FEATURE ARTICLE
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
Plasmidic Plasticity
Bacteria carry some of their genetic information in the form of extrachromosomal elements known as plasmids. These circular DNA molecules are the most dynamic component of the bacterial genome, since they move easily between strains. Plasmids are common in E. coli, although the bacterium can survive without a plasmid or, at the other extreme, store a good percentage of its genome in such elements. Around 300 kinds of plasmids have been described in the species. Within them one can find information for assimilating rare sugars and for producing colicins (substances that kill possible competitors of the same species); resistance to antibiotics and heavy metals; immunity against bacteria-targeting viruses and colicins; genes that code for genetic exchange; and filaments related to pathogenesis and the production of toxins.
In general the distribution of a plasmid depends not only on its range of bacterial hosts, but also on a complex system of incompatibility among plasmids of the same type. A bacterium will not accept new plasmids of a type that it already has. Although the movement of plasmids is not well understood from a molecular point of view, it is known that there are conjugative plasmids, which are capable of moving around by conjugation. These plasmids are relatively large (usually more than 50 kilobases) and contain genes necessary for bacterium-to-bacterium recognition; for forming the projections, or pili, needed for mating; and for allowing the movement of DNA. Nonconjugative plasmids also can be transferred when conjugation takes place.
Many plasmids are capable of transferring themselves among different species. Different models of plasmid distribution are possible in bacteria. Based on studies in 1997 of Salmonella and E. coli by E. Fidelma Boyd and Daniel Hartl of Harvard University, our laboratory has advanced a panmictic, or random-mating, model of plasmid distribution. Some plasmids are extremely successful and, being promiscuous as well, are overrepresented in bacterial populations. These epidemic plasmids allow bacteria to acquire virulence factors or resistance to antibiotics by horizontal transfer. Promiscuous plasmids can contribute to coevolution; as they move between bacterial species, the extrachromosomal genomes of those species may evolve in parallel. However, there are also clonal plasmids, plasmids that are only transferred from parent to child in asexual reproduction, as well as plasmids whose transfer is limited to specific genomes within the same bacterial species.
The great genomic plasticity of E. coli has conferred on it an extraordinary ecological plasticity. E. coli can adapt rapidly to different environments and is capable of existing as a free-living organism or in commensal mutualism in the colons of mammals and birds. Additionally, in the interior of host organisms, it can invade other niches successfully. In this way it can become a dangerous pathogen that successfully colonizes people and animals.
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