An experimental and numerical investigation was conducted to study interactions between microbial dynamics and transport processes in variably saturated porous media. Experiments were conducted with constant, surface-applied water fluxes in duplicate, variably saturated, sand-filled columns that were uniformly inoculated with the bacterium Pseudomonas fluorescens HK44. The permeability of the sand in the columns was reduced by a factor of 45 during 1 wk of growth on glucose. Pressure heads increased (became less negative) at all measured depths, but significant increases in the apparent volumetric water contents were observed in only the upper 5 cm of the columns, corresponding to the areas with the highest concentrations of attached bacteria. A numerical model was used to simulate the experiments. The model accounted for the processes of water flow, solute and bacterial transport, cell growth and accumulation, glucose and O2 consumption, and gas diffusion and exchange. Observed changes in water content and pressure head were reproduced approximately using fluid-media scaling to account for an apparent surface-tension lowering effect. Reasonable correspondence was obtained between observed and simulated effluent data and final attached biomass concentration distributions using first-order reversible cell attachment and detachment kinetics. The attachment rate coefficients were based on particle-filtration theory and time-dependent detachment rate coefficients. The results of this study illustrate the potential importance of using fully coupled multifluid flow and multicomponent reactive transport equations to model coupled biogeochemical and transport processes in soils.