Membrane-potential dynamics mediate bacterial electrical signaling at both intra- and intercellular levels. Membrane potential is also central to cellular proliferation. It is unclear whether the cellular response to external electrical stimuli is influenced by the cellular proliferative capacity. A new strategy enabling electrical stimulation of bacteria with simultaneous monitoring of single-cell membrane-potential dynamics would allow bridging this knowledge gap and further extend electrophysiological studies into the field of microbiology. Here we report that an identical electrical stimulus can cause opposite polarization dynamics depending on cellular proliferation capacity. This was demonstrated using two model organisms, namely Bacillus subtilis and Escherichia coli, and by developing an apparatus enabling exogenous electrical stimulation and single-cell time-lapse microscopy. Using this bespoke apparatus, we show that a 2.5-second electrical stimulation causes hyperpolarization in unperturbed cells. Measurements of intracellular K+ and the deletion of the K+ channel suggested that the hyperpolarization response is caused by the K+ efflux through the channel. When cells are preexposed to 400 ± 8 nm wavelength light, the same electrical stimulation depolarizes cells instead of causing hyperpolarization. A mathematical model extended from the FitzHugh–Nagumo neuron model suggested that the opposite response dynamics are due to the shift in resting membrane potential. As predicted by the model, electrical stimulation only induced depolarization when cells are treated with antibiotics, protonophore, or alcohol. Therefore, electrically induced membrane-potential dynamics offer a reliable approach for rapid detection of proliferative bacteria and determination of their sensitivity to antimicrobial agents at the single-cell level.

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Response to mechanical force is a well characterised phenomenon in eukaryotic organisms, helping to organise multicellular structures. Mechanotactic responses have only rarely been observed in prokaryotic taxa. This work reports on a morphological change due to variations in applied force and surface structure by Bacillus mycoides Flügge. B. mycoides is a ubiquitous soil organism well known among microbiologists for its characteristic spreading colony morphology. An apparent mechanotactic response is elicited by physical deformation of the gel media on which B.mycoides is growing, including applied forces of compression or tension. Variations in the surface such as curvature produced by casting the agar gel in the presence of curved objects also elicited the change. The morphological change in B.mycoides colonies associated with the application of force manifests as a pattern of parallel rhizoid filaments perpendicular to compressing force and parallel to stretching force in the agar medium. The phenomenon is most clearly demonstrated by reversible changes in the orientation of B. mycoides filaments during time-lapse microscopy.

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