Fundamental quantitative insights into bacterial spore electrophysiology

Cells harness out-of-equilibrium electrochemical gradients to power bioenergetics, homeostasis and signaling. In contrast, electrophysiological features and functions in dormant bacterial spores remain obscure.

Here, we introduce and apply a non-invasive combined experimental-theoretical approach that overcomes limitations of classical techniques, thereby enabling fundamental biophysical insights into the electrophysiology of Bacillus subtilis spores. To this end, we monitor individual spore pH dynamics upon microfluidics-controlled environmental fluctuations using genetically encoded fluorescent reporters. Our observations indicate that external pH or salinity changes trigger transitions of internal spore pH to new equilibria within hours to days. Fitting a thermodynamic model of passive proton flux to time series data allowed pH sensor calibration and quantitation of spore pH and voltage changes. Our estimates suggest that ionic environmental changes can alter spore pH and electric potential slowly but substantially.

Thus, dormant spores are open passive systems responding and equilibrating to changing ionic environments on faster-than-lifetime timescales. Thereby, a spore's electrophysiological state is dynamic and relies on environmental factors.