[Role of the conjugative transfer of R-plasmids in competitive interactions between plasmid-containing and plasmid-free strains of Escherichia coli in continuous culture].

Competition of the plasmid-containing strain C600 (RP4) (pBS94) of E. coli and the plasmid-free strain C600 rifr of E. coli in chemostatic and pH-static continuous cultures resulted in displacement of the plasmid-free strain in spite of its higher specific growth rate. Conjugative transfer of RP4 and pBS94 plasmids into the cells of the plasmid-free strain was observed in the experiments. Competition of strains C600 (pBS94) (RP4 Tra-) and C600 rifr in the chemostatic culture in the absence of the plasmid conjugative transfer resulted in displacement of the plasmid-containing strain from the population, while in the pH-static culture, the initial ratio of the plasmid-containing and plasmid-free bacteria was preserved. The conjugative transfer of the plasmids lowered the number of the plasmid-free cells in the population and might be considered as a factor stabilizing the plasmids in the bacterial population of the continuous cultures.