SALMONELLA-VIRULENCE PLASMID - MODULAR ACQUISITION OF THE SPV VIRULENCE REGION BY AN F-PLASMID IN SALMONELLA-ENTERICA SUBSPECIES-I AND INSERTION INTO THE CHROMOSOME OF SUBSPECIE-II, SUBSPECIE-IIIA, SUBSPECIE-IV AND SUBSPECIE-VII ISOLATES

The spv operon is common to all Salmonella virulence plasmids. DNA hyb ridization analysis indicates that the spy region is limited in distri bution to serovars of Snlmonebla enterica subspecies I, II, IIIa, IV, and VII and is absent from Salmonella bongori isolates. Among strains of subspecies II, IIIa, and VII, all isolates examined contained seque nces that hybridized with the sly region. However, among isolates of s ubspecies I, DNA sequences capable of hybridizing with the spv region were found in some isolates of certain serovars. Furthermore, in isola tes of subspecies I, the virulence plasmid was found in the same set o f isolates as an F-related plasmid, as determined by the presence of t he spv region of the virulence plasmid and the finO, traD, and repA se quences of the F-plasmid. The concordance of the virulence plasmid and all three F-plasmid sequences in subspecies I serovar Choleraesuis, P aratyphi, and Typhimurium is most easily explained if the spv, region is carried in an F-related plasmid in these isolates. In contrast, amo ng S. enterica subspecies II, IIIa, IV, and VII, the isolates that con tain spv sequences did not hybridize with an F-related plasmid or any other identifiable plasmid. With the use of pulse-field gel electropho resis, the spv region in subspecies II, IIIa, and VII was found to he encoded on the chromosome. Analysis of the phylogenetic distribution o f spv among Salmonella isolates and comparative nucleotide sequence an alysis of spvA and spvC suggests that the spv region was acquired very recently, after speciation of the salmonellae.