A scarcity of comparable research on the transport of bacteria has forced hydrologic models to assume that bacteria travel as dissolved chemicals. In reality, most bacteria preferentially attach to soil aggregates, and behave very differently from planktonic bacteria. The goal of this research project was to identify and evaluate a laboratory method for partitioning between attached and planktonic bacteria that could be used to improve hydrologic modeling.
Attachment was measured indirectly as the difference between total and planktonic bacterial concentration. Planktonic concentration was defined as the concentration of bacteria that could pass through an 8 μm screen. Total concentration was determined by disaggregating attached bacteria through a dispersion treatment. A randomized complete block design was structured to test for the effects of filtering, two dispersion treatment options, and the presence of soil on concentration. Tween-85 surfactant was selected as the best dispersant for use in further studies. About 78% of bovine E. coli in the laboratory samples were adsorbed/associated with sterile soil particles.
Twenty samples of different bacteria-soil ratios were analyzed using this method to develop an isotherm equation describing E. coli partitioning. The E. coli used to inoculate these samples was cultured using a chemostat reactor to control cell growth stage and control variability. A linear isotherm (R2=0.88) was selected to describe this experimental data; however, future studies characterizing the partitioning behavior of E. coli under different environmental conditions are recommended in order to better understand attachment prior to modeling attached and planktonic E. coli separately.
