Strains of Escherichia coli causing enterohemorrhagic colitis belonging to the O157:H7 lineage are reported to be highly related. Fifteen strains of E. coli O157:H7 and 1 strain of E. coli O46:H ؊ (nonflagellated) were examined for the presence of potassium tellurite resistance (Te r ). Te r genes comprising terABCDEF were shown previously to be part of a pathogenicity island also containing integrase, phage, and urease genes. PCR analysis, both conventional and light cycler based, demonstrated that about one-half of the Te r E. coli O157:H7 strains (6 of 15), including the Sakai strain, which has been sequenced, carried a single copy of the Te r genes. Five of the strains, including EDL933, which has also been sequenced, contained two copies. Three other O157:H7 strains and the O46:H ؊ strain did not contain the Te r genes. In strains containing two copies, the Te r genes were associated with the serW and serX tRNA genes. Five O157:H7 strains resembled the O157 Sakai strain whose sequence contained one copy, close to serX, whereas in one isolate the single copy was associated with serW. There was no correlation between Te r and the ability to produce Shiga toxin ST1 or ST2. The Te r MIC for most strains, containing either one or two copies, was 1,024 g/ml, although for a few the MIC was intermediate, 64 to 128 g/ml, which could be increased to 512 g/ml by pregrowth of strains in subinhibitory concentrations of potassium tellurite. Reverse transcriptase PCR analysis confirmed that in most strains Te r was constitutive but that in the rest it was inducible and involved induction of terB and terC genes. Only the terB, -C, -D, and -E genes are required for Te r . The considerable degree of homology between the ter genes on IncH12 plasmid R478, which originated in Serratia marcescens, and pTE53, from an E. coli clinical isolate, suggests that the pathogenicity island was acquired from a plasmid. This work demonstrates diversity among E. coli O157:H7 isolates, at least as far as the presence of Te r genes is concerned.Escherichia coli O157:H7 is of major interest in clinical practice, food safety, and evolutionary biology. Although only recognized as a human pathogen in 1982 (16), it has rapidly become prominent as an important cause of food-related and waterborne outbreaks of hemorrhagic colitis throughout the world (13). In a proportion of patients infection progresses to hemolytic-uremic syndrome, characterized by acute renal failure, microangiopathic hemolytic anemia, and thrombocytopenia (13). Each of these secondary sequelae carries significant mortality rates (18).The genome sequences of two strains of enterohemorrhagic E. coli O157:H7 were recently completed (6,14). Comparisons between laboratory E. coli strain K-12 MG1655 (2) and EDL933, an E. coli O157:H7 strain (14), demonstrated that they have a complex relationship since their divergence 4.5 million years ago (14). Homology between the two is interrupted by the presence of hundreds of islands of inserted DNA. K islands are DNA segments present in MG1655 ...

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