The existence conditions for bacterial plasmids: Theory and reality

A statistical analysis of the formation of plasmid-free cells in populations of Escherichia coli

By methods analogous to those used in the classical statistical analysis of bacterial mutation, we have analyzed the formation of plasmid-free cells in populations of Escherichia coli harboring pBR322-derived plasmids. Application of fluctuation tests and papilla analysis suggested that there is a high variance in the probability that a plasmid-containing cell will produce a plasmid-free daughter cell. Apparently a subpopulation of plasmid-containing cells gives rise to progeny that produces plasmid-free cells with a high and unpredictable rate. This finding raises the question of whether plasmid maintenance can be adequately described by the conventional mathematical models.

The accessory genetic elements of bacteria: existence conditions and (co)evolution

The accessory elements of bacteria, transposons, plasmids and phages provide tillable as well as fertile ground for studying the ecology and (co)evolution of parasite and symbiont interactions with their hosts. The recent climate has yielded a bountiful harvest of delicious evolutionary food for thought.

The specific growth rate of Pseudomonas putida PAW1 influences the conjugal transfer rate of the TOL plasmid

The kinetics of the conjugal transfer of a TOL plasmid were investigated by using Pseudomonas putida PAW1 as the donor strain and P. aeruginosa PAO 1162 as the recipient strain. Short-term batch mating experiments were performed in a nonselective medium, while the evolution of the different cell types was determined by selective plating techniques. The experimental data were analyzed by using a mass action model that describes plasmid transfer kinetics. This method allowed analysis of the mating experiments by a single intrinsic kinetic parameter for conjugal plasmid transfer. Further results indicated that the specific growth rate of the donor strain antecedent to the mating experiment had a strong impact on the measured intrinsic plasmid transfer rate coefficient, which ranged from 1 x 10(-14) to 5 x 10(-13) ml per cell per min. Preliminary analysis suggested that the transfer rates of the TOL plasmid are large enough to maintain the TOL plasmid in a dense microbial community without selective pressures.

Determination of the mechanism of retrotransfer by mechanistic mathematical modeling

Two mathematical models to elucidate the mechanism of retromobilization (or retrotransfer), that is, the ability of conjugative plasmids to mobilize genes into the cell containing the conjugative plasmid, were developed. This study deals with retromobilization of nonconjugative plasmids (Tra-Mob+). Plasmid transfer was modeled by two mass action models. The first is based on the hypothesis that retromobilization of the Tra-Mob+ vector occurs in one step, by means of the pilus formed by the Tra+ plasmid in the original host. In the second model, retromobilization is considered to be a two-step process involving two transfer events. The first step involves the transfer of the Tra+ plasmid from the recipient cell to the donor of the nonconjugative vector, and during the second encounter the nonconjugative vector is mobilized toward the recipient. Since the relationships between the number of transconjugants and the number of recipients for the two models are different, filter matings were performed for short time periods with different initial densities of the recipient population. Comparison of the numbers of transconjugants with the results of the mathematical equations confirmed the hypothesis that retromobilization is a one-step conjugation process.