Susceptible Escherichia coli cells can actively excrete tetracyclines.

Escherichia coli shows severalfold less susceptibility to tetracyclines when grown in enriched medium than in minimal medium. Transport studies with cells harvested from these media showed different handling of the drugs. Whereas an energy-dependent uptake of tetracycline and minocycline was observed in susceptible K-12 and wild-type E. coli strains grown in minimal medium, an active efflux of minocycline and, to a lesser extent, tetracycline was seen in cells grown in L broth and other enriched media. This efflux was replaced by an active uptake system after treatment of cells grown in L broth with EDTA. When assayed at a lower temperature (27 degrees C), even cells grown in minimal medium showed an efflux of minocycline. Everted membrane vesicles prepared from susceptible cells grown in minimal medium or L broth showed an energy-dependent accumulation of minocycline and tetracycline when supplied with certain divalent cations. These results suggest that an active efflux of tetracyclines occurs in susceptible E. coli but is not detected in cells grown in minimal medium because greater permeability of the outer membrane allows a more rapid active uptake. This efflux system is distinct from that specified by tetracycline resistance determinants. Since the active efflux of minocycline in cells grown in L broth disappeared at external antibiotic concentrations of greater than 100 microM, it may be saturable and so mediated by a membrane carrier.