Uptake of tetracyclines into Escherichia coli was assessed with a strain carrying a tetA-lacZ translational fusion, in which expression of the enzyme is controlled by the pSC101 tetR repressor gene, by examining beta-galactosidase induction. The ability of tetracycline analogues to induce beta-galactosidase synthesis was correlated with their hydrophobicity, such that hydrophobic analogues were poor enzyme inducers. Treatment of E. coli with polymyxin B nonapeptide (PMBN) rendered cells more permeable to minocycline, but not to tetracycline.
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Chopra et al. (1992) studied this question.