CLONAL STRUCTURE AND VIRULENCE FACTORS IN STRAINS OF ESCHERICHIA-COLIOF THE CLASSIC SEROGROUP O55

Virulence properties and genetic variation as determined by multilocus enzyme electrophoresis were studied in 70 strains of Escherichia coli O55, a common serogroup of enteropathogenic E. coli (EPEC), a major c ause of infantile diarrhea in developing countries. Nearly 40% of the strains were originally isolated in Brazil and represented serotypes O 55:H6, O55:H7, and O55:H51 and nonmotile (O55:H-) strains. The analysi s of electrophoretic variants of 20 enzymes defined seven distinct ele ctrophoretic types (ETs). ET 1 was represented by 41% of the strains, including strains which usually hybridized with DNA probes for the int imin gene (eaeA), the EPEC adherence plasmid (EAF), and the gene for t he pilin subunit of the bundle-forming pilus (bfpA). The ET 1 strains were also typically serotype O55:H6, displayed localized adherence (LA ) in tissue culture assays, and were positive in the fluorescent-actin staining test for intimate cell adherence. These same characteristics were observed in the closely related ETs 2 to 4, which clustered in t he same branch as ET 1. No known virulence marker could be identified in ET 6. ET 5 included 23 strains, all of which carried the eaeA gene but otherwise displayed a striking array of distinct virulence traits. This ET was represented by O55:H7 strains with phenotypes as diverse as the simultaneous expression of LA and diffuse adherence and the abi lity to form a newly described adherence pattern, called LA-like adher ence. The results suggest that ET 5 marks a special pathogenic clone w ith a propensity to acquire virulence factors which may facilitate the emergence of new pathogenic strains.