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The Evolutionary Ecology of Escherichia coli

Abundantly studied and much feared, E. coli has more genomic plasticity than once believed and may have followed various routes to become a pathogen

Valeria Souza, Amanda Castillo, Luis Eguiarte

Bacteria are full of surprises. Consider the most familiar and most studied of all cellular life forms: Escherichia coli. Although it has been subjected to scientific scrutiny for more than a century and has occupied center stage in the development of genetic-engineering technologies in the laboratory, E. coli continues to confound our ideas of how bacteria reproduce, adapt and colonize new niches.

In 1994, for example, Stephen Jay Gould wrote that "the most salient feature of life has been the stability of its bacterial mode from the beginning of the fossil record until today." Just eight years later, new insights into the nature of E. coli and its close relatives have made such a view of the bacterium as a stable organism seem superficial. Having now sequenced certain E. coli genomes and studied the population genetics of numerous bacterial species, we know that although the bacteria have undergone little change in morphology, their genome is a small but dynamic and changing entity that has not stopped evolving.

Figure 1. Pathogenic strains of the bacterium <em>Escherichia coli</em> . . .Click to Enlarge Image

Recent advances in our understanding of the genetics and physiology of E. coli have in fact been spectacular. We know the entire genome sequences of three strains of this species, and the E. coli genome is undoubtedly the best understood of any genome (approximately 70 percent of its genes being "annotated," in the terminology of genomics). It has also been used as a model organism in evolutionary studies, both in natural populations and in the laboratory in so-called "experimental evolution" studies. These investigations have allowed us to understand better the action of two evolutionary forces, selection and mutation, over a very long time. These studies were based on the prevailing notion that these bacteria are clonal, passing genes from generation to generation with little scrambling or swapping—a notion that was entirely upset as the 20th century came to a close.

However, we have just begun to investigate bacterial ecology and evolutionary biology in natural populations. Such studies have gained urgency in connection with recent outbreaks of some pathogenic, foodborne strains of E. coli. These strains have virulence factors and genetic "pathogenicity islands" that have made E. coli, long a killer of infants in poor countries, a growing threat everywhere in the world. It is important to understand how this happens.

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