Dual chemotaxis strategy of bacteria in porous media: Run-time and turn-angle bias.

Chemotaxis of bacterial swimmers that move in a run-and-turn pattern is well studied in uniform bulk fluid. It is primarily based on modulating the run time in dependence on the swimming direction with respect to the source of chemoattractant (run-time bias). Here, we provide evidence that the lophotrichously flagellated soil bacterium Pseudomonas putida may also perform chemotaxis in porous media where the free path length is severely restricted. Besides the classical run-time bias, we identify a second chemotaxis strategy, the turn-angle bias: the change in swimming direction upon a turn event is adjusted so that the orientation of the subsequent run phase is more likely to be directed toward the source of chemoattractant. We show that the resulting directional preference of runs arises from active, motor-induced turning events. Passive mechanical trapping in the porous matrix, in contrast, weakens the resulting preference of run orientations. Agent-based simulations, which rely on experimentally observed statistical properties of swimming patterns, indicate that the turn-angle bias is the predominant chemotaxis strategy of bacteria in porous environments.