Characterization of the virulence regions in the plasmids of three live Salmonella vaccines.

Three live Salmonella vaccines, Zoosaloral "Dessau", Bovisaloral "Dessau", and Suisaloral "Dessau" successfully used in veterinary medicine in eastern Germany were analysed for their plasmid DNA content. Plasmids of 60 MDa (S. typhimurium), 55 MDa and 26 MDa (S. dublin) and 34 MDa (S. choleraesuis) were found. The three large plasmids contained a virulence region as shown by restriction enzyme analysis and hybridization studies with virulence-region-specific DNA probes. Restriction enzyme pattern analysis of the whole plasmids revealed differences between vaccine strains and parental strains. We wanted to know if there were alterations within the virulence region of the vaccine strain plasmids compared with parental strains caused by the attenuation procedure. Therefore, the 6 kb-Cla I fragments of several plasmids were cloned and characterized by restriction enzyme fingerprinting. The expression of the Cla I fragment-encoded proteins was analysed in the minicell-producing strain DS 410. The experiments revealed that the restriction pattern of the 6 kb-Cla I fragments of Zoosaloral and Bovisaloral as well as the PAGE pattern of virulence-region-encoded proteins were in complete accordance with one another, between vaccine strains and the parental strain of Zoosaloral, and with a wild-type strain of S. typhimurium.