Light-induced tetracycline accumulation by Rhodopseudomonas sphaeroides

Tetracycline uptake by susceptible Escherichia coli cells

Experiments measuring the initial uptake of commercial (3H) tetracycline exhibit two distinct kinetic phases: a rapid phase followed by a slow phase. (3H) tetracycline purified by chromatography on a Dowex 50WX2 column exhibited only monophasic rapid uptake when tested with susceptible Escherichia coli cells. Cyanide inhibited the uptake of purified (3H) tetracycline only partially while transport of proline and maltose was entirely abolished. Energy independent accumulation of tetracycline may be accounted for by binding to cellular constituents. Uptake of tetracycline--as measured by inhibition of beta-galactosidase synthesis--was strongly affected by a shift in temperature from 37 degrees C to 21 degrees C while carrier-mediated transport systems revealed only minor reductions. Taken together with the non-saturability of tetracycline uptake and the evidence for diffusion of tetracycline through phospholipid bilayers [Argast and Beck (1984) Antimicrob Agents Chemother 26:263-265] these data support the hypothesis that tetracycline enters the cytoplasm by diffusion.

Comparison of kinetics of active tetracycline uptake and active tetracycline efflux in sensitive and plasmid RP4-containing Pseudomonas putida

Membrane vesicles prepared from tetracycline-sensitive cells of Pseudomonas putida took up tetracycline by an active transport system with an apparent Km of 2.5 mM and a Vmax of 50 nmol min-1 mg protein-1. In contrast, resistance determinant RP4-containing P. putida had an active high-affinity efflux system for tetracycline with a Km of 2.0 to 3.54 microM and a Vmax of 0.15 nmol min-1 mg protein-1. Thus, the efflux system of tetracycline-resistant P. putida(RP4) had an average of 1,000-fold greater affinity for tetracycline than the influx system of tetracycline-sensitive cells. From these results, it is clear that a major mechanism of tetracycline resistance in RP4-containing P. putida is an active tetracycline efflux mechanism. There was also evidence for a second tetracycline efflux system with low affinity for tetracycline n P. putida(RP4). This efflux system had a Km of 0.25 mM and a Vmax of 1.45 nmol min-1 protein-1. Whether this low-affinity efflux system was also present in tetracycline-sensitive P. putida could not be discerned from these experiments.

Active uptake of tetracycline by membrane vesicles from susceptible Escherichia coli

A major portion of tetracycline accumulation by susceptible bacterial cells is energy dependent. Inner membrane vesicles prepared from susceptible Escherichia coli cells concentrated tetracycline 2.5 to 5 times above the external concentration when the electron transport substrate D-lactate or reduced phenazine methosulfate was added. This stimulation was reversed by cyanide, 2,4-dinitrophenol, and carbonyl cyanide m-chlorophenyl hydrazone. These vesicles data showed that proton motive force alone could energize tetracycline uptake. The lactate-dependent uptake had a pH optimum of 6.9 and a magnesium optimum of 1 mM and was not saturable up to 400 microM tetracycline. Although the vesicles were not as active as cells in concentrating tetracycline, they were less active to a similar extent in concentrating tetracycline, they were less active to a similar extent in concentrating proline, the transport of which is known to be solely proton motive force dependent. Therefore, we concluded that the active uptake of tetracycline in susceptible cells was largely, if not solely, energized by proton motive force.

Light-induced, carrier-mediated transport of tetracycline by Rhodopseudomonas sphaeroides

Tetracycline accumulation by the phototrophic bacterium Rhodopseudomonas sphaeroides has been studied, using the fluorescence properties of the antibiotic and measuring uptake of [7- 3H]tetracycline. Accumulation was carrier mediated, with a Km of approximately 300 micronM. Efflux also appeared to be carried mediated, with a Km of 25 mM. Chlorotetracycline competitively inhibited tetracycline transport. The transport was energy dependent. Efflux occurred during the influx process, and an energy-requiring steady state was reached when influx balanced efflux. Transport was inhibited by metabolic inhibitors such as antimycin A, cyanide, and iodoacetate. Proton conductors such as carbonylcyanide m-chlorophenyl hydrazone were strongly inhibitory. Efflux was not energy dependent. Efflux is partially blocked by mercuric ions and completely blocked by an external pH of 9 to 11. Although efflux rates increased continuously with lowering of the pH, influx rates have a sharp maximum at pH 7.