Elasticity-mediated nematiclike bacterial organization in model extracellular DNA matrix

DNA is a common extracellular matrix component of bacterial biofilms. We find that bacteria can spontaneously order in a matrix of aligned concentrated DNA, in which rod-shaped cells of Pseudomonas aeruginosa follow the orientation of extended DNA chains. The alignment of bacteria is ensured by elasticity and liquid crystalline properties of the DNA matrix. These findings show how behavior of planktonic bacteria may be modified in extracellular polymeric substances of biofilms and illustrate the potential of using complex fluids to manipulate embedded nanosized and microsized active particles.

Figure 1

Fluorescence images of unidirectionally aligned P. aeruginosa cells in the aligned LC matrix of concentrated DNA; the number density of bacteria increases from (a) to (b). The signal is from the green fluorescent protein in cells.

Figure 2

(Color online) Schematics of the sample preparation procedure. (a) A droplet containing aqueous DNA in the globular states (red) and the rod-shaped bacteria (green) is placed on a clean glass plate. (b) Biopolymers and bacteria are concentrated in a narrow ring next to the contact line; DNA biopolymers are stretched in directions parallel to the contact line and the rod-shaped cells follow their orientations. (c) Residual water from the droplet’s center is removed using a microsyringe. (d) Aqueous DNA with concentric LC director and bacteria orientations, obtained after the sample is covered by a glass plate to prevent further water evaporation.

Figure 3

(Color online) Polarizing microscopy textures of the LC film formed by DNA biopolymers obtained (a) between crossed polarizers, and (b) and (c) with additional retardation plate inserted into the optical path; the orientations of polarizers and “slow” axis of the plate are shown in (c). ${\widehat{n}}_0$ is marked by the white double arrow. The second-order blue and first-order yellow interference colors in (b) and (c) indicate addition and/or subtraction of the optical phase shift when the inserted retardation plate has the “slow” axis perpendicular and/or parallel to ${\widehat{n}}_0$ in the DNA LC with negative optical anisotropy.

Figure 4

(Color online) Rods in the ordered biopolymer matrix. Schematics of a rod oriented (a) parallel, (b) perpendicular, and (c) at an angle $\theta$ with respect to the far-field LC director ${\widehat{n}}_0$. (d), (e) Computer-simulated alignment of the director $\widehat{n}$ (and DNA molecules) around a rod-shaped object (d) parallel and (e) perpendicular to ${\widehat{n}}_0$ in the cross sections marked in (a) and (b).