Sensitivity of geoelectrical measurements to the presence of bacteria in porous media

We investigated the sensitivity of low-frequency electrical measurements (0.1-1000 Hz) to (1) microbial cell density, (2) live and dead cells, and (3) microbial attachment onto mineral surfaces of clean quartz sands and iron oxide-coated sands. Three strains of Pseudomonas aeruginosa PAO1 (wild type and rhlA and pilA mutant) with different motility and attachment properties were used. Varying concentrations of both live and dead cells of P. aeruginosa wild type in sand columns showed no effect on the real conductivity component (sigma'). However, the imaginary conductivity component (sigma ''.) increased linearly with increasing concentrations of live cells in sand columns, whereas minimal changes were observed with different concentrations of dead cells. A strong power law relationship was observed between sigma '' and the number of cells adsorbed onto sand grain surfaces with the rhlA mutant of P. aeruginosa displaying a higher power law exponent compared to the wild type and pilA mutant. In addition, power law exponents were greater in columns with iron oxide-coated sands compared to clean quartz sands. Minimal changes were observed on the sigma' due to the attachment of P. aeruginosa cells onto sands. We relate the measured low-frequency electrical responses to (1) the distinct electrical properties of live cells and (2) the density of cells attached to mineral surfaces enhancing the surface roughness of sand grains and hence the polarization response. The information obtained from this study enhances our interpretation of microbially induced geoelectrical responses in biostimulated geologic media and may have implications for microbial transport studies.