Clinical isolates of harbouring resistance (R) plasmids of mol. wts 4.4 x 10 (Asian) or 3.2 x 10 (African) were grown in prolonged glucose-limited continuous culture to determine the segregation efficiency of each type of plasmid and their expression of penicillinase activity in the absence of antibiotic selective pressure. One strain contained the African plasmid and cryptic and conjugative plasmids, which were all retained after 96 generations in the chemostat. By contrast, the other strain lost all plasmids after 100 generations. Both strains showed increased sensitivity to a range of antibiotics, particularly to the penicillins. Loss of penicillinase activity as minimal inhibitory concentration decreased was confirmed for both strains by assaying the enzyme spectrophotometri-cally. Activity decreased with the number of generations and none was detectable at the time of complete plasmid loss. This decrease was apparently due to individual bacteria ceasing to produce enzyme rather than a gradual decline in production by the whole population. The sensitivities to a broad range of antibiotics also generally increased during glucose-limited growth, but one strain became more resistant to clindamycin and the other to tetracycline.


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