Streptonigrin toxicity in <i>Escherichia coli</i>: oxygen dependence and the role of the intracellular oxidation–reduction state

Abstract

The bacterial physiology of streptonigrin toxicity was further investigated. An optimal oxygen concentration for toxicity was inferred from data showing that streptonigrin at 5 micrograms/mL was rapidly lethal to aerobic cultures of Escherichia coli K12JF361, but was without effect on anaerobic cultures and was bacteriostatic to cultures inhibited in 5 atm of oxygen plus 1 atm of air (5 atm O2 plus air) (1 atm = 101.325 kPa). Escherichia coli were protected from a potentially lethal concentration of streptonigrin during anaerobic incubation, whether previously grown anaerobically, aerobically, or in 5 atm O2 plus air. Superoxide dismutase activity increased with increasing oxygen tension in the medium, but was not significantly changed by a lethal concentration of streptonigrin. Although the superoxide dismutase activity was four times greater in E. coli grown in 5 atm O2 plus air than those grown in air alone, the aerobic survival in 5 micrograms/mL streptonigrin was identical, which suggested that superoxide dismutase was not rate limiting for toxicity. Escherichia coli K12 strains deficient in glutathione (KMBL54-129, AB1157-821, and AB1157-830) were protected from streptonigrin poisoning. Dithiothreotol (5.0 mM), diamide (1 mM), methyl viologen (1 mM), and cyanide (10 mM) protected aerobic E. coli from 5 micrograms/mL streptonigrin. These data are also consistent with a model of in vivo streptonigrin toxicity that requires a favorable intracellular oxidation--reduction state and an optimal concentration of molecular oxygen.