Run and Tumble of \textit{Escherichia coli} in Micropillar Arrays

Responses of microorganisms to emergent environmental conditions compose the dynamic nature of these biological systems. In an aqueous environment, an \textit{Escherichia coli} bacterium responds to a gradient of chemical attractant or repellant by frequently switching between its `run' and `tumble' modes. Despite these extensively studied chemotactic behaviors, it remains unclear how and whether an individual bacterium responds to mechanical signals, such as physical contacts with boundary walls. Such a potential mechanosensing to solid boundaries is associated with bacterial adhesions and thus crucial for formation of their aggregates, known as biofilms. Here, we applied a patterned array of micropillars as well-controlled mechanical stimuli to aqueous bacteria. We examined the run-and-tumble swimming of \textit{E.coli} subjected to these pillars. The long-term behaviors of individual bacteria were captured by a 3D tracking microscope for obtaining cell-specific statistics. By correlating the cellular behaviors to the pillar geometry and the detailed interactions, we explored the mechanisms of bacterial sensing and responding to solid structures by run-tumble statistics.