Immunogenicity and protective efficacy of a Salmonella Enteritidis sptP mutant as a live attenuated vaccine candidate

Background: Salmonella enterica serovar Enteritidis (S. Enteritidis) is a highly adaptive pathogen in both humans and animals. As a Salmonella Type III secretion system (T3SS) effector, Salmonella protein tyrosine phosphatase (SptP) is critical for virulence in this genus. To investigate the feasibility of using C50336 Delta sptP as a live attenuated oral vaccine in mice, we generated the sptP gene deletion mutant C50336 Delta sptP in S. Enteritidis strain C50336 by lambda-Red mediated recombination and evaluated the protective ability of the S. Enteritidis sptP mutant strain C50336.sptP against mice salmonellosis. Results: We found that C50336.sptP was a highly immunogenic, effective, and safe vaccine in mice. Compared to wild-type C50336, C50336.sptP showed reduced virulence as confirmed by the 50% lethal dose (LD50) in orally infected mice. C50336 Delta sptP also showed decreased bacterial colonization both in vivo and in vitro. Immunization with C50336.sptP had no significant effect on body weight and did not result in obvious clinical symptoms relative to control animals treated with phosphate-buffered saline (PBS), but induced humoral and cellular immune responses at 12 and 26 days post inoculation. Immunization with 1 x 10(8) colony-forming units (CFU) C50336 Delta sptP per mouse provided 100% protection against subsequent challenge with the wild-type C50336 strain, and immunized mice showed mild and temporary clinical symptoms as compared to those of control group. Conclusions: These results demonstrate that C50336.sptP can be a live attenuated oral vaccine for salmonellosis.