Tobramycin uptake in Escherichia coli is driven by either electrical potential or ATP

Aminoglycoside antibiotics such as streptomycin and tobramycin must traverse the bacterial cytoplasmic membrane prior to initiating lethal effects. Previous data on Escherichia coli, Staphylococcus aureus, and Bacillus subtilis have demonstrated that transport of aminoglycosides is regulated by ΔΨ, the electrical component of the proton motive force. However, several laboratories have observed that growth of bacterial cells can occur in the apparent absence of ΔΨ, and we wished to confirm these studies with E. coli and further investigate whether transport of aminoglycosides could occur in the absence of a membrane potential. Treatment of acrA strain CL2 with the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) dissipated ΔΨ, decreased intracellular ATP levels, and resulted in cessation of growth; after a variable period of time (3 to 7 h), growth resumed, ultimately achieving growth rates comparable to those of untreated cells. Absence of ΔΨ in these cells was confirmed by absence of [³H]tetraphenyl phosphonium⁺ uptake as measured by membrane filtration, lack of flagellar motion, and inability of these cells to transport proline (but not methionine). Regrowth was associated with restoration of normal intracellar ATP as measured by luciferin-luciferase bioluminesence assay. Unlike unacclimatized CL2 cells treated with CCCP, these cells transported [³H]tobramycin similarly to untreated cells; aminoglycoside-induced killing was seen in association with transport. These studies suggest that under certain circumstances aminoglycoside transport can be driven by ATP (or other high-energy activated phosphate donors) alone, in the absence of a measurable ΔΨ. ΔuncBC mutants of CL2 incapable of interconverting ΔΨ and ATP were treated with CCCP, resulting in dissipation of ΔΨ but no alteration in ATP content. Despite maintenance of normal ATP, there was no transport of [³H]tobramycin, confirming that under normal growth conditions ATP has no role in the transport of aminoglycosides.