Abstract
In order to develop a novel, safe and immunogenic fowl typhoid (FT) vaccine candidate, a Salmonella Gallinarum ghost with controlled expression of the bacteriophage PhiX174 lysis gene E was constructed using pMMP99 plasmid in this study. The formation of the Salmonella Gallinarum ghost with tunnel formation and loss of cytoplasmic contents was observed by scanning electron microscopy and transmission electron microscopy. No viable cells were detectable 24 h after the induction of gene E expression by an increase in temperature from 37 °C to 42 °C. The safety and protective efficacy of the Salmonella Gallinarum ghost vaccine was tested in chickens that were divided into four groups: group A (non-immunized control), group B (orally immunized), group C (subcutaneously immunized) and group D (intramuscularly immunized). The birds were immunized at day 7 of age. None of the immunized animals showed any adverse reactions such as abnormal behavior, mortality, or signs of FT such as anorexia, depression, or diarrhea. These birds were subsequently challenged with a virulent Salmonella Gallinarum strain at 3 weeks post-immunization (wpi). Significant protection against the virulent challenge was observed in all immunized groups based on mortality and post-mortem lesions compared to the non-immunized control group. In addition, immunization with the Salmonella Gallinarum ghosts induced significantly high systemic IgG response in all immunized groups. Among the groups, orally-vaccinated group B showed significantly higher levels of secreted IgA. A potent antigen-specific lymphocyte activation response along with significantly increased percentages of CD4+ and CD8+ T lymphocytes found in all immunized groups clearly indicate the induction of cellular immune responses. Overall, these findings suggest that the newly constructed Salmonella Gallinarum ghost appears to be a safe, highly immunogenic, and efficient non-living bacterial vaccine candidate that protects against FT.
Highlights
The Gram-negative bacterium, Salmonella enterica serovar Gallinarum (Salmonella Gallinarum), is one of the major intracellular bacterial pathogens that causes fowl typhoid (FT), a septicemic disease of domestic birds, primarily chickens [1,2]
The SDM37 ghost system was prepared by introducing a mutation in an operator region encoding the R gene
The strain carrying the SDM37 ghost system could grow at temperatures lower than 37 °C but were unable to grow at temperatures higher than 37 °C due to the induction of lysis
Summary
The Gram-negative bacterium, Salmonella enterica serovar Gallinarum (Salmonella Gallinarum), is one of the major intracellular bacterial pathogens that causes fowl typhoid (FT), a septicemic disease of domestic birds, primarily chickens [1,2]. The use of live Salmonella Gallinarum 9R vaccine is limited to layer breeds older than 6-weeks and is associated with several disadvantages such as insufficient protection, low growth rate [6], and residual virulence that can cause hepatitis and splenic lesions in chicks [10]. The major drawbacks with using these live vaccines are the safety of the animals and environmental contamination via fecal shedding. The alternative for these live vaccines could be the use of subunit or inactivated vaccines. The administration of a subunit vaccine along with an adjuvant has been shown to offer better protection than an experimental live vaccine [6]. A vaccine that can be safely administered to chickens (especially at a young age) to obtain desired immune responses and offer sufficient protection from FT is needed
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