Shigella and enteroinvasive Escherichia coli strains are derived from distinct ancestral strains of Em colr '

INSERM U 458, HBpitaI Robert Debre, Paris, France The differentiation between Shigella subspecies, and the phylogenetic position of Shigella clones within Escherichia coli clones was determined by analysis of restriction fragment length polymorphisms of rDNA (ribotyping). Seventy-f ive Shigella strains belonging to the four subspecies and 13 enteroinvasive Em coli (EIEC) strains were compared with the 72 E. coli strains of the ECOR collection, which have been classified into four phylogenetic groups (A, B1, B2 and D). Seventeen Shigella dysenteriae ri botypes, 12 Shigella flexneri ri botypes, 23 Shigella boydii ribotypes, 12 Shigella sonnei ribotypes and 13 ElEC ribotypes were identified following digestion with Hindlll and EcoRI. Correspondence analysis of the data showed that 5. boydii serotype 13 strains were distantly related to the other Shigella strains, and that 5. sonnei and 5. flexneri were distinct from 5. boydii and 5. dysenteriae. The ribotypes of Shigella and ECOR strains were indistinguishable, and 5. sonnei, 5. flexneri and most 5. dysenteriae strains were closely related to phylogenetic group D, whereas 5. dysenteriae serotype 1 strains belonged to phylogenetic group B1, and 5. boydii strains were evenly distributed between the two groups. The Shigella strains were distantly related to group B2, which contains E. coli strains frequently implicated in extra-intestinal infections in humans. In contrast, the 13 ElEC strains were more widely distributed between phylogenetic groups B1, A and B2. Thus, there was no primordial Shigella species and Shigella and ElEC strains are derived from different ancestral strains.