Attenuated Salmonella as live vaccines: prospects for multivalent poultry vaccines

Application of Molecular Approaches for Understanding Foodborne Salmonella Establishment in Poultry Production

Salmonellosis in the United States is one of the most costly foodborne diseases. Given that Salmonella can originate from a wide variety of environments, reduction of this organism at all stages of poultry production is critical. Salmonella species can encounter various environmental stress conditions which can dramatically influence their survival and colonization. Current knowledge of Salmonella species metabolism and physiology in relation to colonization is traditionally based on studies conducted primarily with tissue culture and animal infection models. Consequently, while there is some information about environmental signals that control Salmonella growth and colonization, much still remains unknown. Genetic tools for comprehensive functional genomic analysis of Salmonella offer new opportunities for not only achieving a better understanding of Salmonella pathogens but also designing more effective intervention strategies. Now the function(s) of each single gene in the Salmonella genome can be directly assessed and previously unknown genetic factors that are required for Salmonella growth and survival in the poultry production cycle can be elucidated. In particular, delineating the host-pathogen relationships involving Salmonella is becoming very helpful for identifying optimal targeted gene mutagenesis strategies to generate improved vaccine strains. This represents an opportunity for development of novel vaccine approaches for limiting Salmonella establishment in early phases of poultry production. In this review, an overview of Salmonella issues in poultry, a general description of functional genomic technologies, and their specific application to poultry vaccine developments are discussed.

Salmonella antibiotic-mutant strains reduce fecal shedding and organ invasion in broiler chicks

We investigated the exposure to antibiotics in the production of antibiotic-mutant strains of Salmonella. Ten isolates of poultry origin were assayed for antibiotic susceptibilities. One strain of Salmonella Enteritidis, one of Salmonella Heidelberg, and one of Salmonella Typhimurium were selected to induce antimicrobial resistance. Each strain was exposed to high concentrations of streptomycin, rifampicin, and nalidixic acid, respectively. Parent and antibiotic-mutant strains were assayed for antibiotic susceptibilities using a commercial microdilution test and the disk susceptibility test. The strains were assessed for virulence genes and evaluated for fecal shedding, cecal colonization, organ invasion, and mean Salmonella counts after inoculation in 1-day-old chicks. The study revealed that exposure to high concentrations of streptomycin produced the antibiotic-mutant strain SE/LABOR/USP/08 and the exposure to rifampicin produced the antibiotic-mutant SH/LABOR/USP/08. These strains showed significantly reduced fecal shedding (P≤0.05) and organ invasion, persisting less than the parental strains and showing no clinical signs in inoculated chicks. High concentrations of nalidixic acid produced the antibiotic-mutant strain ST/LABOR/USP/08, which did not show any differences compared with the parent strain. Likewise, SE/LABOR/USP/08 did not show the expression of plasmid-encoded fimbriae (pefA) and plasmid virulence protein (spvC), suggesting that after exposure to streptomycin, the parent isolate lost the original gene expression, reducing fecal shedding and organ invasion in inoculated chicks.

LPS-based conjugate vaccines composed of saccharide antigens of smooth-type Salmonella enteritidis and rough-type S. gallinarum 9R bound to bovine serum albumin

Lipopolysaccahrides (LPSs) from Salmonella enteritidis, S. gallinarum, and S. enterica Typhimurium showed an identical electrophoretic banding pattern and serological cross-reactions among each other. LPSs from wild-type Salmonella enteritidis and rough mutant S. gallinarum 9R were detoxified by cleavage of lipid A moieties using mild acid hydrolysis. The non-toxic saccharide moieties from both strains were coupled directly to bovine serum albumin (BSA). The conjugates were injected in mice in combination with monophosphory lipid A (MPL), Freund, and Alum adjuvants. The highest IgM and IgG titres were obtained when the conjugates were emulsified with MPL adjuvant, followed by Freund adjuvant. The antisera raised against the conjugates in combination with MPL and Freund adjuvants showed high complement-mediated lysis to the homologous strains. A correlation was observed between IgG titres and bactericidal activities against homologous strains. Low in vivo protection was obtained when mice immunized with the conjugates were challenged with 10 times the LD50 of the wild S. enteritidis.

Nonrecombinant and recombinant avirulent Salmonella vaccines for poultry

We have constructed and evaluated a live avirulent Salmonella typhimurium vaccine strain, attenuated by deletion (delta) mutations in genes for adenylate cyclase (cya) and the cAMP receptor protein (crp). Immunization of chicks can preclude Salmonella colonization and invasion of challenged vaccinated chickens when compared with the non vaccinated control. Immunization induces significant cross-protective immunity against various Salmonella serotypes and protects laying hens from transmission of Salmonella in or on eggs following challenge with S. enteritidis or S. typhimurium. Immunization of chicks destined to be breeders and then with a booster immunization at 16-18 weeks of age leads to maternal transfer of immunity to chicks which then can be immunized either orally or by coarse spray to display an enhanced immunity to prevent infection of visceral organs by and shedding of Salmonella. The attenuated S. typhimurium vaccine can be genetically manipulated to express foreign antigens specified by cloned genes from other pathogens. Immunization with such recombinant vaccines not only induces immunity to Salmonella but to infection by the pathogen that supplied the genes specifying the protective antigens expressed by the recombinant vaccine.

Genetic aspects of aromatic amino acid biosynthesis in Lactococcus lactis

Polymerase chain reaction (PCR) primers designed from a multiple alignment of predicted amino acid sequences from bacterial aroA genes were used to amplify a fragment of Lactococcus lactis DNA. An 8 kb fragment was then cloned from a lambda library and the DNA sequence of a 4.4 kb region determined. This region was found to contain the genes tyrA, aroA, aroK, and pheA, which are involved in aromatic amino acid biosynthesis and folate metabolism. TyrA has been shown to be secreted and AroK also has a signal sequence, suggesting that these proteins have a secondary function, possibly in the transport of amino acids. The aroA gene from L. lactis has been shown to complement an E. coli mutant strain deficient in this gene. The arrangement of genes involved in aromatic amino acid biosynthesis in L. lactis appears to differ from that in other organisms.

Cloning and characterization of a surface antigen of Eimeria tenella merozoites and expression using a recombinant vaccinia virus

A rabbit serum raised against Eimeria tenella merozoites was used to screen a lambda gt11 cDNA library made from merozoite mRNA of E. tenella. The insert of the phage clone lambda Mz 5-7 revealed an open reading frame consisting of 945 nucleotides, encoding a 33-kDa protein. This size is consistent with the size of a protein translated in vitro from merozoite mRNA and immunoprecipitated with monospecific anti-Mzp 5-7 antibodies. A smaller protein of 24 kDa, located on the surface of the parasite, also reacted with the monospecific antiserum and is the potential processed form of the Mzp 5-7. Furthermore, a recombinant vaccinia virus expressing the Mzp 5-7 antigen was constructed and used to immunize chickens.

Protection of turkeys against haemorrhagic enteritis by monoclonal antibody and hexon immunization

Virus-neutralizing monoclonal antibodies specific for the hexon of haemorrhagic enteritis virus (HEV), a turkey adenovirus, were examined for their ability to confer passive protection against haemorrhagic enteritis (HE) in turkeys. A high dose of antibody prevented clinical disease and reduced virus replication in experimentally infected birds. This suggests that virus neutralization might be an important mechanism for protection against HE. Subsequently, the use of the hexon protein as a subunit vaccine was investigated by immunizing birds with affinity-purified HEV hexon. The birds were tested for the appearance of hexon-specific antibodies in their sera, for protection from clinical disease, and prevention of virus replication after challenge with virulent HEV (HEV-V). Regardless of whether birds were immunized with native or denatured hexon, high ELISA antibody titres were produced to each immunogen. A virus-neutralizing antibody response was induced by immunization with the native hexon but not by immunization with the denatured protein. All turkeys twice immunized with a dose of at least 1 microgram, and four out of five birds immunized with two doses of 0.3 micrograms of purified native hexon, were protected against virus-induced disease and virus replication. In contrast, birds inoculated with denatured hexon were not protected. These results demonstrate the importance of the native (trimeric) structure of the hexon protein for eliciting a protective immune response. The impact of these results on the development of a vaccine for HE in turkeys produced by recombinant DNA technology is discussed.

Construction of an aroA mutant of Salmonella serotype Gallinarum: its effectiveness in immunization against experimental fowl typhoid

An aroA mutant was produced from a virulent strain of Salmonella Gallinarum, the causative agent of fowl typhoid in poultry, by Tn10 insertional inactivation and deletion. The mutant was highly attenuated for chickens. A single intramuscular immunization of 2-week-old chickens with 10(7) mutant organisms reduced mortality following oral challenge with 10(8) organisms of the parent strain from 63 to 30%. In a second experiment multiple immunizations with 10(8) mutant organisms reduced mortality after challenge with 10(7) organisms of the virulent strain from 77 to 0%. By oral inoculation the mutant was not able to immunize chickens against oral challenge.