BACTERIAL ENRICHMENT AT THE GAS-WATER INTERFACE OF A LABORATORY APPARATUS

The gas-water interface (GWI) is likely to have important effects on b acterial adsorption and transport in unsaturated porous media. A glass apparatus that isolated GWIs in ports above a flowthrough suspension of a groundwater bacterial isolate was used to represent unsaturated p orous media. The surface microlayer was collected by placing a polycar bonate filter on the GWI. The filter was stained, and the bacteria wer e enumerated by direct count. The significance of five independent var iables on the surface density of cells (s, in cells per square millime ter) was determined by nonlinear multiple regression. Three of the var iables were shown to be significant: surfactant concentration (d), tim e (t), and bulk bacterial concentration (B). The surface density decre ased with increasing d and increased with increasing t and B. When sur factant was absent, the GWI became highly enriched in bacteria. For ex ample, when d = 0, 48 h < t < 72 h, and 5,000 cells mm(-3) < B < 10,00 0 cells mm(-3), s averaged 3.0 x 10(4) cells mm(-2). This surface dens ity occupied about 6.0% of the GWI, and the surface microlayer concent ration of cells was 190 times the bulk concentration. The other two va riables: pH (p) and ionic strength (I) were shown to be insignificant. The strong effect of d and the lack of effect of I and p support the hypothesis that hydrophobic interaction dominates bacterial adsorption to the GWI.