The Origin of Bacterial Resistance to Proflavine

SUMMARY: It was reported (Baskett, 1952) that small additions of proflavine made at intervals to a growing culture of Bacterium lactis aerogenes (synonyms Aerobacter aerogenes, Klebsiella pneumoniae), did not prevent continued growth; this was regarded as proof that drug resistance might be acquired by adaptation. This phenomenon does not occur with Escherichia coli grown in broth, but we confirm that it occurs with B. lactis aerogenes grown in a chemically defined medium containing glucose. Growth of the aerogenes organism ceased when all the proflavine was added at once. Examination of the aerogenes organisms at the end of the experiments showed that a high proportion had become slightly more resistant, and a few had become highly resistant. These changes were not sufficient to account for continued growth of the aerogenes organisms in increasing concentrations of proflavine. In high concentrations of proflavine, growth of sensitive organisms did occur when some filtrate from a drug-free culture was added to the culture medium. It also occurred when the growing culture was kept acid, or when the medium was made acid before inoculation. On the other hand, growth did not occur in the presence of proflavine when the culture was kept neutral. It is concluded that the growth of B. lads aerogenes in cultures to which proflavine is added gradually is chiefly due to a decrease of the inhibitory action of the drug by the acid formed during growth in a glucose + salts medium. The absence of the phenomenon when E. coli was grown in broth can be explained by the absence of glucose so that there was no lowering of pH value. There is thus no evidence of a rapid adaptation to proflavine resistance in B. lactis aerogenes.