Transport of tetracyclines through the bacterial cell membrane assayed by fluorescence: a study with susceptible and resistant strains of Staphylococcus aureus and Escherichia coli.

The fluorescence of 12 tetracyclines in buffered solutions was measured by excitation at 400 nm and emission at 520 nm. The fluorescence varied markedly for different tetracyclines at equivalent concentrations. Demethylchlortetracycline exhibited more fluorescence than both chlortetracycline and demthyltetracycline; minocycline was virtually non-fluorescent at the conditions of the study. Fluorescence was highly dependent on the polarity of the solvent; when the buffer solution in water was replaced by a solvent containing 50% methanol, fluorescence increased significantly, but to various degrees for different tetracyclines. The most striking influence of the addition of methanol was observed for doxycycline (5-oxy 6-deoxy-tetracycline), whereas the influence on anhydrotetracycline was negligible. When suspensions of a susceptible strain of Staphylococcus aureus were added to solutions of tetracyclines, membrane permeation of the drugs could be monitored by an increase in fluorescence. This increase varied strikingly with the different drugs and could not be correlated with the concentrations for 50% growth inhibition (Ki). This might be due to the quantitative variations in the intracellular level corresponding to a certain external concentration of the respective drug. When tetracycline-resistant strains of S. aureus and Escherichia coli were exposed to tetracycline, the intensity of fluorescence observed was less than for the corresponding susceptible strains; in spite of this, the quantitative differences of fluorescence exhibited by the susceptible and resistant strains seemed slight in relation to the differences in susceptibility. It was demonstrated that minocycline inhibits the membrane transport of tetracycline in S. aureus and E. coli. This inhibition seems to be competitive for S. aureus, but not for E. coli.