RESIDENCE TIME CALCULATION FOR CHEMOTACTIC BACTERIA WITHIN POROUS-MEDIA

Local chemical gradients can have a significant impact on bacterial po pulation distributions within subsurface environments by evoking chemo tactic responses. These local gradients may be created by consumption of a slowly diffusing nutrient, generation of a local food source from cell lysis, or dissolution of nonaqueous phase liquids trapped within the interstices of a soil matrix. We used a random walk simulation al gorithm to study the effect of a local microscopic gradient on the swi mming behavior of bacteria in a porous medium. The model porous medium was constructed using molecular dynamics simulations applied to a flu id of equal-sized spheres. The chemoattractant gradient was approximat ed with spherical symmetry, and the parameters for the swimming behavi or of soil bacterium Pseudomonas putida were based on literature value s. Two different mechanisms for bacterial chemotaxis, one in which the bacteria responded to both positive and negative gradients, and the o ther in which they responded only to positive gradients, were compared . The results of the computer simulations showed that chemotaxis can i ncrease migration through a porous medium in response to microscopic-s cale gradients. The simulation results also suggested that a more sign ificant role of chemotaxis may be to increase the residence time of th e bacteria in the vicinity of an attractant source.