Virulence characterization of a strain of Salmonella enterica subspecies houten (subspecies IV) with chromosomal integrated Salmonella plasmid virulence (spv) genes

The Salmonella plasmid virulence genes (spv) are commonly found on plasmids contained in a small number of serotypes of Salmonella belonging to subspecies I, where they are important for survival within macrophages and the establishment of successful systemic infection. However, in this study, spy genes were detected by the polymerase chain reaction in the chromosome of a plasmid-free strain of S. IV 16:z(4), z(32):- (Salmonella subspecies IV). The full range of spy genes (spvR, spvA, spvB, spvC and spvD) was demonstrated, but a 216-bp deletion, accompanied by an insertion of 59-bp cryptic DNA, was present in spvA. S. IV 16:z(4), z(32):- was avirulent in mice and did not become virulent with the introduction of a fully functionally serotype-associated virulence plasmid (SAP) from S. typhimurium. By use of an spvRAB'-chloramphenicol acetyl transferase fusion gene, it was demonstrated that S. IV 16:z(4), z(32):- did not express the spv genes. Salmonella subspecies IV is monophasic, and in phylogenetic analyses it clusters distantly to Salmonella subspecies I, where all the serotypes that normally carry SAPs are found. The mechanisms by which spv genes have been transferred to this serotype remain unknown.