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Genotypic and Phenotypic Changes in the Emergence of Escherichia coli O 157 : H 7

Escherichia coli O157:H7 is a foodborne pathogen distinguished from typical E. coli by the production of Shiga toxins (Stx) and the inability to ferment sorbitol (SOR) and to express bglucuronidase (GUD) activity. An allele-specific probe for the GUD gene (uidA) and multilocus enzyme electrophoresis were used to elucidate stages in the evolutionary emergence of E. coli O157: H7. A point mutation at /92 in uidA was found only in O157:H7 and its nonmotile relatives, including a SOR/ O157:H0 clone implicated in outbreaks of hemolytic-uremic syndrome in Germany. The results support a model in which O157:H7 evolved sequentially from an O55:H7 ancestor, first by acquiring the Stx2 gene and then by diverging into two branches; one became GUD0 SOR0, resulting in the O157:H7 clone that spread worldwide, and the other lost motility, leading to the O157:H0 clone that is an increasing public health problem in Europe.