Response of the flagellar rotary motor to abrupt changes in extracellular pH

Cells of a motile Streptococcus were starved, tethered to a quartz coverslip, energized with a potassium diffusion potential, and exposed to sudden decrements in external pH generated by flash photolysis of 2-hydroxyphenyl-1-(2-nitro)phenyl phosphate. The rotation rate of the cells increased following the flash but only after a brief time lag. Lags of the order of 0.1 second were observed in a dilute buffer (0.05 mM), confirming results obtained earlier. These lags were longer when the buffer was prepared in D2O. However, lags as short as 0.01 second were found in more concentrated buffers (1 and 3 mM). In this case, there was no deuterium solvent isotope effect. These differences arise from the extra time required for diffusion of protons from a dilute medium into the cell wall, which has a large buffering capacity. The short lags observed in concentrated media could be inherent to the flagellar motor, but the possibility that they are due to buffered diffusion through the cell wall or to elastic filtering by the tether has not been ruled out.