Bacterial chemotaxis in crowded environments

Swimming bacteria can adapt their motility in the face of environmental cues and stresses. In response to nutrient gradients, bacteria lower their tumbling frequency when going up the gradients, leading to a net drift toward the nutrient source. In the presence of flows and in crowded environments, however, bacterial swimming patterns change. They get trapped near surfaces due to flow shear, and around obstacles due to hydrodynamic/steric interactions. While bacteria often live in such complex environments, our understanding of bacterial chemotaxis has mostly remained limited to simple environments with 1D steady nutrient gradients. Here, using microfluidic experiments and numerical simulations, we probe the role of chemotaxis in dynamic environments in the presence of flows and obstacles on the bacterial colonization of nutrient sources. We discuss the implications of our observations for bacterial colonization of marine snow in the oceans and nutrient hotspots in the soil.