Evidence for detachment of indigenous bacteria from aquifer sediment in response to arrival of injected bacteria.

Two bacterial strains isolated from the aquifer underlying Oyster, Va., were recently injected into the aquifer and monitored using ferrographic capture, a high-resolution immunomagnetic technique. Injected cells were enumerated on the basis of a vital fluorescence stain, whereas total cell numbers (stained target cells plus unstained target and antigenically similar indigenous bacteria) were identified by cell outlines emanating from fluorophore-conjugated antibodies to the two target strains. The arrival of injected bacteria at the majority of monitored sampling ports was accompanied by simultaneous temporary increases in unstained cell counts that outnumbered the injected bacteria by 2- to 100-fold. The origin and mechanism of appearance of the unstained cells are considered.