Résumé
Plasmids are nucleic acid molecules that can drive their own replication
in a living cell. They can be transmitted horizontally and can thrive in
the host cell to high copy numbers. Plasmid replication and gene
expression consume cellular resources and cells carrying plasmids incur
fitness costs. But many plasmids carry genes that can be beneficial under
certain conditions, allowing the cell to endure in the presence of
antibiotics, toxins, competitors or parasites. Horizontal transfer of
plasmid-encoded genes can thus instantaneously confer differential
adaptation to local or transient selection conditions. This conflict
between cellular fitness and plasmid spread sets the scene for multilevel
selection processes. We have engineered a system to study the short term
evolutionary impact of different synonymous versions of a plasmid-encoded
antibiotic resistance gene. Applying experimental evolution under
different selection conditions and deep sequencing allowed us to show
rapid local adaptation to the presence of antibiotic and to the specific
version of the resistance gene transferred. We describe the presence of
clonal interference at two different levels: at the within-cell level,
because a single cell can carry several plasmids, and at the between-cell
level, because a bacterial population may contain several clones carrying
different plasmids and displaying different fitness in the
presence|absence of antibiotic. Understanding the within-cell and
between-cell dynamics of plasmids after horizontal gene transfer is
essential to unravel the dense network of mobile elements underlying the
worldwide threat to public health of antibiotic resistance.