We previously reported that Salmonella typhimurium SR-11 mutants with deletion mutations in the genes encoding
adenylate cyclase (cya) and the
cAMP receptor protein (crp) are avirulent and protective in mice. Salmonella typhimurium
UK-1 is highly virulent for chicks (oral LD50 of 3x10(3) CFU) and mice (oral LD50 of 8.5x10(3) CFU) and is capable of lethal
infections in pigs, calves and horses. We postulated that attenuated derivatives of this lethal strain would probably induce a higher level of protective immunity than achieved with attenuated derivatives of less virulent S. typhimurium strains such as SR11. To test this hypothesis, we have constructed S. typhimurium
UK-1 Deltacya-12Deltacrp-11 mutant strain chi3985 and its virulence plasmid cured derivative chi4095 to investigate their avirulence and immunogenicity in mice. We found that the mutants are avirulent and able to induce protective immune responses in BALB/c mice. These mutant strains retained wild-type ability to colonize the gut associated lymphoid tissue but reach and persist in spleen and liver at a significantly lower level than the wild-type parent strain. Mice survived oral
infection with >1x10(9) CFU of chi3985 (the equivalent to 10(5) 50% lethal doses of wild-type S. typhimurium
UK-1) and were fully protected against challenge with 10(5)times the LD50 of the wild-type parent. Immunized mice developed a high level of serum
IgG titre to Salmonella LPS and delayed-type
hypersensitivity (DTH) response to S. typhimurium outer
membrane proteins. Compared to the virulence plasmid-containing strain chi3985, the virulence plasmid cured DeltacyaDeltacrp mutant strain chi4095 was more attenuated and less protective, as some mice immunized with chi4095 died when challenged with the wild-type
UK-1 strain. This work demonstrates that S. typhimurium
UK-1 Deltacrp Deltacya mutant strain may be a potential live
vaccine to induce protective immunity against
Salmonella infection or to deliver foreign
antigens to the immune system.