Study of polymorphic variable-number of tandem repeats loci in the ECOR collection and in a set of pathogenic Escherichia coli and Shigella isolates for use in a genotyping assay

The Escherichia coli (E. coli) reference collection, ECOR, consists of 72 strains that are representative of the genotypic diversity, as indexed by multilocus enzyme electrophoresis (MLEE), in the species as a whole. MLEE revealed 4 main phylogenetic groups designated A, B1, B2 and D. We present a study of the relationship between the ECOR strains as determined by polymorphisms in seven variable-number of tandem repeats (VNTR) loci. Seven tandem repeats that were present in more than one of the fully sequenced E. coli strains were selected, and primers were constructed in order to amplify the targets in all species where the loci were present. The combined result for all VNTR loci was adapted as a multiple-locus variable-number tandem repeats analysis (MLVA) and showed that the ECOR collection was divided into 63 distinct genotypes. The ECOR phylogenetic groups defined by MLEE were not well conserved by MLVA. A set of 61 pathogenic isolates of both E. coli and Shigella spp. was then tested with the same set of VNTR loci, and revealed 54 distinct genotypes. In addition, the MLVA method was used to genotype isolates from patients and suspected sources in a recent outbreak of E. coli O103 in Norway. The pathogenic E. coli isolates contained the diarrhea causing categories EIEC, EAEC, STEC, ETEC and EPEC. Shigella isolates were of species S. flexneri, S. boydii, S. sonnei and S. dysenteriae. The MLVA method rapidly genotyped all isolates in the study at a Simpson's index of diversity of D=0.98.