Multiple insertions of fimbrial operons correlate with the evolution of Salmonella serovars responsible for human disease

On centisome 7, Salmonella spp. contain a large region not present in the c orresponding region of Escherichia coli. This region is flanked by sequence s with significant homology to the E. coli tRNA gene aspV and the hypotheti cal E. coli open reading frame yafV. The locus consists of a mosaic of diff erentially acquired inserts forming a dynamic cs7 region of horizontally tr ansferred inserts. Salmonella enterica subspecies I, responsible for most S almonella infections in warm-blooded animals, carries a fimbrial gene clust er (saf) in this region as well as a regulatory gene (sinR). These genes ar e flanked by inverted repeats and are inserted in another laterally transfe rred region present in most members of Salmonella spp. encoding a putative invasin (pagN). S. enterica subspecies I serovar Typhi, the Salmonella sero var that causes the most severe form of human salmonellosis, contains an ad ditional insert of at least 8 kb in the sinR-pagN intergenic region harbour ing a novel fimbrial operon (tcf) similar to the coo operon encoding the CS 1 fimbrial adhesin expressed by human-specific enterotoxigenic E. coli. It is suggested that the multiple insertions of fimbrial genes that have occur red in the cs7 region have contributed to phylogenetic diversity and host a daptation of Salmonella spp.