A wild-type strain of PPK1, carrying a non-conjugative TOL plasmid, was grown in continuous culture under carbon-limitation with -toluate as growth substrate. When the medium was changed to benzoate a plasmid-free strain appeared after about 100 h. After this event the proportion of plasmid-containing bacteria declined rapidly but in several experiments this was followed by oscillations in the relative frequencies of the plasmid-free and plasmid-containing populations. About 1% of the total population retained the plasmid after 600 h growth under benzoate-limitation. When the input medium was returned to -toluate the plasmid-free bacteria disappeared and the TOL population recovered to 100%. The plasmid in the wild-type strain was stably maintained during 500 h of chemostat growth under succinate-limitation. Tol strains re-isolated from continuous culture under benzoate-limitation retained their TOL plasmids for longer periods. It is suggested that plasmid loss is related to a failure in the control of partitioning at cell division but that this is not absolute and allows the maintenance of a residual low level of plasmid-containing bacteria in the population.


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