The positive role of voids in the plasma membrane in growth and energetics of Escherichia coli

Abstract

Bacterial respiration, endogenous as well as induced respiration by glucose, lactose and glycine betaine, was found to be sensitive to external solute concentration. Permeability of hydrogen peroxide, a non-electrolyte of molecular size between water and urea, through the bacterial membranes changed directly with the rate of respiration (an activity residing in the bacterial plasma membrane) in E. coli and the enhanced permeability and respiratory activity were highly correlated. Hydrogen peroxide permeability and induction of voids (spaces in the matrix of the bilayer into which hydrophobic fluorescent probes partition, which in turn were used to assess the modulation of these cavities) were shown to be a direct and excellent measure of leak conductance. Fluorescence intensity and anisotropy of the extrinsic fluorescent probes (incorporated by growing bacteria in their presence) decreased with increased respiration in bacteria, consistent with lowered molecular restriction and enhanced hydration in the membrane phase for these probes as seen in dimyristoylphosphatidylcholine bilayers due to phase transition. The physical basis of osmotic phenomena, as a relevant (thermodynamic) volume, could relate to water exchange or compression, depending on the osmotic domain. In the domain of compression in bacteria, i.e. well above the isotonic range, the computed activation volume was consistent with voids in the membrane. This study emphasises a major role of leak conductance in bacterial physiology and growth.