Enhanced competitiveness of drug-resistant Escherichia coli

Strains of Escherichia coli K-12 J621-harbouring the antibiotic-resistance plasmids R1 and R39 were competed against an isogenic plasmid-free strain i n glucose-limited chemostats. In duplicate experiments performed for both p lasmid-containing strains an initial drop in the number of R1- and R39-cont aining bacteria was followed by a rapid increase in their numbers relative to the plasmid-free cells. In each experiment plasmid-containing cells out- competed their plasmid-free counterpart's and resulted in the disappearance of plasmid-free cells from the chemostat population.