540 The Journal of Clinical Investigation| March 2001| Volume 107| Number 5 first with patterns of the major outer proteins and then through the broad application of multilocus enzyme electrophoresis (1). Recent sequence comparisons have shown that a phylogenetic approach based on the clone concept, however, is complicated by recombination events, which, like mutations, contribute to the divergence of bacterial genomes in nature (reviewed in refs. 6, 7). The diversity of pathotypes and their genetic relatedness are illustrated in the dendrogram (Figure 1). This analysis, based on multilocus enzyme electrophoresis, includes strains of the pathotypes associated with enteric disease and strains representing the major phylogenetic groups (groups A, B1, B2, and D) of the E. coli Reference (ECOR) collection, a set of natural isolates chosen to represent genetic variation in the E. coli species as a whole. The dendrogram includes pathogenic strains of the most common clones of five serogroups (O26, O111, O55, O128, and O157) associated with infectious diarrheal disease; these widespread clones are referred to as the DEC (diarrheagenic E. coli) clones. In addition, there are representatives of the common clones of enteroinvasive E. coli (8). The genetic distance between clones based on alleles detected by enzyme electrophoresis strongly correlates with the amount of sequence divergence in housekeeping genes (Figure 1b). The sequence data indicate that the deepest branches in the dendrogram reflect about 8% divergence at synonymous sites. It should be emphasized that because of past recombination, the dendrogram cannot be a true phylogeny but can only serve as a framework for investigating the evolution of the various clones. Pathotypes of E. coli are concentrated in clonal groups, although some pathotypes are found in multiple lineages (Figure 1). In particular, there are two clusters of enteropathogenic E. coli (EPEC) that are associated with infantile diarrhea and two clusters of enterohemorrhagic E. coli (EHEC) that are associated with hemorrhagic colitis. The EPEC 1 and EHEC 1 clusters are highly divergent, whereas both EPEC 2 and EHEC 2 are more closely related to one another and fall into the B1 group of ECOR. The finding that independent lineages harbor the same virulence factors and cause clinically similar disease indicates that certain pathotypes have evolved multiple times in different clonal groups (7). EPEC and EHEC groups are phylogenetically distinct from the enteroinvasive E. coli (EIEC), bacteria that cause dysentery and are most closely related to strains of the ECOR group A. The clonal groups associated with enteric diseases are also different from those recovered in extraintestinal infections including uropathogenicE. coli (UPEC), which are found near the bottom of the dendrogram in the B2 and D groups of ECOR (9, 10). Below, we focus on the virulence factors and pathogenic mechanisms of two major pathotypes, EPEC and

EHEC, for which data exist both on the genetic basis of disease and on the phylogenetic history of the strains. These examples are put forward to demonstrate how genetic polymorphisms among E. coli strains profoundly influence disease. The reader is referred elsewhere for reviews of diarrheagenic (11) and extraintestinal E. coli pathogenesis (12).