Distribution and structural variation of the she pathogenicity island in enteric bacterial pathogens

Shigella flexneri serotype 2a carries a chromosomal pathogenicity island (PAI), termed the she PAI, that has been implicated in the pathogenesis of diarrhoeal disease. The complete nucleotide sequence and genetic organisation of the she PAI of S. flexneri 2a strain YSH6000T was determined recently. In the current study the distribution and structure of the she PAI was investigated by PCR and Southern analysis in 65 isolates of enteric pathogens including Shigella spp., enterohaemorrhagic Escherichia coli (EHEC), enteropathogenic E. coli (EPEC), enteroinvasive E. coli (EIEC), Yersinia enterocolitica and Salmonella enterica serovar Typhimurium. The study showed that the she PAI has undergone a variety of structural changes, defined by the presence or absence of specific marker genes in the PAI. The she PAI or structural variants of this element were found in all species of Shigella as well as in EIEC, EHEC and EPEC. No evidence of the PAI was found in Y. enterocolitica or Sal. Typhimurium. The structural form of the she PAI that exists in strain YSH6000T was present in all strains of S. flexneri serotype 2a and in some strains of S. flexneri serotypes 2b and 3c. Variants of the PAI that were missing one or more marker regions were found in all species of Shigella and in pathogenic strains of E. coli. In all strains, the PAIs have inserted into either pheV or a phe tRNA gene in another location on the chromosome. It was concluded that the she PAI is one of several closely related genetic elements that have disseminated throughout Shigella and pathogenic strains of E. coli and diverged into distinct stuctural forms.