Orbiting of Flagellated Bacteria within a Thin Fluid Film around Micrometer-Sized Particles

Abstract

Bacterial motility under confinement is relevant to both environmental control and the spread of infection. Here, we report observations on Escherichia coli, Enterobacter sp., Pseudomonas aeruginosa, and Bacillus subtilis when they are confined within a thin layer of water around dispersed micrometer-sized particles sprinkled over a semisolid agar gel. In this setting, E. coli and Enterobacteria orbit around the dispersed particles. The liquid layer is shaped like a shallow tent with its height at the center set by the seeding particle, and the meniscus profile set by the strong surface tension of water. The tentshaped confinement and the left handedness of the flagellar filaments result in exclusively clockwise circular trajectories. The thin fluid layer is resilient because of a balance between evaporation and reinforcement of fluid that permeated out of the agar. The latter is driven by the Laplace pressure caused by the concave meniscus. In short, we explain the physical mechanism of a convenient method to entrap bacteria within localized thin fluid film near a permeable surface.

Confinement Discerns Swarmers from Planktonic Bacteria

“…A novel bacterial strain Enterobacter sp. SM3 (NCBI BioProject PRJNA558971), isolated in 2014 from mice with colitis induced with dextran sulfate sodium (DSS), has been previously studied for motility(Araujo, Chen, Mani, & Tang, 2019) and host phenotype(Chen et al, 2020). SM3 expands rapidly on 0.5% agar with the collective motion of multilayers of cells at the edge.…”

Confinement discerns swarmers from planktonic bacteria

“…Similarly, there is a sideway component acting on the flagella bundle at the tail end of the cell in the opposite direction. These two sideway forces result in a net torque on the bacterium, causing it to make gradual right turns (Araujo, Chen, Mani, & Tang, 2019;DiLuzio et al, 2005). In our case, the agar surface is the bottom surface, and the bacteria tend to follow a clockwise curved trajectory near the bottom (when viewed from the top).…”

“…A novel bacterial strain Enterobacter sp. SM3 (NCBI BioProject PRJNA558971), isolated in 2014 from mice with colitis induced with dextran sulfate sodium (DSS), has been previously studied for motility (Araujo, Chen, Mani, & Tang, 2019) and host phenotype (De et al, 2021). SM3 expands rapidly on 0.5% agar with the collective motion of multilayers of cells at the edge.…”