Locus of enterocyte effacement from Citrobacter rodentium: Sequence analysis and evidence for horizontal transfer among attaching and effacing pathogens

The family of attaching and effacing (A/E) bacterial pathogens, which inclu des diarrheagenic enteropathogenic Escherichia coli (EPEC) and enterohemorr hagic E. coli (EHEC), remains a significant threat to human and animal heal th. These bacteria intimately attach to host intestinal cells, causing the effacement of brush border microvilli. The genes responsible for this pheno type are encoded in a pathogenicity island called the locus of enterocyte e ffacement (LEE). Citrobacter rodentium is the only known murine A/E pathoge n and serves as a small animal model for EPEC and EHEC infections. Here we report the full DNA sequence of C. rodentium LEE and provide a comparative analysis with the published LEEs from EPEC, EHEC, and the rabbit diarrheage nic E. coli strain RDEC-1. Although C. rodentium LEE shows high similaritie s throughout the entire sequence and shares all 41 open reading frames with the LEE from EPEC, EHEC, and RDEC-1, it is unique in its location of the r orf1 and rorf2/espG genes and the presence of several insertion sequences ( IS) and IS remnants. The LEE of EPEC and EHEC is inserted into the selC tRN A gene. In contrast, the Citrobacter LEE is flanked on one side by an opero n encoding an ABC transport system, and an IS element and sequences homolog ous to Shigella plasmid R100 and EHEC pO157 flank the other. The presence o f plasmid sequences next to C. rodentium LEE suggests that the prototype LE E resided on a horizontally transferable plasmid. Additional sequence analy sis reveals that the 3-kb plasmid in C. rodentium is nearly identical to p9 705 in EHEC O157:H7, suggesting that horizontal plasmid transfer among A/E pathogens has occurred. Our results indicate that the LEE has been acquired by C. rodentium and A/E E. coli strains independently during evolution.