The effects of paraquat on Escherichia coli: distinction between bacteriostasis and lethality.

Paraquat exerted a progressively more pronounced bacteriostatic effect on Escherichia coli as its concentration was raised in the range 0-1.0 microM. In contrast, concentrations of 100 microM or greater were required before significant lethality could be observed. This bacteriostatic effect of paraquat could be eliminated by supplementation of the glucose-plus-salts medium with either yeast extract or a casein hydrolysate. This protection was seen whether the supplement was added a few minutes prior to or following the addition of paraquat and was thus not due to the inhibition of active uptake of paraquat by the cells. The lethal effect of high levels of paraquat was not influenced by supplementation of the medium with yeast extract. It follows that the bacteriostatic and lethal effects of paraquat involve attack upon distinct targets within the cell.