Characterization and immunogenicity of Salmonella typhimurium SL1344 and UK-1 Δcrp and Δcdt deletion mutants

S. typhimurium SL1344 and UK-1 mutants with deletions of the crp (cyclic AMP receptor protein) and cdt (colonization of deep tissues) genes have been constructed and characterized, and their levels of virulence and immunogenicity have been determined for BALB/c mice. All Crp^- Cdt^- and Crp⁺ Cdt^- mutants were avirulent, as mice survived oral doses of 10⁹ cells without illness. All the mutants colonized the gut-associated lymphoid tissue efficiently, but capacities to colonize deeper tissues, such as those of the spleen and liver, and blood were significantly reduced for the Crp^- Cdt^- and Crp⁺ Cdt^- mutants compared with the Crp^- Cdt⁺ mutant and the wild- type parent strain. The Crp^- Cdt^- and Crp⁺ Cdt^- SL1344 strains induced complete protection, as all mice immunized with the mutants survived challenge with ~10⁴ times the 50% lethal dose of the wild-type SL1344 strain. The Crp^- UK-1 strain was least attenuated yet induced the highest level of protective immunity against challenge with the wild-type UK-1 strain. The Crp⁺ Cdt^- and Crp^- Cdt^- strains, although totally attenuated, differed in immunogenicity to challenge with the highly virulent UK-1 parent, with the apparently hyperattenuated Crp^- Cdt^- strain inducing a lower level of protective immunity than the Crp⁺ Cdt^- strain. Nevertheless, these UK-1 Crp^- Cdt^- and Crp⁺ Cdt^- strains induced complete protective immunity to challenge with the less-virulent SL1344 wild-type strain. Taken collectively, the results indicate that the attenuation of a highly virulent S. typhimurium strain can yield a vaccine that induces excellent protective immunity to challenge with less-virulent S. typhimurium strains.