Virulence attenuation and live vaccine potential of aroA, crp cdt cya, and plasmid-cured mutants of Salmonella enterica serovar Abortusovis in mice and sheep

Abstract

Three live vaccine candidates of Salmonella enterica subspecies I serotype Abortusovis (aroA, cya crp cdt, and plasmid-cured strains) have been developed, and their efficacies in inducing humoral antibodies and protecting against abortion after challenge with wild-type strain SS44 were evaluated in sheep. Following estrus synchronization, animals were immunized 3 weeks after fertilization and boosted once 3 weeks later. Following challenge with wild-type SS44, pregnancy failure of vaccinated ewes was reduced compared to that of nonimmunized controls. Attenuation of each vaccine was also assessed in challenge experiments with nonimmunized pregnant ewes and in BALB/c mice. All three vaccine candidates appear to be safe for use in sheep and provide a model for the development of live vaccine candidates against naturally occurring ovine salmonellosis.

FIG. 1.

Mean rectal temperatures of ewes immunized with aroA strain SU304 (solid circles), plasmid-cured strain SU40 (squares), or crp cdt cya strain SSM189 (triangles) or nonimmunized (open circles). Temperatures were recorded weekly from the day of first immunization up to 7 weeks postchallenge with wild-type strain SS44.

FIG. 2.

Anti-OMP IgM values in pregnant ewes immunized with vaccine strain SU304 (solid circles), plasmid-cured strain SU40 (squares), or crp cdt cya strain SSM189 (triangles) and in nonimmunized ewes (open circles). All animals were challenged with wild-type SS44 10 weeks after the second vaccine administration. Arrows indicate times of first immunization, second immunization, and challenge with wild-type strain SS44. Values are expressed as means and standard deviations of OD readings at 450 nm. The horizontal line represents the cutoff value (0.424), calculated as described in Materials and Methods.

FIG. 3.

Anti-OMP IgG values in pregnant ewes immunized with vaccine strain SU304 (solid circles), plasmid-cured strain SU40 (squares), or crp cdt cya strain SSM189 (triangles) and in nonimmunized ewes (open circles). All animals were challenged with wild-type SS44 10 weeks after the second vaccine administration. Arrows indicate times of first immunization, second immunization, and challenge with wild-type strain SS44. Values are expressed as means and standard deviations of OD readings at 450 nm. The horizontal line represents the cutoff value (0.254), calculated as described in Materials and Methods.