Effect of growth rate and selection pressure on rates of transfer of an antibiotic resistance plasmid between E. coli strains

• Transfer of tetracycline resistance plasmids between E. coli strains was studied.
• The rate of plasmid transfer depends on growth rate and on the concentration of tetracycline.
• Modelling suggests takeover of the recombinant in continuous culture is slow.
• Two functional mutations in the acceptor genome coincided with transfer of the plasmid.
• The plasmid was transferred without mutations.

Antibiotic resistance increases costs for health care and causes therapy failure. An important mechanism for spreading resistance is transfer of plasmids containing resistance genes and subsequent selection. Yet the factors that influence the rate of transfer are poorly known. Rates of plasmid transfer were measured in co-cultures in chemostats of a donor and a acceptor strain under various selective pressures. To document whether specific mutations in either plasmid or acceptor genome are associated with the plasmid transfer, whole genome sequencing was performed. The DM0133 TetR tetracycline resistance plasmid was transferred between Escherichia coli K-12 strains during co-culture at frequencies that seemed higher at increased growth rate. Modeling of the take-over of the culture by the transformed strain suggests that in reality more transfer events occurred at low growth rates. At moderate selection pressure due to an antibiotic concentration that still allowed growth, a maximum transfer frequency was determined of once per 1011 cell divisions. In the absence of tetracycline or in the presence of high concentrations the frequency of transfer was sometimes zero, but otherwise reduced by at least a factor of 5. Whole genome sequencing showed that the plasmid was transferred without mutations, but two functional mutations in the genome of the recipient strain accompanied this transfer. Exposure to concentrations of antibiotics that fall within the mutant selection window stimulated transfer of the resistance plasmid most.