Gold Ions Hyperpolarize Bacteria

Abstract

Background: The study of bacterial electrophysiology is important for understanding antibacterial resistance, biofilm formation, and fundamental questions of cell growth and division. These experiments require new tools to modulate the resting membrane potential of bacteria. The use of potassium and magnesium ions, antibiotics, short exposures to blue light, and electric field have been used to hyperpolarize or depolarize bacteria. Our research characterizes the cellular response to a new reagent, gold ions, using single-cell imaging. Methods: We use fluorescence microscopy to monitor, in real time, the change to the membrane potential when Bacillus subtilis (B. subtilis) and Escherichia coli (E. coli) are exposed to gold ions. Membrane potential is measured with two different Nernstian dyes, thioflavin T and tetramethylrhodamine methyl ester. Gold ions are introduced to cell by adding gold salt solutions or through the electrochemical generation of gold ions. Inductively coupled plasma mass spectrometry (ICP-MS) is used to determine the concentration of gold ions generated electrochemically. Standard growth curves are used to study the effect of gold ions on bacterial growth. Results: Single cell fluorescence imaging shows that a solution of gold ions hyperpolarizes B. subtilis and E. coli in a concentration-dependent manner. ICP-MS confirms that applied voltage and frequency controls the concentration of gold ions generated electrochemically. Electrochemically generated gold ions diffuse through the imaging chamber creating a wave of hyperpolarization. The speed of the hyperpolarization wave can be modulated by voltage and frequency. Conclusions: Our research describes a new tool, gold ions, for the hyperpolarization of bacteria. Gold ions can be introduced to bacteria as a salt solution or generated electrochemically, providing spatial and temporal control of bacterial membrane potential. Beyond this work, it is possible that hyperpolarization may play a role in the use of gold as an antibacterial agent.