen to 3 treatment groups: oral vaccination, intranasal vaccination, and an unvaccinated control group. Administration of the vaccine through oral and intranasal routes did not have a significant effect on pneumonia incidence risk or weight gain; however, calves vaccinated orally and intranasally had lower mortality risk as compared with control calves. Among calves tested using a Salmonella Dublin ELISA, vaccination did not induce an increase in antibody production relative to control calves, indicating that oral and intranasal administration will not hinder diagnosis based on this assay.

Brucella abortus lysed cells using GI24 induce robust immune response and provide effective protection in Beagles

The aim of the present study is to estimate the protective efficacy of Brucella abortus lysed cells by GI24 against brucellosis in Beagles. Group A was subcutaneously (sc) immunized with sterile phosphate-buffered saline, and group B was sc immunized with approximately 3 × 109 of the lysed cells. Brucella-LPS-specific serum IgG titers and IL-4, TNF-α and IFN-γ concentrations were investigated by enzyme linked immunosorbent assay. All dogs were intraconjunctivally challenged with B. abortus strain 544 at 6 weeks post-prime immunization. The serum IgG titers were considerably higher in group B than in group A. The levels of IL-4, TNF-α and IFN-γ in group B than in group A were significantly higher. Following challenge, no challenge strain was observed from all tissues of three dogs of group B. However, challenge strain was detected from spleen, uterus (except one Beagle) and inguinal and retropharyngeal lymph nodes of all group A Beagles. The results of this study demonstrated that sc immunization with the lysed cells induced robust antibody and cell-mediated immune responses in Beagles. The lysed cells also conferred protection against infection with B. abortus. These results suggest that sc immunization with B. abortus lysed cells by GI24 is a good vaccine candidate against brucellosis in dogs.

HU-Lacking Mutants of Salmonella enterica Enteritidis Are Highly Attenuated and Can Induce Protection in Murine Model of Infection

Salmonella enterica infection is a major public health concern worldwide, particularly when associated with other medical conditions. The serovars Typhimurium and Enteritidis are frequently associated with an invasive illness that primarily affects immunocompromised adults and children with HIV, malaria, or malnutrition. These serovars can also cause infections in a variety of animal hosts, and they are the most common isolates in poultry materials. Here, we described S. Enteritidis mutants, where hupA and hupB genes were deleted, and evaluated their potential use as live-attenuated vaccine candidates. In vitro, the mutants behaved like S. Typhimurium described previously, but there were some particularities in macrophage invasion and survival experiments. The virulence and immunogenicity of the mutant lacking both hupA and hupB (PT4ΔhupAB) were evaluated in a BALB/c mice model. This mutant was highly attenuated and could, therefore, be administrated at doses higher than 10⁹ CFU/treatment, which was sufficient to protect all treated mice challenged with the wild-type parental strain with a single dose. Additionally, the PT4ΔhupAB strain induced production of specific IgG and IgA antibodies against Salmonella and TH1-related cytokines (IFN-γ and TNF-α), indicating that this strain can induce systemic and mucosal protection in the murine model. Additional studies are needed to better understand the mechanisms that lead to attenuation of the double-mutant PT4ΔhupAB and to elucidate the immune response induced by immunization using this strain. However, our data allow us to state that hupAB mutants could be potential candidates to be explore as live-attenuated vaccines.

Protective efficacy of a canine brucellosis vaccine candidate based on live attenuated Salmonella expressing recombinant Brucella BCSP31, Omp3b and SOD proteins in Beagles

The aim of this study was to establish a proof-of-concept of protective efficacy of Salmonella-based B. abortus vaccine candidate in Beagles. Group A Beagles (n=10) were subcutaneously (SC) inoculated with S. Typhimurium delivery strain containing pMMP65 (vector to deliver antigens) only as vector control. Group B Beagles (n=10) were SC vaccinated with the mixture of the three Salmonella delivery strains expressing the recombinant B. abortus BCSP31, Outer membrane protein 3b (Omp3b), and superoxide dismutase (SOD) proteins, respectively. No Salmonella delivery strains were isolated from all tissues tested. Serum IgG, interleukin-4, tumor necrosis factor-alpha, and interferon-gamma concentrations were significantly higher in group B than in group A. Following intraconjunctival challenge with B. abortus 544, among 5 group B Beagles, the challenge strain was isolated from mandibular, and retropharyngeal lymph nodes of three Beagles, and no isolates were observed from all tissues of two Beagle. However, the challenge strains were detected from spleen, uterus (except two Beagles), and mandibular, prescapular, retropharyngeal, and superficial inguinal lymph nodes of all group A Beagles. These results suggest that the mixture of three S. Typhimurium delivery strains be a good vaccine candidate against brucellosis by B. abortus in dogs. Further investigations are needed to improve the protective efficacy of the Salmonella-based B. abortus vaccine candidate and explore its practical application in dogs.

Vaccines as alternatives to antibiotics for food producing animals. Part 1: challenges and needs

Vaccines and other alternative products can help minimize the need for antibiotics by preventing and controlling infectious diseases in animal populations, and are central to the future success of animal agriculture. To assess scientific advancements related to alternatives to antibiotics and provide actionable strategies to support their development, the United States Department of Agriculture, with support from the World Organisation for Animal Health, organized the second International Symposium on Alternatives to Antibiotics. It focused on six key areas: vaccines; microbial-derived products; non-nutritive phytochemicals; immune-related products; chemicals, enzymes, and innovative drugs; and regulatory pathways to enable the development and licensure of alternatives to antibiotics. This article, part of a two-part series, synthesizes and expands on the expert panel discussions regarding opportunities, challenges and needs for the development of vaccines that may reduce the need for use of antibiotics in animals; new approaches and potential solutions will be discussed in part 2 of this series. Vaccines are widely used to prevent infections in food animals. Various studies have demonstrated that their animal agricultural use can lead to significant reductions in antibiotic consumption, making them promising alternatives to antibiotics. To be widely used in food producing animals, vaccines have to be safe, effective, easy to use, and cost-effective. Many current vaccines fall short in one or more of these respects. Scientific advancements may allow many of these limitations to be overcome, but progress is funding-dependent. Research will have to be prioritized to ensure scarce public resources are dedicated to areas of potentially greatest impact first, and private investments into vaccine development constantly compete with other investment opportunities. Although vaccines have the potential to improve animal health, safeguard agricultural productivity, and reduce antibiotic consumption and resulting resistance risks, targeted research and development investments and concerted efforts by all affected are needed to realize that potential.

Why Is Eradicating Typhoid Fever So Challenging: Implications for Vaccine and Therapeutic Design

Salmonella enterica serovar Typhi (S. Typhi) and S. Paratyphi, namely typhoidal Salmonellae, are the cause of (para) typhoid fever, which is a devastating systemic infectious disease in humans. In addition, the spread of multidrug-resistant (MDR) and extensively drug-resistant (XDR) S. Typhi in many low and middle-income countries poses a significant risk to human health. While currently available typhoid vaccines and therapeutics are efficacious, they have some limitations. One important limitation is the lack of controlling individuals who chronically carry S. Typhi. However, due to the strict host specificity of S. Typhi to humans, S. Typhi research is hampered. As a result, our understanding of S. Typhi pathogenesis is incomplete, thereby delaying the development and improvement of prevention and treatment strategies. Nonetheless, to better combat and contain S. Typhi, it is vital to develop a vaccine and therapy for controlling both acutely and chronically infected individuals. This review discusses how scientists are trying to combat typhoid fever, why it is so challenging to do so, which approaches show promise, and what we know about the pathogenesis of S. Typhi chronic infection.

Evaluation of recombinant Salmonella vaccines to provide cross-serovar and cross-serogroup protection

Historically, Salmonella vaccines have been either live attenuated or killed bacterin vaccines that fail to offer cross-serogroup protection, which limits risk mitigation and protection of consumers. Subunit recombinant vaccines which possess highly conserved antigens offer potential to provide cross-serogroup protection, and the ability to express immune-enhancing molecules that promote recognition by the immune system. Six Salmonella subunit vaccine candidates were developed in either attenuated S. Enteritidis (SE) or S. Typhimurium (ST) that cell-surface express antigenic epitopes of high mobility group box 1 immune-enhancing sequence (H), peptidoglycan associated lipoprotein (P), and Omp18 protein Cj0113 (C) in different pattern arrangements for evaluation against S. Heidelberg (SH) challenge in broilers. In exp. 1, chicks were orally vaccinated with SE-CPH, SE-HCP, SE-CHP, ST-CPH, ST-HCP, or ST-CHP at 1 × 107 cfu/chick, or saline on d 1 and d 14. On d 17 all birds were challenged with 6 × 106 cfu/chick SH, and ceca collected on d 23 and d 28. On d 23 only SE-CPH reduced (P < 0.05) SH recovery at 0.34 ± 0.23 log10 cfu when compared to control at 1.19 ± 0.26 log10 cfu. On d 28, SE-CPH and ST-HCP reduced SH recovery at 0.40 ± 0.40 and 0.51 ± 0.26 log10 cfu, respectively in comparison to control at 1.36 ± 0.23 log10 cfu. For exp. 2, chicks were orally vaccinated with 1 × 108 cfu/chick SE-CPH, SE-HCP, SE-CHP or saline on d 1. At d 7 all chicks were orally challenged with 7 × 106 cfu/chick SH and ceca collected on d 28 and d 35. SE-CPH reduced (P < 0.05) SH recovery on d 28 when compared to control (6.16 ± 0.13 vs. 4.71 ± 0.55 log10 cfu). In exp 3, chicks were vaccinated by spray in a commercial vaccination cabinet with SE-CPH vaccination, 1.6 × 107 cfu/chick, or saline. Birds were challenged on d 14 with 3 × 107 cfu/chick SH and ceca collected on d 18 and d 25. SE-CPH reduced SH recovery (P < 0.05) on d 18, 2.75 ± 0.05 log10 cfu, and d 25, 1.89 ± 0.43 log10 cfu, as compared to control chickens at 5.6 ± 0.37 (d 18) and 3.98 ± 0.5 log10 cfu (d 25). The results of these experiments suggest that cross-serogroup protection is possible using these SE and ST-vectored subunit vaccines.

Protective efficacy by various doses of a new brucellosis vaccine candidate based on Salmonella strains expressing Brucella abortus BSCP31, Omp3b and superoxide dismutase against brucellosis in murine model

Brucella species are important etiological agents of zoonotic diseases. Attenuated Salmonella strains expressing Brucella abortus BCSP31, Omp3b and superoxide dismutase proteins were tested as vaccine candidates in this study. In order to determine the optimal dose for intraperitoneal (IP) inoculation required to obtain effective protection against brucellosis, mice were immunized with various doses of a mixture of the three vaccine strains. Fifty BALB/c mice were divided into five equal groups (groups A–E). Group A mice were intraperitoneally inoculated with 100 μL of sterile phosphate-buffered saline. Group B, C, D and E mice were intraperitoneally immunized with approximately 1.2 × 10⁵ colony-forming units (CFU) mL⁻¹ of Salmonella containing pMMP65 in 100 μL and with 1.2 × 10⁴ CFU mL⁻¹, 1.2 × 10⁵ CFU mL⁻¹ and 1.2 × 10⁶ CFU mL⁻¹ of the mixture of the three strains in 100 μL, respectively. Serum IgG, tumor necrosis factor alpha and interferon gamma concentrations were significantly higher in group E than in groups A–D. Following challenge with B. abortus 544, the challenge strain was not detected in the spleen of any mouse from group E. Thus, IP immunization with 1.2 × 10⁶ CFU mL⁻¹ of the mixture of the three vaccine strains induced immune responses and provided effective protection against brucellosis in mice.

Vaccination strategies

Currently all prion diseases are without effective treatment and are universally fatal. It is increasingly being recognized that the pathogenesis of many neurodegenerative diseases, such as Alzheimer disease (AD), includes "prion-like" properties. Hence, any effective therapeutic intervention for prion disease could have significant implications for other neurodegenerative diseases. Conversely, therapies that are effective in AD might also be therapeutically beneficial for prion disease. AD-like prion disease has no effective therapy. However, various vaccine and immunomodulatory approaches have shown great success in animal models of AD, with numerous ongoing clinical trials of these potential immunotherapies. More limited evidence suggests that immunotherapies may be effective in prion models and in naturally occurring prion disease. In particular, experimental data suggest that mucosal vaccination against prions can be effective for protection against orally acquired prion infection. Many prion diseases, including natural sheep scrapie, bovine spongiform encephalopathy, chronic wasting disease, and variant Creutzfeldt-Jakob disease, are thought to be acquired peripherally, mainly by oral exposure. Mucosal vaccination would be most applicable to this form of transmission. In this chapter we review various immunologically based strategies which are under development for prion infection.

Maternal vaccination as a Salmonella Typhimurium reduction strategy on pig farms

AIMS

The control of Salmonella in pig production is necessary for public and animal health, and vaccination was evaluated as a strategy to decrease pig prevalence.

METHODS AND RESULTS

The study examined the efficacy of a live Salmonella Typhimurium vaccine, administered to sows on eight commercial farrow-to-finish herds experiencing clinical salmonellosis or Salmonella carriage associated with S. Typhimurium or its monophasic variants. Results of longitudinal Salmonella sampling were compared against eight similarly selected and studied control farms. At the last visit (~14 months after the start of vaccination), when all finishing stock had been born to vaccinated sows, both faecal shedding and environmental prevalence of Salmonella substantially declined on the majority of vaccinated farms in comparison to the controls. A higher proportion of vaccine farms resolved clinical salmonellosis than controls. However, Salmonella counts in positive faeces samples were similar between nonvaccinated and vaccinated herds.

CONCLUSIONS

The results suggest that maternal vaccination is a suitable option for a Salmonella Typhimurium reduction strategy in farrow-to-finish pig herds.

SIGNIFICANCE AND IMPACT OF THE STUDY

Salmonella vaccines have the potential to reduce the prevalence of Salmonella in pigs and result in a reduction of human cases attributed to pork.

Guanosine 5'-monophosphate-chelated calcium and iron feed additives maintains egg production and prevents Salmonella Gallinarum in experimentally infected layers

We evaluated the effects of guanosine 5'-monophosphate (GMP)-chelated calcium and iron (CaFe-GMP) on health and egg quality in layers experimentally infected with Salmonella Gallinarum. In this study, a CaFe-GMP feed additive was added to a commercial layer feed and fed to layers over a four-week period. All were inoculated with Salmonella Gallinarum. Body weight, mortality, clinical symptoms, and poultry production including feed intake, egg production, egg loss, and feed conversion rate were observed, and Salmonella Gallinarum was re-isolated from the liver, spleen, and cecum of the layers. All tested internal organs for the CaFe-GMP additive group exhibited significantly lower re-isolation numbers of Salmonella Gallinarum and less severe pathological changes than those in the control group, indicating that the CaFe-GMP feed supplement induced bacterial clearance and increased resistance to Salmonella Gallinarum. Additionally, due to the inhibitory action of CaFe-GMP on the growth of Salmonella Gallinarum, the CaFe-GMP additive group exhibited better egg production, including a higher laying rate and fewer broken eggs. The results suggest that a 0.16% CaFe-GMP additive may help prevent salmonellosis in the poultry industry.

Prime-boost vaccination with attenuated Salmonella Typhimurium ΔznuABC and inactivated Salmonella Choleraesuis is protective against Salmonella Choleraesuis challenge infection in piglets

Background

Salmonella enterica serovar Choleraesuis (S. Choleraesuis) infection causes a systemic disease in pigs. Vaccination could represent a solution to reduce prevalence in farms. In this study, we aimed to assess the efficacy of an attenuated strain of Salmonella enterica serovar Typhimurium (S. Typhimurium ΔznuABC) against S. Choleraesuis infection. The vaccination protocol combined priming with attenuated S. Typhimurium ΔznuABC vaccine and boost with an inactivated S. Choleraesuis vaccine and we compared the protection conferred to that induced by an inactivated S. Choleraesuis vaccine.

Methods

The first group of piglets was orally vaccinated with S. Typhimurium ΔznuABC and boosted with inactivated S. Choleraesuis, the second one was intramuscularly vaccinated with S. Choleraesuis inactivated vaccine and the third group of piglets was unvaccinated. All groups of animals were challenged with a virulent S. Choleraesuis strain at day 35 post vaccination.

Results

The results showed that the vaccination protocol, priming with S. Typhimurium ΔznuABC and boosted with inactivated S. Choleraesuis, applied to group A was able to limit weight loss, fever and organs colonization, arising from infection with virulent S. Choleraesuis, more effectively, than the prime-boost vaccination with homologous S. Choleraesuis inactivated vaccine (group B).

Conclusion

In conclusion, these research findings extend the validity of attenuated S. Typhimurium ΔznuABC strain as a useful mucosal vaccine against S. Typhimurium and S. Choleraesuis pig infection. The development of combined vaccination protocols can have a diffuse administration in field conditions because animals are generally infected with different concomitant serovars.

Salmonella infection: Interplay between the bacteria and host immune system

Salmonella infection causes morbidity and mortality throughout the world with the host immune response varying depending on whether the infection is acute and limited, or systemic and chronic. Additionally, Salmonella bacteria have evolved multiple mechanisms to avoid or subvert immunity to its own benefit and often the anatomical location of infection plays a role in both the immune response and bacterial fate. Here, we provide an overview of the interplay between the immune system and Salmonella, while discussing how different host and bacterial factors influence the outcome of infection.

Immunogenicity and protective efficacy of a Salmonella Enteritidis sptP mutant as a live attenuated vaccine candidate

Background

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a highly adaptive pathogen in both humans and animals. As a Salmonella Type III secretion system (T3SS) effector, Salmonella protein tyrosine phosphatase (SptP) is critical for virulence in this genus. To investigate the feasibility of using C50336ΔsptP as a live attenuated oral vaccine in mice, we generated the sptP gene deletion mutant C50336ΔsptP in S. Enteritidis strain C50336 by λ-Red mediated recombination and evaluated the protective ability of the S. Enteritidis sptP mutant strain C50336ΔsptP against mice salmonellosis.

Results

We found that C50336ΔsptP was a highly immunogenic, effective, and safe vaccine in mice. Compared to wild-type C50336, C50336ΔsptP showed reduced virulence as confirmed by the 50% lethal dose (LD₅₀) in orally infected mice. C50336ΔsptP also showed decreased bacterial colonization both in vivo and in vitro. Immunization with C50336ΔsptP had no significant effect on body weight and did not result in obvious clinical symptoms relative to control animals treated with phosphate-buffered saline (PBS), but induced humoral and cellular immune responses at 12 and 26 days post inoculation. Immunization with 1 × 10⁸ colony-forming units (CFU) C50336ΔsptP per mouse provided 100% protection against subsequent challenge with the wild-type C50336 strain, and immunized mice showed mild and temporary clinical symptoms as compared to those of control group.

Conclusions

These results demonstrate that C50336ΔsptP can be a live attenuated oral vaccine for salmonellosis.

Vaccination as a control strategy against Salmonella infection in pigs: A systematic review and meta-analysis of the literature

Consumption or handling of improperly processed or cooked pork is considered one of the top sources for foodborne salmonellosis, a common cause of intestinal disease worldwide. Asymptomatic carrier pigs may contaminate pork at slaughtering; therefore, pre-harvest reduction of Salmonella load can contribute to reduce public health risk. Multiple studies have evaluated the impact of vaccination on controlling Salmonella in swine farms, but results are highly variable due to the heterogeneity in vaccines and vaccination protocols. Here, we report the results of an inclusive systematic review and a meta-analysis of the peer-reviewed scientific literature to provide updated knowledge on the potential effectiveness of Salmonella vaccination. A total of 126 articles describing the use of Salmonella vaccines in swine were identified, of which 44 fulfilled the inclusion criteria. Most of the studies (36/44) used live vaccines, and S. Typhimurium and S. Choleraesuis were the predominant serotypes evaluated. Vaccine efficacy was most often measured through bacteriological isolation, and pooled estimates of vaccine efficacy were obtained as the difference in the percentage of positive animals when available. Attenuated and inactivated vaccines had similar efficacy [Risk Difference=-26.8% (-33.8, -19.71) and -29.5% (-44.4, -14.5), respectively]. No serotype effect was observed on the efficacy recorded for attenuated vaccines; however, a higher efficacy of inactivated vaccines against S. Choleraesuis was observed, though in a reduced sample. Results from the meta-analysis here demonstrate the impact that vaccination may have on the control of Salmonella in swine farms and could help in the design of programs to minimize the risk of transmission of certain serotypes through the food chain.

Cinobufagin enhances the protective efficacy of formalin-inactivated Salmonella typhimurium vaccine through Th1 immune response

Cinobufagin (CBG), one active ingredient isolated from Venenum Bufonis, has been demonstrated to have immunoregulatory effect. The aim of this study was to investigate whether CBG can enhance the protective efficacy of formalin-inactivated Salmonella typhimurium (FIST) in mice. ICR mice were immunized with FIST (10⁶ CFU/mouse) alone or mixed with CBG (10, 20, and 40 μg) or alum (200 μg) on day 1 and day 15. Two weeks after the second immunization, serum and spleen were sampled for measuring FIST-specific antibody levels, cytokine levels, and splenocyte proliferation. The results showed that CBG enhanced FIST-specific IgG and IgG2a, the levels of interferon-gamma (IFNγ) and nitric oxide (NO), and the splenocyte proliferation response induced by concanavalin A, lipopolysaccharide, and FIST. In vivo protection studies showed that CBG significantly decreased the bacterial burdens in the spleen and prolonged the survival time of FIST-immunized mice challenged with live Salmonella typhimurium. In vivo IFNγ neutralization led to a significant reduction in FIST-specific IgG2a and IFNγ levels, and in the protective efficacy in CBG/FIST-immunized mice. In conclusion, CBG enhances the protective efficacy of formalin-inactivated Salmonella typhimurium vaccine by promoting the Th1 immune response.

Prevention of egg contamination by Salmonella Enteritidis after oral vaccination of laying hens with Salmonella Enteritidis ΔtolC and ΔacrABacrEFmdtABC mutants

Vaccination of laying hens has been successfully used to reduce egg contamination by Salmonella Enteritidis, decreasing human salmonellosis cases worldwide. Currently used vaccines for layers are either inactivated vaccines or live attenuated strains produced by mutagenesis. Targeted gene deletion mutants hold promise for future vaccines, because specific bacterial functions can be removed that may improve safety and allow differentiation from field strains. In this study, the efficacy of Salmonella Enteritidis ΔtolC and ΔacrABacrEFmdtABC strains in laying hens as live vaccines was evaluated. The mutants are deficient in either the membrane channel TolC (ΔtolC) or the multi-drug efflux systems acrAB, acrEF and mdtABC (ΔacrABacrEFmdtABC). These strains have a decreased ability for gut and tissue colonization and are unable to survive in egg white, the latter preventing transmission of the vaccine strains to humans. Two groups of 30 laying hens were orally inoculated at day 1, 6 weeks and 16 weeks of age with 10⁸ cfu of either vaccine strain, while a third group was left unvaccinated. At 24 weeks of age, the birds were intravenously challenged with 5 × 10⁷ cfu Salmonella Enteritidis PT4 S1400/94. The vaccine strains were not shed or detected in the gut, internal organs or eggs, 2 weeks after the third vaccination. The strains significantly protected against gut and internal organ colonization, and completely prevented egg contamination by Salmonella Enteritidis under the conditions of this study. This indicates that Salmonella Enteritidis ΔtolC and ΔacrABacrEFmdtABC strains might be valuable strains for vaccination of layers against Salmonella Enteritidis.

Immunology, epidemiology and mathematical modelling towards a better understanding of invasive non-typhoidal Salmonella disease and rational vaccination approaches

INTRODUCTION

Invasive non-typhoidal Salmonella (iNTS) infections cause a high burden of lethal sepsis in young children and HIV patients, often associated with malaria, anaemia, malnutrition and sickle-cell disease. Vaccines against iNTS are urgently needed but none are licensed yet. Areas covered: This review illustrates how immunology, epidemiology and within-host pathogen behaviour affect invasive Salmonella infections and highlights how this knowledge can assist the improvement and choice of vaccines. Expert Commentary: Control of iNTS disease requires approaches that reduce transmission and improve diagnosis and treatment. These are often difficult to implement due to the fragile ecology and economies in endemic countries. Vaccines will be key tools in the fight against iNTS disease. To optimise vaccine design, we need to better define protective antigens and mechanisms of resistance to disease in susceptible populations even in those individuals where innate immunity may be impaired by widespread comorbidities.

Safety and protective efficacy of a spiC and crp deletion mutant of Salmonella gallinarum as a live attenuated vaccine for fowl typhoid

With an aim to develop a safe, immunogenic fowl typhoid (FT) vaccine, the safety and efficacy of 1009ΔspiCΔcrp, a spiC and crp deletion mutant of Salmonella gallinarum, were evaluated in chickens. Three-day-old chickens were intramuscularly immunized with 1009ΔspiCΔcrp (1×10(7)CFU) and boosted 7days later (at 10-days old) with the same dose and via the same route (vaccinated group). The vaccinated group showed no clinical symptoms and no differences in body weight compared to the unvaccinated control group. 1009ΔspiCΔcrp bacteria colonized and persisted in the liver and spleen of vaccinated chickens for >14days, and significant specific humoral and cellular immune responses were induced. Vaccinated chickens were challenged with S. gallinarum strain SG9 at 21days post-immunization (24-day-old chickens), and efficient protection was observed based on the mortality and clinical symptoms, as compared to those in the control group. These results demonstrate that 1009ΔspiCΔcrp can be used as a live attenuated vaccine.

Immunogenicity and Cross-Protective Efficacy Induced by Outer Membrane Proteins from Salmonella Typhimurium Mutants with Truncated LPS in Mice

Lipopolysaccharide (LPS) is a major virulence factor present in the outer membrane of Salmonella enterica serovar Typhimurium (S. Typhimurium). Outer membrane proteins (OMPs) from Salmonella show high immunogenicity and provide protection against Salmonella infection, and truncated LPS alters the outer membrane composition of the cell wall. In our previous study, we demonstrated that Salmonella mutants carrying truncated LPS failed to induce strong immune responses and cross-reaction to other enteric bacteria, due to their high attenuation and low colonization in the host. Therefore, we plan to investigate whether outer membrane proteins from Salmonella mutants with truncated LPS resulting from a series of nonpolar mutations, including ∆waaC12, ∆waaF15, ∆waaG42, ∆rfaH49, ∆waaI43, ∆waaJ44, ∆waaL46, ∆wbaP45 and ∆wzy-48, affect immunogenicity and provide protection against diverse Salmonella challenge. In this study, the immunogenicity and cross-protection efficiency of purified OMPs from all mutants were investigated to explore a potential OMP vaccine to protect against homologous or heterologous serotype Salmonella challenge. The results demonstrated that OMPs from three Salmonella mutants (∆waaC12, ∆waaJ44 and ∆waaL46) induced higher immune responses and provided good protection against homologous S. Typhimurium. The OMPs from these three mutants were also selected to determine the cross-protective efficacy against homologous and heterologous serotype Salmonella. Our results indicated that the mutant ∆waaC12 can elicit higher cross-reactivity and can provide good protection against S. Choleraesuis and S. Enteritidis infection and that the cross-reactivity may be ascribed to an antigen of approximately 18.4–30 kDa.

Live and inactivated Salmonella enterica serovar Typhimurium stimulate similar but distinct transcriptome profiles in bovine macrophages and dendritic cells

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a major cause of gastroenteritis in cattle and humans. Dendritic cells (DC) and macrophages (Mø) are major players in early immunity to Salmonella, and their response could influence the course of infection. Therefore, the global transcriptional response of bovine monocyte-derived DC and Mø to stimulation with live and inactivated S. Typhimurium was compared. Both cell types mount a major response 2 h post infection, with a core common response conserved across cell-type and stimuli. However, three of the most affected pathways; inflammatory response, regulation of transcription and regulation of programmed cell death, exhibited cell-type and stimuli-specific differences. The expression of a subset of genes associated with these pathways was investigated further. The inflammatory response was greater in Mø than DC, in the number of genes and the enhanced expression of common genes, e.g., interleukin (IL) 1B and IL6, while the opposite pattern was observed with interferon gamma. Furthermore, a large proportion of the investigated genes exhibited stimuli-specific differential expression, e.g., Mediterranean fever. Two-thirds of the investigated transcription factors were significantly differentially expressed in response to live and inactivated Salmonella. Therefore the transcriptional responses of bovine DC and Mø during early S. Typhimurium infection are similar but distinct, potentially due to the overall function of these cell-types. The differences in response of the host cell will influence down-stream events, thus impacting on the subsequent immune response generated during the course of the infection.

Utilization of a Modified Phage E Protein Lysis System Accounts for Increased Biomass in Salmonella Gallinarum Ghosts

A major limiting issue of bacterial ghost technology involves the stable maintenance of Phix174 lysis gene E expression. Unwanted leaky expression of gene E in the absence of induction temperature results in reduced biomass production of host bacterium, consequently leading to the lower yield of bacterial ghost. To mitigate the leaky expression status of lysis gene E, we utilized a novel E-lysis system in which gene E is located between sense λpR promoter with a CI857 regulator and antisense ParaBAD promoter with the AraC regulator. In the presence of L-arabinose at 28 C, unwanted transcription of lysis gene E from λpR promoter is repressed by a simultaneous transcription event from ParaBAD promoter by means of anti-sense RNA-mediated inhibition. Tight repression of lysis gene E in the absence of induction temperature resulted in higher bacterial cell number in culture suspension and, consequently, higher production of Salmonella Gallinarum (SG) ghost biomass. The safety and protective efficacy of the SG ghost vaccine were further examined in chickens. All of the immunized chickens showed significantly higher mucosal and systemic antibody responses accompanied by a potent antigen-specific lymphocyte proliferative response. Vaccination of chickens with SG ghost preparation offered efficient protection against wild-type SG challenge.