The resistance ofBact. lactis aerogenesto proflavine (2:8-diaminoacridine). I. The applicability of the statistical fluctuation test

Abstract

The number of colonies ofBact. lactis aerogenesappearing on solid media containing proflavine (2:8-diaminoacridine) under different conditions has been determined. The statistical variation is greater with samples taken from different cultures than with the same number taken from a given culture. This might appear to correspond to the Luria-Delbrück criterion that the resistant colonies are formed from spontaneously appearing mutants in the cultures under test. It is pointed out, however, that this greater variation may in the present example depend equally well upon other factors than mutations, since separate cultures cannot be so closely controlled as any single culture in respect of age, pH and degree of aeration. The effects of these factors are studied, and it is shown that varying efficiency of aeration has a specially marked influence on the number of resistant survivors. It is shown, moreover, that there is a competition between the development of resistance and death of the population, and that the times at which the last surviving cell in a proflavine culture can be detected itself shows a very great variation. Thus the variation in the colony number can be a function of survival rather than of mutation. The form of the curve showing the initial decline of population followed by growth of resistant survivors is difficult to reconcile quantitatively with a simple form of the mutation hypothesis.