The Effect of Growth at Elevated Temperatures on Some Heritable Properties of Staphylococcus aureus

SUMMARY: Populations of a predominantly tetracycline-resistant, penicillinase-positive strain of Staphylococcus aureus grown at 43.44° gave rise to progressively increasing proportions of tetracycline-sensitive and penicillinase-negative cocci. 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 cocci; that the replication-rates of the plasmids at elevated temperature are less than that of the cocci; 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 cocci, is maintained in a population growing at 37° and observed to change in one direction but never in the reverse direction.