SUMMARY: Mutants resistant to high levels of chloramphenicol can be obtained in B by one or two mutational events. All of 144 high-level resistant mutant clones examined were powerful inactivators of the drug. Growth of this kind of mutants in nutrient media containing chloramphenicol 100 µg./ml. or more depended on the inoculum size, the composition of the medium, and the concentration of the drug. No growth was observed with lactose as sole energy source unless the organisms had been previously induced for β-galactosidase production.

With a strain of K 12 an entirely different type of resistant mutants occurred. High-level resistant derivatives were obtainable only through several serial mutations. None of 36 high-level resistant mutants was able to inactivate the drug. Growth of these bacteria was extremely slow, even in the absence of drug. This resistance was due to a decreased rate of permeation, which probably was non-specific and concerned many species of micromolecules.


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