A kinetic model of gene transfer via natural transformation of Azotobacter vinelandii

Horizontal gene transfer allows antibiotic resistance and other genetic traits to spread among bacteria in the aquatic environment. Despite this important role, quantitative models are lacking for one mechanism of horizontal gene transfer, which is natural transformation. The rates of horizontal gene transfer of a tetracycline resistance gene through natural transformation were experimentally determined for motile and non-motile strains of Azotobacter vinelandii. We developed a mathematical model adapted from the mass action law that successfully described the experimentally determined rates of natural transformation of a tetracycline resistance gene for motile and non-motile strains of Azotobacter vinelandii. Transformation rates showed a rapid initial increase, followed by a decrease in the first 30 minutes of the experiment, and then a constant rate was maintained at a given cell and DNA concentration. The proposed model also described the relationship between transformation frequency and varied DNA or cell concentrations. The modeling results revealed that under the given experimental conditions, the gene transformation rate was limited both by the abundance of the tetracycline resistance gene and by cellular activities associated with cell-DNA interactions. This work establishes a quantitative model of natural transformation, suggests a need to further investigate the cell properties affecting transformation rates, and provides a basis for development of comprehensive models of horizontal gene transfer and quantitative risk assessment of antibiotic resistance gene dissemination in the aquatic environment.