Proton‐motive force during growth of Escherichia coli in the recycling fermentor

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Abstract
The proton‐motive force (Δp) has been determined during growth of Escherichia coli in a recycling fermentor and in a chemostat. The recycling fermentor has proven to be an easy and suitable technique for measuring Δp in growing cells, especially at low specific growth rates. Δp was determined by measuring the transmembrane gradient of protons, ΔpH, from the accumulation of benzoate anions, and the electrical gradient, ΔΨ, from the accumulation of the lipophilic cation tetraphenylphosphonium (TPP+). The accumulation of lactose was also used to calculate Δp in two lactose operon constitutive, β‐galactosidase negative relaxed mutants. In the recycling fermentor the calculated Δp values were independent of the specific growth rate and of the growth phase with both relaxed mutants. Δp values measured in the chemostat at different specific growth rates yielded the same result. Results with a wild‐type E. coli that expresses the stringent response at specific growth rates lower than 0.01 h−, corresponding to generation times greater than 70 h, showed that Δp varies inversely with specific growth rate. These results indicate that a relationship exists between Δpand the expression of the stringent response.