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The mechanics of motion and confinement of flagellated bacteria at air-water interface

Michael Morse (Brown University), Jay Tang (Brown University)

Hydrodynamics of Swimming Microorganisms

Tue 4:20 - 5:40

Barus-Holley 191

Flagellated bacteria have adapted to diverse environments, including interfaces of multiple media. We study the behavior of Caulobacter crescentus, a singly flagellated bacterium, at the air/water interface. Forward swimming C. crescentus swarmer cells tend to get trapped at the surface when swimming in a nutrient rich growth medium but not in a minimal salt motility medium. Trapped cells move in tight, clockwise circles when viewed from the air with reduced speed. Trace amounts of surfactant release the trapped cells from these circular trajectories. We show, by tracing the motion of positively charged colloidal beads near the interface that organic molecules in the growth medium adsorb at the interface, creating a high viscosity film. Consequently, the air/water interface no longer acts as a free surface. Instead, forward swimming cells become trapped hydrodynamically. Added surfactants efficiently partition to the surface, replacing the viscous layer of molecules and reestablishing free surface behavior. These findings help explain similar behavior of E. coli, moving in trajectories of variable curvature depending on media chemistry. The consistent behavior of these two distinct microbial species provides insights on how microbes have evolved to cope with challenging interfacial mechanics.