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Bacterial evolution: Resistance is a numbers game

Nature Microbiology volume 1, Article number: 16235 (2016) Cite this article

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Plasmids are well known for spreading antibiotic-resistance genes between bacterial strains. Recent experiments show that they can also act as catalysts for evolutionary innovation, promoting rapid evolution of novel antibiotic resistance.

Plasmids — non-essential loops of DNA — are well known for their ability to fuel bacterial adaptation by moving genes between bacterial lineages, a process known as horizontal gene transfer. This is particularly important in the case of antibiotic resistance, where acquisition of plasmid-borne resistance genes can instantly render a strain impervious to antibiotic treatment. However, in a recent article in Nature Ecology & Evolution, San Millan and colleagues show that plasmids — specifically small, multicopy plasmids — can facilitate bacterial adaptation through an alternative and complementary mechanism1. Many plasmids, particularly those that are small (<10 kb), do not carry sophisticated partitioning systems, instead ensuring transmission to daughter cells by existing in multiple copies within a cell. A consequence of this strategy is that any resistance genes they carry will also be multicopy, leading to both amplification of the encoded phenotype through greater gene expression, and plasticity through random fluctuations in plasmid, and therefore gene, copy number. Most intriguingly, however, is the prediction that multicopy plasmids act as catalysts for evolutionary innovation due to the combined effect of gene duplication and freedom from linkage between alleles, accelerating the appearance, and exaggerating the impact, of novel mutations.

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Figure 1: Comparing the adaptive potential of a resistance gene carried on the chromosome, on a multicopy plasmid and, hypothetically, in multiple copies on the chromosome.

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  1. James P. J. Hall and Ellie Harrison are at the Department of Biology, University of York, York, YO10 5DD, UK.,

    James P. J. Hall & Ellie Harrison

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  1. James P. J. Hall
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  2. Ellie Harrison
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Correspondence to Ellie Harrison.

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Hall, J., Harrison, E. Bacterial evolution: Resistance is a numbers game. Nat Microbiol 1, 16235 (2016). https://doi.org/10.1038/nmicrobiol.2016.235

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