Shiga toxin-producing Escherichia coli (STEC) strains belonging to serogroup O26 have been associated with sporadic cases and outbreaks of hemorrhagic colitis and hemolytic uremic syndrome. In addition to chromosomal virulence genes, STEC strains usually harbor a large plasmid that carries genes associated with pathogenicity. The complete nucleotide sequence and genetic organization of 6 plasmids carried by STEC O26:H11 strain H30 were determined. The large virulence plasmid (pO26-Vir) was approximately 168 kb in size and contained 196 open reading frames (ORFs). pO26-Vir possesses a mosaic structure and shows similarity to the virulence plasmids in locus of enterocyte effacement (LEE)-negative STEC O113:H21 EH41 (pO113), in E. coli clinical strain C1096 (pSERB1), and in E. coli O157:H7 RIMD 0509952 (pO157). Plasmid pO26-Vir shares several highly conserved regions with pO157 and carries important virulence genes, including toxB, katP, espP, and the hly gene cluster. In addition, pO26-Vir possesses genes encoding for type IV pili (pilL-V). The second largest plasmid, pO26-L (73 kb) contains 101 ORFs. pO26-L carries the tetracycline resistance gene and has regions that show similarity to the E. coli conjugative resistance plasmid NR1. The third largest plasmid, pO26-S4 (5.8 kb), is homologous to the ColE2 colicinogenic plasmid that encodes for colicin E2. The remaining 3 plasmids, pO26-S1 (1.5 kb), pO26-S2 (3.1 kb), and pO26-S3 (4.2 kb), carry very little genetic information except for putative proteins involved in plasmid replication and DNA maintenance. The data presented underscore the diversity among the STEC virulence plasmids and provide insights into the evolution of these plasmids in STEC strains that cause serious human illness.
Published by Elsevier GmbH.