Evaluating the use of Spectral Induced Conductivity to Detect Biofilm Development within Porous Media

Abstract:

Microbial biomass accumulation in subsurface sediments dynamically alters porosity/permeability; factors critical to contaminant transport and management of bioremediation efforts. Current methodologies (i.e. plate counts, tracer/slug tests) offer some understanding of biofilm effect on subsurface hydrology, yet do not provide real-time information regarding biofilm development. Due to these limitations there is interest in assessing the near surface geophysical technique Spectral Induced Polarization (SIP), to measure biofilm formation. Our study assesses the influence of cell density and biofilm production on SIP response. Laboratory experiments monitored changes in SIP, measured colony forming units (CFU), and cellular protein levels on sand packed columns inoculated with either Pseudomonas aeruginosa PAO1 (non-mucoid strain) or Pseudomonas aeruginosa FRD1 (biofilm-overproducing mucoid strain) cells over one month. Confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) were used to confirm the presence of biofilm. Our results indicate that phase and imaginary conductivity remained stable in PAO1 treatments as cell densities and cellular protein levels remained low (1.7x10⁵ CFUml^-1; 111 µg ml^-1). However, we observed a significant decrease in both phase (0.5 to -0.20 mrad) and imaginary conductivity (0.0 to -3.0x10^-5 S m^-1) when both cell densities and cellular protein levels increased. In FRD1 treatments we observed an immediate decrease in phase (0.1 mrad) and imaginary conductivity (-2.0x10^-6 S m^-1) as cell densities were an order of magnitude greater then PAO1 treatments and cellular protein levels surpassed 500 µg ml^-1. CLSM and SEM analysis confirmed the presence of biofilm and cells within both PAO1 and FRD1 treatments. Our findings suggest that the ratio of cells to cellular protein production is an important factor influencing both phase and imaginary conductivity response. However, our results are not in agreement with previous studies suggesting large phase shift (~50 mrads) and imaginary conductivity (5.5 S m^-1) values due to biofilm development. We hypothesize that large phase shift and imaginary conductivity response are due to trapping of conductive materials within the biofilm matrix, studies are currently underway to address this hypothesis.