Tracking of injected and resident (previously injected) bacterial cells in groundwater using ferrographic capture

A high-resolution bacterial tracking technique, ferrographic capture, was used to enumerate fluorescent-stained bacterial cells that were injected into groundwater during a field experiment. The goal of the experiment was to investigate whether detachment of previously injected stained resident cells attached to aquifer sediment was enhanced in the presence of the newly injected mobile cells. This injection was an improvement on past experiments in that the attached (resident) cells were stained, allowing their concentrations to be enumerated directly by ferrographic capture (upon detachment). Contrary to expectations based on previous experiments, enhanced detachment of stained resident cells did not occur upon the arrival of injected cells. Consistent with previous experiments, however, was the observation of ephemeral increases in unstained cell concentrations coincident with the arrival of the stained injected cells. The ephemeral pulses of unstained cells were previously speculated to represent enhanced detachment of unstained indigenous cells in response to hydrodynamic collision with injected cells. The lack of enhanced detachment of stained resident cells in the present experiments indicates that increased concentrations of unstained cells may have occurred by mechanisms other than hydrodynamic collision. Visually observed variations in stain intensity indicated that increased unstained cell concentrations may have resulted from cell division at the low-concentration fringe of the injected plume.