Measurements of net fluxes and extracellular changes of H⁺, Ca²⁺, K⁺, and NH₄⁺ in Escherichia coli using ion-selective microelectrodes

This study introduced the use of a non-invasive ion-selective microelectrode (MIFE) technique to study membrane-transport processes in bacteria. Net ion fluxes and changes in the extracellular concentrations of H⁺, Ca²⁺, K⁺ and NH₄⁺ in adherent bacteria, isolated from cultures at different growth stages (exponential, late exponential, and stationary phases), were monitored. With the exception of Ca²⁺, a significant (P = 0.05) difference was found in the magnitude of net fluxes of the ions measured from bacterial cells at different stages of the population growth curve. The magnitude of the H⁺ response was glucose-dependent with maximum changes occurring at the highest concentration. There was a progressive increase in H⁺ extrusion followed by a gradual return to zero at late stationary phase. Measurements of net ion fluxes crossing the bacterial cytoplasmic membrane, demonstrated here for the first time, may offer insight into underlying mechanisms of ion transport kinetics. Applications of the non-invasive ion-selective microelectrode technique in microbiology are discussed.