Transitory plasmid instability during the exponential phase of growth of a recombinantEscherichia coli

Summary The present results concern the recombinant bacteriaEscherichia coli HB101(GAPDH) which produces glyceraldehyde 3-phosphate dehydrogenase. An unusual phenomenon was noticed concerning the plasmid stability of this strain growing in batch culture. The determination of sensitivity to ampicillin, whose resistance is carried by the plasmid, has been tested as a function of time on Petri dishes containing increasing concentrations of ampicillin in a batch culture in a complex medium without any selection pressure. A transitory decrease in the percentage of resistant cells has been noted during the exponential phase of growth. This phenomenon corresponds to a momentary plasmid instability probably due to a transitory gap between the growth rate of the cell and the duplication rate of the plasmid.