When performing comparisons between pathogenic and non-pathogenic strains of E.

Question

When performing comparisons between pathogenic and non-pathogenic strains of E. coli, you find that the pathogenic strains have a large amount of additional genes, that are absent in the non-pathogenic strains.

A) Describe how you would try to determine which DNA was acquired by the pathogenic strains, and which was lost by the non-pathogenic strains, relative to their last common ancestor?

B) Upon further research, you learn that many of the unique DNA in the pathogenic E. coli is thought to have been acquired by “horizontal gene transfer.” Explain what this is.

Explanation / Answer

A. The virulence factors that distinguish the various E. coli pathotypes were acquired from numerous sources, including plasmids, bacteriophages, and the genomes of other bacteria. Pathogenicity islands, relatively large (>10 kb) genetic elements that encode virulence factors and are found specifically in the genomes of pathogenic strains, frequently have base compositions that differ drastically from that of the content of the rest of the E. coli genome, indicating that they were acquired from another species.

In discussing the diversity of pathogenic forms of this versatile species, we distinguish between an isolate’s pathotype, a classification of E. coli into groups that have a similar mode of pathogenesis and cause clinically similar forms of disease, and the pathogenic clone, bacteria of a genetic lineage within a bacterial species that share similarities because of recent descent from a common ancestral cell.

The dendrogram analysis of polymorphism at protein loci studied by multilocus enzyme electrophoresis. The number of differences between strains is converted to a genetic distance assuming that each difference results from at least one amino acid–altering mutation at the DNA level. The diagram can be interpreted as a hypothetical phylogeny of strains that can be tested by gathering independent data. Early evidence for the clonal nature of pathogenic E. coli was seen in the repeated recovery of identical serotypes and biotypes from separate outbreaks of disease. The idea of widespread pathogenic clones gained support from the study of protein polymorphisms, first with patterns of the major outer proteins and then through the broad application of multilocus enzyme electrophoresis.

B. Bacterial genomes evolve through mutations, rearrangements or horizontal gene transfer. Besides the core genes encoding essential metabolic functions, bacterial genomes also harbour a number of accessory genes acquired by horizontal gene transfer that might be beneficial under certain environmental conditions. The horizontal gene transfer contributes to the diversification and adaptation of microorganisms, thus having an impact on the genome plasticity.

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