Low-Frequency Charge Noise of Bacteria as Indicator of Ion Concentration Regulation*

Bacteria rely on the free energy provided by ionic concentration gradients across the bacterial

membrane to power various cellular processes. These concentration gradients are regulated by

ion channels and ion pumps within the membrane. Due to the stochasticity of the transport and

the physical dimensions of a bacterium, intracellular ion concentrations are bound to fluctuate.

Thus, bacterial ion homeostasis is expected to be highly dynamic and dominated by strong charge

noise. By employing sensitive electrical measurement techniques in a microfluidic system, we

investigate the charge noise of several hundred nonmotile bacteria. The power spectral density

of these fluctuations displays 1/f² behavior for frequencies 0.05 ≤ f ≤ 1 Hz. By fitting our

data to a simple noise model, we estimate the charge fluctuations of a single bacterium to be

±1.30 × 10⁶e (e ≈ 1.60 × 10⁻¹⁹ C). The charge noise is directly related to the fluctuations in the

membrane potential. However, the limited understanding of intracellular concentration gradients

makes current predictions unreliable.