Expression and loss of the pBR322 plasmid in Klebsiella aerogenes NCTC 418, grown in chemostat culture.

Klebsiella aerogenes harbouring the plasmid pBR322 was grown in continuous culture at various growth rates under glucose, phosphate or ammonia limitation. With tetracycline in the medium, the maximum culture beta-lactamase activity was found at the higher growth rates. When tetracycline was absent, loss of resistance to the drug occurred. Concomitant with the occurrence of drug-sensitive cells, the culture beta-lactamase activity decreased. At the higher growth rates the enzyme activity decreased at a slightly higher rate than did the resistance to tetracycline. From this it was concluded that the beta-lactamase activity per mg cellular dry weight of the drug-resistant fraction of the population was still decreasing during the appearance of drug-sensitive cells. At the higher growth rates, this decrease was independent of the nutrient that was growth-limiting.