Transverse Migration of Chemotactic Bacteria Toward Chemical Attractants in a Two Dimensional Microcosm with Converctive Flow

Bioremediation is often limited by the inability to bring populations of bacteria capable of degradation into contact with groundwater contaminants. Chemotaxis, the ability of bacteria to sense a chemical gradient and swim preferentially towards locations of high concentration, can enhance the biased transport of bacteria. Improving bacterial transport to contaminant sources in groundwater aquifers offers a way to potentially improve the overall effectiveness of in situ bioremediation. A two-dimensional microcosm packed with quartz sand was used to quantify the effect of chemotaxis on the migration of bacteria in porous media. Aqueous media was pumped across the microcosm at approximately 1 m/day to simulate the conditions found naturally in a groundwater aquifer. A plume of sodium benzoate was continuously injected into the microcosm to create an attractant gradient transverse to flow. The chemotactic bacteria, Pseudomonas putida F1, or the nonchemotactic mutant, Pseudomonas putida F1 CheA, were injected with a fluorescent tracer above or below the attractant. A moment analysis was implemented to estimate the center of mass, variance, and skewness of the concentration profiles. The transverse dispersion coefficient and the transverse dispersivity transport parameters were also determined.
Results show that the center of mass for the chemotactic bacteria was closer to the attractant source on average than the nonchemotactic control when compared to the uranine tracer for experiments using a pulse injection of bacteria. Experiments were also performed using a continuous injection of bacteria and the center of mass for chemotactic bacteria was closer to the attractant source on average than the nonchemotactic control when compared to the uranine tracer. These results showed that chemotaxis can increase bacterial transport toward contaminants, potentially enhancing in situ bioremediation. Experiments with 3 cm and 2 cm spacing between bacteria and attractant were performed to explore the relationship between the exposure time of the bacteria to attractant and the transverse migration of bacteria due to chemotaxis. A difference was not found between the experimental results for 3 cm spacing and 2 cm spacing.
Experimentally determined transport parameters were used as input to a two-dimensional mathematical model. Modeling was used to test the effects of changing the chemotactic sensitivity coefficient and the chemotaxis receptor constant at three different bacteria and attractant separation distances: 4 cm, 3 cm, and 2 cm. A chemotactic sensitivity coefficient of 10-4 cm2/s was found to match the change in center of mass determined experimentally for 3 cm and 2 cm separation distances. Model results showed the center of mass shift for chemotactic bacteria was greater for 3 cm and 2 cm spacing than 4 cm spacing at constant chemotactic sensitivity coefficient values, which shows that increasing the exposure time of the bacteria to the attractant can increase the transverse migration of bacteria. Mathematical modeling is a valuable tool that can be used to predict which values of chemotactic sensitivity coefficient, chemotaxis receptor constant, and injection spacing will provide the greatest transverse migration of chemotactic bacteria.