Measurement of the Low-Frequency Charge Noise of Bacteria

TL;DR

This study measures the electrical charge fluctuations of bacteria using microfluidics and physics-based techniques, revealing significant charge noise and dynamic ion regulation at the single-cell level.

Contribution

It introduces a novel method combining electrical measurements and microfluidics to quantify bacterial charge noise, providing new insights into bacterial ion homeostasis.

Findings

Charge fluctuations follow a 1/f^2 power spectral density.

Bacteria exhibit a charge fluctuation of approximately ±1.30×10^6 elementary charges.

Bacterial charge noise indicates highly dynamic ion regulation.

Abstract

Bacteria meticulously regulate their intracellular ion concentrations and create ionic concentration gradients across the bacterial membrane. These ionic concentration gradients provide free energy for many cellular processes and are maintained by transmembrane transport. Given the physical dimensions of a bacterium and the stochasticity in transmembrane transport, intracellular ion concentrations and hence the charge state of a bacterium are bound to fluctuate. Here, we investigate the charge noise of 100s of non-motile bacteria by combining electrical measurement techniques from condensed matter physics with microfluidics. In our experiments, bacteria in a microchannel generate charge density fluctuations in the embedding electrolyte due to random influx and efflux of ions. Detected as electrical resistance noise, these charge density fluctuations display a power spectral density proportional to $1/f^2$ for frequencies $0.05~{\rm Hz} \leq f \leq 1 ~{\rm Hz}$. Fits to a simple noise model suggest that the steady-state charge of a bacterium fluctuates by $\pm 1.30 \times 10^6 {e}~({e} \approx 1.60 \times 10^{-19}~{\rm C})$, indicating that bacterial ion homeostasis is highly dynamic and dominated by strong charge noise. The rms charge noise can then be used to estimate the fluctuations in the membrane potential; however, the estimates are unreliable due to our limited understanding of the intracellular concentration gradients.