MATHEMATICAL MODELING OF THE EXPERIMENTAL EVOLUTION OF TRANSGENIC BACTERIA: “PLASMID PARADOX”

In the present work, a mathematical model for the distribution of transgenic bacteria cells by the number of plasmids contained in them is constructed. This makes it possible to study the patterns of survival of transgenic microorganisms under certain environmental conditions, taking into account various characteristics of transgenic strains, for example, the stability of plasmids in new cells, the “population cost” of maintaining conjugative and non-conjugative plasmids, the efficiency of expression of genes cloned on plasmids, and others. The "plasmid paradox" lies in the fact that although the presence of plasmids containing cloned genes usually reduces the specific growth rate of transgenic bacteria, but with prolonged growth of bacteria under non-selective conditions ("experimental evolution", 20-30 or more generations of the original form without antibiotics) “cost of fitness” is reduced by an order of magnitude or two. An analysis of the experimental results leads to the conclusion that this is due to a change in the difference in the growth rates of plasmid and plasmid-free cells, with a change in the probability of formation of plasmid-free cells, the copy number of plasmids, and the expression of cloned genes in cells of transgenic bacteria under various environmental conditions.