  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.
