Measuring the rate of conjugal plasmid transfer in a bacterial population using quantitative PCR |
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Authors: | Wan Zhenmao Varshavsky Joseph Teegala Sushma McLawrence Jamille Goddard Noel L |
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Affiliation: | †Department of Physics & Astronomy, Hunter College, City University of New York, New York, New York;‡Department of Physics, The Graduate Center, City University of New York, New York, New York;§Department of Biology, The Graduate Center, City University of New York, New York, New York |
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Abstract: | Horizontal transfer of genes between species is an important mechanism for bacterial genome evolution. In Escherichia coli, conjugation is the transfer from a donor (F+) to a recipient (F−) cell through cell-to-cell contact. We demonstrate what we believe to be a novel qPCR method for quantifying the transfer kinetics of the F plasmid in a population by enumerating the relative abundance of genetic loci unique to the plasmid and the chromosome. This approach allows us to query the plasmid transfer rate without the need for selective culturing with unprecedented single locus resolution. We fit the results to a mass action model where the rate of plasmid growth includes the lag time of newly formed F+ transconjugants and the recovery time between successive conjugation events of the F+ donors. By assaying defined mixtures of genotypically identical donor and recipient cells at constant inoculation densities, we extract an F plasmid transfer rate of 5 × 10−10 (cells/mL · min)−1. We confirm a plasmid/chromosome ratio of 1:1 in homogenous F+ populations throughout batch growth. Surprisingly, in some mixture experiments we observe an excess of F plasmid in the early saturation phase that equilibrates to a final ratio of one plasmid per chromosome. |
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