Physiological function of superoxide dismutase in glucose-limited chemostat cultures of Escherichia coli

Abstract

Conditions for continuous culture of E. coli K-12 His- Thi- under glucose limitation were established. The capacity for respiration, at D [dilution rate] > 0.2/h, and specific activity of superoxide dismutase increased as a function of specific growth rate, but peroxidase and catalase were invariant with or inversely related to this growth rate. The abrupt increase in the availability of glucose, as a means of elevating the growth rate, was followed by an increase in superoxide dismutase, which reached a plateau before there was a significant increase in the growth rate. Thus, an increase in superoxide dismutase appeared to be a prerequisite for an increase in the rate of growth. Cells that had higher levels of superoxide dismutase, because of varying specific growth rates, were more resistant to the toxicity of hyperbaric O2. Superoxide dismutase thus behaved like an essential defense against the toxicity of O2. Sensitivity towards streptonigrin increased with specific growth rate in the range of 0.09-0.25/h but decreased with further increases in the growth rate. Since this antibiotic was shown to shunt electrons to O2, with concomitant production of O2-, these results indicated a progressive deficiency of reducing power at growth rates < 0.25/h and a surfeit of reducing power with progressively greater protection against O2- by superoxide dismutase at growth rates > 0.25/h.