Collective bacterial condensation is fundamentally constrained by the emergence of active turbulence

Collective bacterial condensation arises from positive feedback between the ability of bacteria to generate chemical gradients in their environment and their chemotactic ability to follow those gradients. This feedback drives the spontaneous formation of local cell accumulations, characterized by sharp cell-density gradients, even in the absence of physical boundaries. By following the dynamics of bacterial condensation in uniform acidic environments, we show that condensation is critically constrained by the spontaneous emergence of correlated bacterial swimming and the associated active turbulence. These collective behaviors generate vortex-like cell motion with a pronounced radial component directed down the cell-density gradient. This, in turn, induces local fluid motion that broadens the condensate and expels non-chemotactic bacteria. When condensates are strongly confined in thin layers, the radial component of fluid motion diminishes and condensation is enhanced. Moreover, in porous environments, correlated bacterial motion is strongly suppressed, allowing spontaneous condensation to progress even further until it reaches the limits imposed by the non-local nature of bacterial chemotaxis. Overall, these findings highlight the fundamental interplay between self-generated bacterial condensation and correlated swimming. Significance statement Self-induced condensation and active turbulence are two prominent forms of collective bacterial behaviors. However, their interplay has not been experimentally studied. Here, we show that collective bacterial condensation gives rise to correlated swimming, which in turn fundamentally limits further condensation. Furthermore, we demonstrate that in dilute porous environments—common in natural bacterial habitats—correlated swimming is strongly suppressed, enabling the spontaneous formation of extremely dense cell condensates. These condensates may serve as a basis for the development of more structured bacterial communities.