Preparation and comparative evaluation of conjugate and whole-cell killed vaccine candidates against Salmonella enterica serovar Typhimurium.

Poultry is one of the largest and rapidly growing industries in Pakistan. Avian salmonellosis is among the most prevalent bacterial infections in poultry. A preventive vaccine in chicken is needed against food-borne zoonotic Salmonella enterica serovar Typhimurium. The surface polysaccharide-protein conjugate vaccine and whole-cell killed vaccine candidates were prepared against a poultry isolate of S. Typhimurium followed by immunogenicity evaluation in mice and protection studies in chicken. The O-specific polysaccharide (OSP) antigen of S. Typhimurium was directly conjugated to two carrier proteins: diphtheria toxoid (DT) and bovine serum albumin (BSA) using reductive amination chemistry whereas, whole-cell killed vaccine candidates were prepared using formalin and heat. Direct conjugation of S. Typhimurium OSP with DT and protection studies of the prepared conjugates in chicken have not been reported before. This enabled conjugate vaccine candidates to produce significantly (p < 0.05) better immunity in mice than the control group and comparable to that of killed vaccines, that presented a very high response. In chicken, the prepared vaccine candidates elicited adequate immunogenicity comparable to the available commercial live attenuated vaccine. The layers in vaccinated groups, specifically in conjugate vaccinated groups, were well protected against the given pathogenic challenge as compared to the unvaccinated birds which showed reduced growth and feed consumption, oviduct atrophy and decreased egg production in addition to other gross and histopathological lesions. The current study reports the first successful comparison of the S. Typhimurium OSP-Protein conjugate vaccine candidates with live and killed Salmonella vaccines in poultry.