(539d) Cargo Carrying Bacteria at Interfaces

We have been studying the motion of passive colloids at the interface between oil and bacterial suspensions. The bacterial strain Pseudomonas aeruginosa PA14DpelA does not restructure the interface to form a viscoelastic film. Rather, these bacteria remain motile over several hours. This allows us to study bacteria interactions with passive colloidal particles trapped at the interface. The colloids have enhanced mobilities; these are commonly observed, and typically ascribed to hydrodynamic interactions with the swimming bacteria. However, inspection of trajectories of hundreds of particles over time scales of seconds reveals directed colloid displacements that are inconsistent with purely hydrodynamic interactions with swimming bacteria. The individual colloid paths reveals new mechanism of transport caused by direct adhesion of the bacteria to the colloids; the bacteria, adhered to colloids, are carrying them as cargo. We sort colloid trajectories into four different categories, including diffusive, persistent, curly, and mixed trajectory types. We show that trajectories in each category have distinctive characteristics. This study was performed in collaboration with Liana Vaccari, Mehdi Moelai and Robert Leheny.