From order to chaos: Understanding the principles behind collective motion in bacteria

The collective motion of bacteria -- from stable swirling patterns to chaotic turbulent flows -- has intrigued scientists for decades. When a bacterial swarm is confined in small circular space, stable rotating vortices are formed. However, as the radius of this confined space increases, the organized swirling pattern breaks down into a turbulent state. This transition from ordered to chaotic flow has remained a long-standing mystery. It represents a fundamental question not only in the study of bacterial behavior but also in classical fluid dynamics, where understanding the emergence of turbulence is crucial for both controlling and utilizing complex flows.

In a recent study published in Proceedings of the National Academy of Sciences (PNAS)on March 14,2025, a research team led by Associate Professor Daiki Nishiguchi from Institute of Science Tokyo (Science Tokyo), Japan, has revealed in detail how bacterial swarms transition from organized movement to chaotic flow. Combining large-scale experiments, computer modeling, and mathematical analysis, the team observed and explained previously unknown intermediate states that emerge between order and turbulence.

Their experimental approach involved creating numerous circular wells of different sizes using advanced microfabrication technology and acquiring high-quality video footage, allowing them to observe bacterial population behaviors across various confinement conditions. The experiments revealed that vortex reversal is the first sign of destabilization; simply put, as the confinement radius increases beyond a critical size, the initial stable vortex gives way to two competing vortices that periodically reverse their rotation direction. As the space grows larger, this pattern evolves into a four-vortex configuration with pulsating fluctuations, before finally transitioning into fully developed turbulence. These observations provide the first detailed view of how bacterial swarm vortices gradually lose their orderly movement patterns as a result of changes in their confinement.

The research team also conducted theoretical analyses and simulations, which revealed that these transitions arise from the interplay of specific mathematical patterns called azimuthal modes that become unstable as the confinement radius increases. "Our findings shed light on the universal properties of confined bacterial active matter, and can be applied to various other biological and synthetic active matter systems," says Nishiguchi. The remarkable agreement between their experimental observations, computer simulations, and mathematical predictions validates their comprehensive approach to understanding this complex phenomenon.

In the future, this intriguing discovery could be translated into sophisticated applications. "The insights revealed in our study provide novel design principles for functioning active devices, such as biosensors or micro-robotics swarms, and have elucidated how geometrical confinements can modify the collective motion of active matter," notes Nishiguchi. Moreover, this newfound understanding could be particularly valuable for developing active fluid-based systems on a microscopic scale that exploit controlled collective motion.

Overall, this work represents a significant advance in active matter physics, a field that seeks to shed light on the governing mechanisms behind self-propelled systems ranging from bacterial colonies to bird flocks and fish schools. Future studies will focus on characterizing transitions in different geometries beyond circular confinement and quantifying the effects of environmental noise, pushing the boundaries of what's possible in active matter engineering.