Escherichia coli intracellular pH, membrane potential, and cell growth

We studied the changes in various cell functions during the shift to alkaline extracellular pH in wild-type Escherichia coli and in strain DZ3, a mutant defective in pH homeostasis. A rapid increase in membrane potential (Δψ) was detected in both the wild type and the mutant immediately upon the shift, when both cell types failed to control intracellular pH. Upon reestablishment of intracellular pH - extracellular pH and growth in the wild type, Δψ decreased to a new steady-state value. The electrochemical proton gradient (Δμ(H+)) was similar in magnitude to that observed before the pH shift. In the mutant DZ3, Δψ remained elevated, and even though Δμ(H+) was higher than in the wild type, growth was impaired. Cessation of growth in the mutant is not a result of cell death. Hence, the mutant affords an interesting system to explore the intracellular-pH-sensitive steps that arrest growth without affecting viability. In addition to Δμ(H+), we measured respiration rates, synthesis, cell viability, induction of β-galactosidase, DNA synthesis, and cell elongation upon failure of pH homeostasis. Cell division was the only function arrested after the shift in extracellular pH. The cells formed long chains with no increase in colony-forming capacity.