Virulence Plasmid-Borne spvB and spvC Genes Can Replace the 90-Kilobase Plasmid in Conferring Virulence to Salmonella enterica Serovar Typhimurium in Subcutaneously Inoculated Mice

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Abstract
In a mouse model of systemic infection, the spv genes carried on the Salmonella enterica serovar Typhimurium virulence plasmid increase the replication rate of salmonellae in host cells of the reticuloendothelial system, most likely within macrophages. A nonpolar deletion in the spvB gene greatly decreased virulence but could not be complemented by spvBalone. However, a low-copy-number plasmid expressing spvBCfrom a constitutive lacUV5 promoter did complement thespvB deletion. By examining a series of spvmutations and cloned spv sequences, we deduced thatspvB and spvC could be sufficient to confer plasmid-mediated virulence to S. enterica serovar Typhimurium. The spvBC-bearing plasmid was capable of replacing all of the spv genes, as well as the entire virulence plasmid, of serovar Typhimurium for causing systemic infection in BALB/c mice after subcutaneous, but not oral, inoculation. A point mutation in the spvBC plasmid preventing translation but not transcription of spvC eliminated the ability of the plasmid to confer virulence. Therefore, it appears that both spvB and spvC encode the principal effector factors for Spv- and plasmid-mediated virulence of serovar Typhimurium.