Safety evaluation and immunogenicity of arabinose-based conditional lethal Salmonella Gallinarum mutant unable to survive Ex vivo as a vaccine candidate for protection against fowl typhoid

In seeking to develop a safe fowl typhoid (FT) vaccine, a novel candidate lacking cpxR, lon, and asd Salmonella Gallinarum (SG) genes was constructed with the plasmid-containing araC::ParaBAD::asd system. A balanced-lethal host-vector system based on the essential bacterial gene for aspartate βI-semialdehyde dehydrogenase (asd) was used to construct the SG mutant strain. A plasmid (p15A ori) with an araC::ParaBAD::asd cassette was introduced into an auxotrophic mutant to prevent ex vivo survival. The safety, immunity, and protective properties of the SG mutant were evaluated. Inoculation of the mutant at 10 ⁶ colony-forming units (CFU) did not result in recovery in feces and internal organs, whereas inoculation at 10⁸ and 10¹⁰ CFU resulted in moderate bacterial recovery from feces and organs. Birds immunized with the mutant were challenged with a virulent SG strain at day 14 postimmunization; significantly reduced mortality and induced plasma immunoglobulin (Ig)G and mucosal IgA responses were noted. Cellular immune responses as evaluated by a peripheral lymphocyte proliferation assay were also significantly induced. The balanced-lethal host-vector system for construction of SG mutants is an effective and improved approach for safe vaccine construction against FT.