Growth of E. coli K12 in the presence of chlorpromazine or promethazine resulted in the appearance of a special kind of mutational change with a frequency of 5–10%. These cells were UV-sensitive and very mucoid in their colony formation. The strains were characterized as colanic acid producing on the basis of phage m-59 sensitivity and we tentatively conclude that these mutants are Lon−. In another series of experiments drug treatment of E. coli failed to yield significant numbers of auxotrophs and the back mutation rate of a number of S. typhimurium His− strains was not significantly increased.
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