SUMMARY: Populations of a predominantly tetracycline-resistant, penicillinase-positive strain of grown at 43-44° gave rise to progressively increasing proportions of tetracycline-sensitive and penicillinase-negative . The losses did not appear until after the elapse of a number of generations at the elevated temperature, and then apparently proceeded independently, tetracycline resistance being lost more rapidly than the ability to produce penicillinase. Tetracycline-sensitive and penicillinase-negative variants were extremely stable and the growth rates at 44° of the parent strain and the tetracycline-sensitive variants were indistinguishable. Screening for numerous other ‘marker’ properties revealed no changes under the test conditions. The evidence suggests that tetracycline resistance and penicillinase-forming ability probably depend on the presence of two different plasmids in the ; that the replication-rates of the plasmids at elevated temperature are less than that of the ; and that the plasmids, once lost, are not spontaneously resynthesized nor, at least for that conferring tetracycline resistance, regained by infection. However, such a hypothesis raises the question of how equilibrium, particularly between the tetracycline-resistant and -sensitive , is maintained in a population growing at 37° and observed to change in one direction but never in the reverse direction.


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