Effects of toluene permeabilization and cell deenergization on tetracycline resistance in Escherichia coli.

Resistance to tetracycline (Tcr) mediated by Tn10 and related Tcr determinants involves an inner membrane protein, TET (similar but not identical for different determinants), and a proton motive force-dependent efflux of tetracycline which keeps the drug away from its intracellular target, the ribosome (L. M. McMurry, R. E. Petrucci, Jr., and S. B. Levy, Proc. Natl. Acad. Sci. USA 77:3974-3977, 1980). However, the amount of tetracycline accumulated by bacteria does not always correlate with their resistance levels, suggesting that an additional resistance mechanism may be present. When we permeabilized susceptible and resistant Tn10-bearing cells with toluene, we found that protein synthesis in the two strains became equally sensitive to tetracycline. Therefore, the protein synthesis machinery was not a source of resistance, and an intact membrane was required for resistance. To determine whether resistance was entirely dependent on energy, we measured susceptibility to tetracycline after inhibition of proton motive force by starvation and specific inhibitors. An 80 to 90% loss of Tcr (measured by protein synthesis) resulted from partial deenergization of resistant cells. A remaining resistance (10- to 20-fold greater than that of susceptible cells) could not be eliminated by further deenergization. These findings indicated that, to a major extent, expression of Tn10 resistance required energy, presumably for tetracycline efflux. They also suggested the existence of a small component of Tcr having little or no energy dependence. Whether this component depends on tetracycline efflux or some other mechanism is not known, but presumably both high- and low-energy components of resistance reflect activity of TET protein.