Comparison of different attenuation strategies in development of a Salmonella hadar vaccine

The purpose of this work was to develop a live, attenuated vaccine strain to protect chickens against colonization by group C Salmonella. We constructed two candidate vaccines: a Δcya Δcrp derivative and a ΔphoP derivative of Salmonella hadar. White Leghorn chickens were vaccinated at day of age and at 2 wk with one of the two strains. A nonvaccinated group served as a control. At 4 wk of age, all birds were challenged with wild-type S. hadar and necropsied 6 days later. Numbers of S. hadar in the ceca were determined. Enzyme-linked immunosorbent assay-derived serum immunoglobulin G responses against S. hadar lipopolysaccharide indicated that both strains induced a serum antibody response. The average optical density₄₅₀ for birds vaccinated with the ΔphoP or Δcya Δcrp derivatives was 0.456 and 0.881, respectively. Although the Δcya Δcrp derivative induced higher levels of serum antibody, it did not provide an immune response protective against colonization by S. hadar. Conversely, birds vaccinated with the ΔphoP strain showed significant protection against 5. hadar challenge. Seventy percent of the nonvaccinates, 60% of the Δcya Δcrp vaccinates, and 15% of ΔphoP vaccinates were positive for S. hadar in tissues. In a second experiment, birds were vaccinated with either the ΔphoP strain or buffer and challenged with a 10-fold higher dose than in the first experiment. After challenge, all of the birds in both groups were colonized. The geometric mean number of cecal S. hadar isolated from the control group was 1.0 × 10⁶ colony-forming units (CFU)/g, and from the vaccinated group, this value was 32 CFU/g, indicating a four to five log reduction in colonization by the challenge strain.