Estimating the rate of plasmid transfer: An end-point method

We describe a method for determining the rate parameter of conjugative plasmid transfer that is based on single
estimates of donor, recipient and transconjugant densities and the growth rate in exponential phase of the mating
culture. The formula for estimating the plasmid transfer rate, y, was derived from a mathematical model describing
cell growth and plasmid transfer in batch culture. Computer simulations were used to explore the sensitivity of this
method to the realities of bacterial life, such as growth rate differences, plasmid segregation and transitory
derepression of pilus synthesis. As predicted by the theory, mating experiments with the plasmid Rl in Eschericlria
coli K12 demonstrated that the estimate y is unaffected by cell density, donocrecipient ratio and mating time.
Unlike previous techniques, our method allows us to investigate the effect of environmental factors on plasmid
transfer rates when these factors also influence population growth rates. To illustrate this, we examined the effect
of temperature on the rate of plasmid transfer.