Chemotactic behavior of Escherichia coli at high cell density

Dense suspensions of swimming bacteria are known to exhibit self-organized collective motility patterns, observed both in artificial and natural situations, e.g. during swarming, pellicle biofilm formation or in travelling chemotactic bands. Coincidentally, bacteria must perform physiologically relevant functions, such as chemotaxis, which allows finding optimal ecological niches by biasing their otherwise random walk according to gradients of environmental conditions. Here we studied the influence of collective behavior on chemotaxis of the model organism Escherichia coli in attractant gradients created in microchannels, and varying cell density, cell length and the degree of two-dimensional confinement of bacterial suspension. The typical eddy size of swirling collective motion observed at high cell density was set by the channel height, while its magnitude scaled with cell volume fraction and swimming speed but also depended on the channel height. This indicated that the collective motion predominantly emerged as a result of hydrodynamic interactions between swimming bacteria. After reaching a maximum at an intermediate volume fraction of about 1%, chemotaxis was strongly impaired by the collective motion, with the chemotactic drift decreasing exponentially with its magnitude. At moderate confinement, elongated cells retained their chemotactic ability until higher density than normally-sized cells. Simulations of self-propelled, chemotactic and sterically-interacting rods were in qualitative agreement with the experiments, suggesting that reorientations induced by the physical interactions between cells were responsible for chemotaxis impairment. Collective motion thus exerts a strong physical control on the ability of bacteria to navigate chemical gradients, with important implications for high-density behaviors of motile bacteria.