Two-dimensional and three-dimensional motion of Pseudomonas aeruginosa at the droplet interface

Abstract:

[Objective] The ubiquitous air-liquid interfaces affect bacterial motility and nutrient transport, thereby regulating the interaction among microbial populations and microbial community structure. Therefore, it is of vital importance for understanding and elucidating the mechanisms of microbial diversity generation and maintenance as well as the ecological functions to clarify the movement characteristics of microorganisms at microscopic interfaces. [Methods] With microfluidic microscopes (ultra-high speed fluorescence microscope and digital holographic microscope), we quantified the movement patterns of Pseudomonas aeruginosa (PAO1) cells near air-liquid-solid and air-liquid interfaces of droplet. [Results] Below the air-liquid interfaces, the trajectories of PAO1 are as follows:straight lines, clockwise circles, or counterclockwise circles with R_min(minimum radius of curvature)=3 µm. At the air-liquid-solid interfaces, 6.45% of the immobile cells accumulated at the edge of the interfaces and completed irreversible attachment directly. Meanwhile, due to capillary flow and Marangoni effect inside the droplet, mobile cells returned in the direction perpendicular to the interface or moved in the direction approximately parallel to the interface and attached after swimming in a straight line to a region within about 40 µm from the interface. These behaviors significantly modulated the spatial distribution of PAO1, promoting the migration toward the air-liquid-solid interface. Therefore, the active flagellar motility played a little role in the process. [Conclusion] With similar trajectories in both the solid-liquid interfaces and the air-liquid interfaces, PAO1 can move towards and subsequently attach onto the air-liquid-solid interfaces under the complex cell-surface interfacial forces.