FIG. 4.

Antibiotic accumulation in E. coli. A—Ofloxacin uptake in JM109 bearing pT7-7 CGmdr (▾), pT7-7 MSmdr (▴), pT7-7 HSmdr (•), and the control (▪). Data represent relative uptake (uptake divided by the maximum uptake in the presence of CCCP). Ofloxacin was added to 200 μl of bacterial culture (OD₄₂₀ = 20) to reach the final concentration of 30 μM. At the indicated times bacteria were separated from the medium by rapid filtration and ofloxacin was measured by fluorescence. CCCP was added to parallel tubes to a final concentration of 0.5 mM. B— Chloramphenicol uptake in the same strains of JM109 relative to uptake after addition of CCCP. Chloramphenicol uptake was measured as described above for ofloxacin except that the latter was replaced with [³H]chloramphenicol at a final concentration of 0.5 μM. The experiment was repeated three times. One representative experiment is shown.