Survival of, and plasmid transfer between modified Enterobacter cloacae strains under long-term starvation conditions in a continuous-flow system

Continuous-flow, packed-bed column reactors, which provide an experimental model of a soil profile, were used to investigate survival of, and plasmid transfer between, strains of Enterobacter cloacae. When columns, inoculated with nutrient-sufficient donor and recipient strains, were provided with a minimal salts medium with no added carbon source, transconjugant cells app eared in their effluents. During the first few days of such experiments, th e concentration of cells in the effluent declined but then the donor popula tion stabilized, while the recipient and transconjugant populations continu ed to decrease. The results indicate that the amount of nutrient required t o maintain and transfer plasmids is very low. No transconjugants were obser ved in the effluent from columns inoculated with pre-starved donor and reci pient strains.