Cellular morphology and density control kinetic energy spectra in bacterial monolayer swarms

In contrast to the universal power-law scaling observed in energy spectra of classical inertial turbulence, previous theoretical and experimental studies have revealed that active matter systems exhibit a pronounced system-specific dependence in their energy spectra. In this study, we experimentally investigate how cellular properties influence the kinetic energy spectra in bacterial monolayer swarms. We mapped a two-dimensional dynamical state diagram of bacterial swarming parameterized by cell area fraction and aspect ratio, with each state exhibiting distinct dynamical features. The scaling exponents of the power law regimes in the kinetic energy spectra show significant dependence on these parameters. Furthermore, our findings demonstrate that densely packed, elongated monolayer bacteria align into an active nematic state characterized by the formation of half-integer topological defects. These results highlight the critical role of cellular properties and population density in shaping the dynamics of bacterial swarming.