Rewiring of one carbon metabolism in Salmonella serves as an excellent live vaccine against systemic salmonellosis

Live attenuated vaccines are superior to the killed or subunit vaccines. We designed a Salmonella Typhimurium strain by deleting folD gene (encoding methylenetetrahydrofolate dehydrogenase-cyclohydrolase) in the presence of a heterologous fhs gene (encoding formyltetrahydrofolate synthetase) and tested its vaccine potential under stringent conditions of lethal and sub-lethal challenges with virulent Salmonella in the murine model. The efficacy of the vaccine in conferring protection against Salmonella infection was determined in a wide range of host conditions of systemic infection, corresponding to human young adults, neonates, geriatric age and, importantly, to the immune compromised state of pregnancy. The standardized vaccination regime comprised a primary dose of 10 ⁴ CFU/animal followed by a booster dose of 10 ² CFU/animal on day 7. Challenge with the virulent pathogen was done at day 7 post-administration of the booster. Subsequently, the mortality, morbidity, systemic colonization, antibody response and cytokine profiling were determined. The vaccinated cohort showed a strong protection against virulent pathogen in all models tested. The serum anti-Salmonella antibody titers and cytokine levels were significantly higher in the vaccinated cohort compared to the mock vaccinated cohort. Thus, we report the development and validation of a live attenuated vaccine candidate conferring excellent protection against Salmonellosis and typhoid fever.