Salmonella enterica serovar Typhi strains from which SP17, a 134-kilobase island with genes for Vi exopolysaccharide and other functions, has been deleted

Salmonella enterica serovar Typhi has a 134-kb island of DNA identified as salmonella pathogenicity island 7 (SPI7), inserted between pheU and 'pheU (truncated), two genes for tRNA(Phe). SPI7 has genes for Vi exopolysaccharide, for type IVB pili, for putative conjugal transfer, and for sopE bacteriophage. Pulsed-field gel electrophoresis following digestion with the endonuclease I-CeuI, using DNA from a set of 120 wild-type strains of serovar Typhi assembled from several sources, identified eight strains in which the I-CeuI G fragment, which contains SPI7, had a large deletion. In addition, agglutination tests with Vi antiserum and phage typing with Vi phages show that all eight strains are Vi negative. We therefore tested these strains for deletion of SPI7 by multiplex PCR, by microarray analysis, and by sequencing of PCR amplicons. Data show that seven of the eight strains are precise deletions of SPI7: a primer pair flanking SPI7 results in a PCR amplicon containing a single pheU gene; microarrays show that all SPI7 genes are deleted. Two of the strains produce amplicons which have A derived from pheU at bp 27, while five have C derived from 'pheU at this position; thus, the position of the crossover which results in the deletion can be inferred. The deletion in the eighth strain, TYT1669, removes 175 kb with junction points in genes STY4465 and STY4664; the left junction of SPI7 and adjacent genes, as well as part of SPI7 including the vial? operon for Vi exopolysaccharide, was removed, while the right junction of SPI7 was retained. We propose that these deletions occurred during storage following isolation.