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Tetracyclines of various hydrophobicities as a probe for permeability of Escherichia coli outer membranes

The outer membrane of gram-negative cells excludes hydrophobic molecules and is responsible for the resistance of these cells to a number of dyes, detergents, and antibiotics. We describe a test for hydrophobic permeability in which a series of tetracyclines with various hydrophobicities are used. Normal Escherichia coli cells became more resistant as hydrophobicity was increased in this series, but mutants altered in outer membrane permeability remained susceptible. A mutant lacking all polysaccharide except 2-keto-3- deoxyoctonic acid in its lipopolysaccharide is virtually as susceptible to the hydrophobic drug 13- phenylmercapto -alpha-6- deoxytetracycline as to oxytetracycline (MIC 100 times lower than that of the wild type), and a mutant with another, as yet undefined outer membrane defect, acrA , also shows increased, although somewhat lesser, susceptibility (MIC 20 times lower than that of the wild type). Increased susceptibility to this tetracycline derivative is associated with greater fluorescence of the derivative when added to the cells, which we interpret as increased interaction of the derivative with hydrophobic domains, such as membranes, in the mutants. This series of tetracyclines may provide an assay for measuring the permeability of gram-negative organisms and their mutants to hydrophobic molecules.