Multicopy plasmids facilitate evolutionary innova-tion in bacteria

Álvaro San Millán Cruz
Plasmid Biology and Evolution lab, Hospital Universitario Ramón y Cajal, Madrid, Spain  alvaro.sanmillan@hrc.es

(Seminar in English)

Plasmids play a key role in prokaryotic ecology and evolution because they disseminate adaptive genes across microbial populations thorough horizontal gene transfer. For example, large conjugative plasmids (>20kb) are responsible for the spread of antibiotic resistance among bacterial pathogens, one of the most important problems in modern medicine. However, approximately half of the plasmids in bacteria are non-conjugative, small (<20kb), multicopy plasmids (MCP), which usually present between 10 and 30 copies per cell. Interestingly, these small MCP provide an island of polyploidy in an otherwise haploid bacterial genome. Intrigued by this neglected but fascinating feature, we explored the evolutionary advantages associated with carrying a beneficial gene on a MCP. To this end, we constructed an experimental model system using Escherichia coli MG1655 and the β-lactamase gene blaTEM-1. Using this system we were able to show that MCP accelerate resistance evolution. Moreover, we also showed that MCP promoted the coexistence of ancestral and novel blaTEM alleles in the same cells, allowing bacteria to escape from the trade-offs classically associated with different blaTEM alleles. In summary, our results suggest that MCP are excellent platforms for the creation and maintenance of evolutionary innovations in bacteria, contributing to explain their extreme abundance in nature.

Recent publications:

San Millan et al. (2016) Multicopy plasmids potentiate the evolution of antibiotic resistance in bacteria. Nature Ecology & Evolution 1, 0010.

Rodriguez Beltran et al. (2018) Multicopy plasmids allow bacteria to escape from fitness trade-offs during evolutionary innovation. Nature Ecology & Evolution https://doi.org/10.1038/s41559-018-0529-z.