Abstract
Transmissible plasmids spread genes encoding antibiotic resistance and other traits to new bacterial species. Here we report that laboratory populations of Escherichia coli with a newly acquired IncQ plasmid often evolve ‘satellite plasmids’ with deletions of accessory genes and genes required for plasmid replication. Satellite plasmids are molecular parasites: their presence reduces the copy number of the full-length plasmid on which they rely for their continued replication. Cells with satellite plasmids gain an immediate fitness advantage from reducing burdensome expression of accessory genes. Yet, they maintain copies of these genes and the complete plasmid, which potentially enables them to benefit from and transmit the traits they encode in the future. Evolution of satellite plasmids is transient. Cells that entirely lose accessory gene function or plasmid mobility dominate in the long run. Satellite plasmids also evolve in Snodgrassella alvi colonizing the honey bee gut, suggesting that this mechanism may broadly contribute to the importance of IncQ plasmids as agents of bacterial gene transfer in nature.
Highlights
Transmissible plasmids spread genes encoding antibiotic resistance and other traits to new bacterial species
We added the plasmid pQGS (Fig. 1a), which has an RSF1010-derived backbone from pMMB67EH36, to E. coli K-12 strain BW2511337. pQGS has three genes inserted into the RSF1010 backbone at a site where accessory genes are found in natural IncQ plasmids[33]
We found that E. coli cells with accessory gene deletion plasmids (DPs) have a much higher fitness than cells containing a mixture of satellite and ancestor plasmids
Summary
Transmissible plasmids spread genes encoding antibiotic resistance and other traits to new bacterial species. With some notable exceptions[16,19], it usually takes a long time—at least several hundred cell generations—and constant selection for plasmid function for compensatory mutations to arise when these mechanisms have been directly observed in laboratory populations of microbes[20]. These long timescales and stringent conditions may be unrealistic in most natural environments.
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