GENOTYPIC AND PHENOTYPIC CHANGES IN THE EMERGENCE OF ESCHERICHIA-COLIO157-H7

Escherichia toil 0157:H7 is a foodborne pathogen distinguished from ty pical E. coli by the production of Shiga toxins (Stx) and the inabilit y to ferment sorbitol (SOR) and to express beta-glucuronidase (GUD) ac tivity. An allele-specific probe for the GUD gene (uidA) and multilocu s enzyme electrophoresis were used to elucidate stages in the evolutio nary emergence off. coli 0157: H7. A point mutation at +92 in uidA was found only in 0157:H7 and its nonmotile relatives, including a SOR+ 0 157:H- clone implicated in outbreaks of hemolytic-uremic syndrome in G ermany. The results support a model in which 0157:H7 evolved sequentia lly from an 055:H7 ancestor, first by acquiring the Stx2 gene and then by diverging into two branches; one became GUD(-) SOR-, resulting in the O157:H7 clone that spread worldwide, and the other lost motility, leading to the 0157:H- clone that is an increasing public health probl em in Europe.