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Category: Bacterial Pathogenesis
The Evolutionary Model of Escherichia Coli O157:H7, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555817114/9781555819354_Chap13-1.gif /docserver/preview/fulltext/10.1128/9781555817114/9781555819354_Chap13-2.gifAbstract:
The first model for the evolution of Escherichia coli O157:H7 was proposed over a decade ago. This model was based on data from multilocus enzyme electrophoresis, sorbitol fermentation, β-glucuronidase activity, and the presence/absence of the genes encoding the two main antigenic forms of Shiga toxin (Stx): stx1 and stx2. With the higher-resolution, single nucleotide polymorphisms (SNPs)-based phylogenetic framework in place, the stepwise model can be expanded further through the examination of the Stx profiles present within the clades. With clade 7a, the model once again bifurcates with one line losing stx1 (clade 7b) and the other evolving into clade 5a. The observed changes in the tolA locus emphasize the utility of the revised model to elucidate the complex relationships among otherwise closely related isolates. The updated stepwise evolution model highlights the dynamic nature of the phage-encoded Stx variants, as each was independently acquired and lost multiple times during the diversification of O157:H7. Finally, this model reflects a collective effort on the part of multiple investigators to describe the nature, population genetic diversity, and emergence of pathogenic bacteria.
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The original stepwise model for the evolution of O157:H7. This model is based on multilocus enzyme electrophoretic profiles, GUD and SOR phenotypes, and Stx production. Phenotypes of ancestors A1 to A6 are shown. A strain with the traits of ancestor A3 (black circle) has not been reported.
Phylogenetic relationships among SNP genotypes. Nodes are labeled with their corresponding SNP genotype (SG). (A) Phylogenetic network generated using the neighbor-net algorithm with 96 SNPs and 39 SGs from Manning et al. ( 11 ). (B) Phylogenetic network generated using the neighbor-net algorithm with 96 SNPs and 36 SGs from Manning et al. ( 11 ). Genotypes 21, 22, and 27 were excluded from this analysis since the isolates were found to be mixed cultures. (C) Bifurcating tree generated using the neighbor-net algorithm with 89 SNPs and 36 SGs from Manning et al. ( 11 ). The nine previously described clades are marked by the shaded ellipses. SG23 has been reclassified as the new clade 5 based on the number of SNP differences between it and neighboring SGs. Clade 9 has been subdivided into its major phenotypic variants; see text for details.
Expanded model for the stepwise evolution of O157:H7. Phenotypic and genotypic changes predicted to have occurred are indicated. A strain with the traits of clade 5a (shaded circle) has not been reported, but is predicted by the model. Abbreviations: 1, stx 1; 2, stx 2′; 2c, stx 2c; cl, clade. (Inset) Simplified view of clade evolution.
Alignment of tolA alleles with differing numbers of repeats. The 15-bp repeats are located between positions 637 and 792 of the Sakai allele and are labeled A through O. Repeats involved in duplication events are shaded. The proposed directionality of the observed expansion and contraction is indicated.
Stx profiles observed among the clades of O157:H7 and its close relatives
Nucleotide and amino acid sequences of tolA repeats