We performed electrical measurements on sands flushed with bacterial suspensions of varying concentration. The first experiment was conducted with Shewanella putrefaciens (biomass 0–0.5 mg/L) and the second with Escherichia coli (biomass 0–42 mg/L). We measured a biomass-dependent low-frequency (10 Hz) polarization. At cell density <12 mg/L polarization decreased (up to 60 %) relative to before introduction of cells; the decrease was greater when the sand was artificially Fe-coated to enhance the affinity of cells to the mineral surface. At cell density >12 mg/L polarization increased (up to 15%). We attribute the decrease in polarization at low cell density to alteration of the mineral-fluid interface due to mineral-cell interactions. The polarization enhancement at higher cell density is possibly a pore throat mechanism resulting from decreased ionic mobility and/or electron transfer due to cell accumulation in pores.