Topological defects as quantitative probes in bacterial active matter

Bacterial colonies provide a natural setting for testing ideas in active matter physics. In a thin layer of the rod-shaped bacterium Myxococcus xanthus, the director field exhibits nematic order with half-integer defects, whose geometry and motion reflect the dynamics of the system. Building on prior agreement between theory and experiment at the level of average properties (director order, flows, mechanical stresses), we focus on the relationship between detailed structure of individual defects and their properties, and what they reveal about both time-averaged and instantaneous behaviors. We outline a framework that translates local geometric features into interpretable dynamical signatures, enabling richer information extraction from standard imaging data. This links single-cell activity to mesoscale pattern formation, and positions topological defects as quantitative probes for understanding and, eventually, controlling living systems.