The resistance ofBact. lactis aerogenesto proflavine (2:8-diaminoacridine). Il. The direct induction of adaptation

Abstract

The adaptation of cultures ofBact. lactis aerogenesto proflavine (2:8-diaminoacridine) is examined in the light of the theories of selection and induced adaptation. As is already known the presence of proflavine increases the lag before growth, and reduces the number of colonies formed by unadapted cells in a solid medium. Fractions of mutants calculated on the selection theory from these two effects are very different, particularly in the concentration range where growth is completely inhibited in liquid medium but not in solid medium. Adaptation is very greatly accelerated if the drug is added gradually to an actively growing culture. Cells can become adjusted to concentrations which are normally completely inhibitory to unadapted cells subcultured into liquid medium in times too short for any appreciable selection to occur. The interaction of drug and cell clearly brings about adjustments which enable growth to occur under previously inhibitory conditions. The rate of adaptation is dependent on the metabolic activity of the culture. When growth is halted in the presence of proflavine the reducing power declines steadily. Such secondary decay processes are probably responsible for the major effects of inhibition both in solid and in liquid media. A study of filament formation confirms that most cells rather than selected mutants do, in fact, grow in gradually increasing concentrations of proflavine.