Swimming and rafting of E.coli microcolonies at air-liquid interfaces

Abstract

The dynamics of swimming microorganisms is strongly affected by solid-liquid and air-liquid interfaces. In this paper, we characterize the motion of both single bacteria and microcolonies at an air-liquid interface. Both of them follow circular trajectories. Single bacteria preferentially show a counter-clockwise motion, in agreement with previous experimental and theoretical findings. Instead, no preferential rotation direction is observed for microcolonies suggesting that their motion is due to a different physical mechanism. We propose a simple mechanical model where the microcolonies move like rafts constrained to the air-liquid interface. Finally, we observed that the microcolony growth is due to the aggregation of colliding single-swimmers, suggesting that the microcolony formation resembles a condensation process where the first nucleus originates by the collision between two single-swimmers. Implications of microcolony splitting and aggregation on biofilm growth and dispersion at air-liquid interface are discussed.

Keywords: E.coli; biofilms; microcolony; modeling; motility.

Figure 1

(a) Sketch of the experimental set‐up. The E.coli suspension is dropped onto the coverslip stuck on the cavity slide. The cavity slide is kept upside‐down in the inverted microscope slide holder. (b) Typical snapshots. Single E.coli and microcolonies coexist. Different microcolonies configuration can be observed

Figure 2

Single E.coli swimmers. The microswimmer trajectories are constituted by a sequence of circular arcs (a,b). The cusps between two consecutive arcs correspond to tumbling phases. In few cases, complete circles are apparent. C) Radius of curvature R versus swimming velocity v. Each point corresponds to a single circular arc. Red and blue points refer to CW and CCW trajectories, respectively. Horizontal and vertical dashed lines are the mean values. CCW swimmers move significantly faster than CW swimmers (p < 10⁻⁶) while radius of curvature difference is not statistically significant (p > .05)

Figure 3

Microcolonies move like 2D rigid rafts suspended at the air–liquid interface and exhibit both CCW (a–c) and CW motion (d–f). Panel G reports the scatter plot of the speed versus the radius of curvature of the microcolony center. Red and blue symbols refer to CW and CCW motion of the raft center, respectively. CW motion occurs 49% of the cases while CCW 51%, the difference is not significant (p = .91). Horizontal and vertical lines correspond to the average CW and CCW radius of curvature and speed. No significant difference is observed between CW and CCW for both average speed and radius of curvature R (p > .2 for both comparisons)

Figure 4

Microcolony growth and splitting. (a) A single swimmer (yellow dashed circle) hits a microcolony and merges with it. (b) A small microcolony (yellow dashed circle) collides with a large raft and merges with it while a single swimmer (white continuous circle) hits the large raft, it is trapped for a while on the raft contour and, finally, escapes. (c) A small portion of a large microcolony (yellow dashed circle) splits off and starts to raft independently while a single bacterium (white continuous circle) first swims at a constant distance form the microcolony and then is trapped by it

Figure 5

Schematic model of the microcolony. The colony is represented as a raft moving on the air–water interface (panel A). The motion is described by three degrees of freedom, namely, the position of the center of mass x_cm = (x_cm, y_cm) and the angle θ between the unit vector e ₁ of the body reference frame and the x‐axes of the fixed reference frame. The bacteria at the contour of the raft exert a force on the raft, see, for example, f₁ and f₂ in panel B. Panels C, D, and E report the average velocity of the microcolony center v _cm, the angular velocity of the microcolony ω, and the radius of curvature of the trajectory of the microcolony center as functions of the number of bacteria forming the microcolony (N). Red and blue circles correspond to raw data for CW and CCW rotation of the microcolony center, respectively. Black points represent binned data while dashed lines correspond to power‐law fits