Bacterial enrichment at the gas-water interface of a laboratory apparatus.

The gas-water interface (GWI) is likely to have important effects on bacterial 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 porous media. The surface microlayer was collected by placing a polycarbonate filter on the GWI. The filter was stained, and the bacteria were enumerated by direct count. The significance of five independent variables on the surface density of cells (s, in cells per square millimeter) was determined by nonlinear multiple regression. Three of the variables were shown to be significant: surfactant concentration (d), time (t), and bulk bacterial concentration (B). The surface density decreased with increasing d and increased with increasing t and B. When surfactant was absent, the GWI became highly enriched in bacteria. For example, when d = 0, 48 h < t < 72 h, and 5,000 cells mm(sup-3) < B < 10,000 cells mm(sup-3), s averaged 3.0 x 10(sup4) cells mm(sup-2). This surface density occupied about 6.0% of the GWI, and the surface microlayer concentration of cells was 190 times the bulk concentration. The other two variables: 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.