Molecular Motors Govern Liquidlike Ordering and Fusion Dynamics of Bacterial Colonies

Bacteria can adjust the structure of colonies and biofilms to enhance their survival rate under external stress. Here, we explore the link between bacterial interaction forces and colony structure. We show that the activity of extracellular pilus motors enhances local ordering and accelerates fusion dynamics of bacterial colonies. The radial distribution function of mature colonies shows local fluidlike order. The degree and dynamics of ordering are dependent on motor activity. At a larger scale, the fusion dynamics of two colonies shows liquidlike behavior whereby motor activity strongly affects surface tension and viscosity.

Figure 1

(a) Confocal section of gonococcal $\mathrm{wt}*$ (Ng150) microcolony after 6 h. Reconstruction of $2\mu \mathrm{m}$ slices of bacterial coordinates after (b) 6 and (c) 24 h of growth. The colors encode the number of nearest neighbors $n$. (d) Radial distribution functions $g(r)$ after 6 h (black) and 24 h (red). Shaded lines: $g(r)$ of individual colonies. Full line: fit to Eq. (1). Dots and error bars: mean and standard deviation of 15 colonies from three independent experiments. Scale bar: $10\mu \mathrm{m}$. (e) Hypothetical sketch of how T4P retraction supports local ordering.

Figure 2

Tuning T4P motor dynamics. (a) Experimental setup for characterizing the velocity of pilus retraction. (b) Velocity distribution of T4P retraction for force clamped at $F=30\mathrm{pN}$. Gray: $\mathrm{wt}*$ (Ng170); red: $pil{T}_{WB1}$ (Ng171); green: $pil{T}_{WB2}$ (Ng176).

Figure 3

Tuning bacterial interaction dynamics. (a) Experimental setup. A spherical cell is trapped in each of two laser traps. The deflection $d$ of the cell body from the center of the trap yields the force acting on both cell bodies $F\sim d$. When pili of both cells bind to each other and one of them retracts, both cell bodies are pulled towards each other. (b) Typical force $F$ as a function of time $t$. (c) Distribution of rupture forces for gray: $\mathrm{wt}*$ (Ng170); red: $pil{T}_{WB1}$ (Ng171); green: $pil{T}_{WB2}$ (Ng176). Forces were measurable up to 80 pN. ($N>300$ for each strain). (d) Frequencies of T4P retraction, elongation, pausing, bundling, and rupture. Color codes as in (c). $N=38–844$ per strain and condition; error bars: bootstrapping with $N=100$. (e) Probability that at least one pair of pili from different cells are attached to each other. Error bars: bootstrapping.

Figure 4

(a) Typical confocal section after 6 h and (b) radial distribution functions $g(r)$ of T4P retraction reduced $pil{T}_{WB2}$ (Ng176) microcolony after 6 h (black) and 24 h (red). (c) Typical confocal section after 6 h and (d) radial distribution functions $g(r)$ of T4P retraction inhibited $\mathrm{\Delta }pilT$ (Ng178) microcolony after 6 h (black) and 24 h (red). Shaded lines: $g(r)$ of individual colonies. Full line: fit to Eq. (1). Dots and error bars: mean and standard deviation of at least 15 colonies from three independent experiments. Scale bar: $10\mu \mathrm{m}$. (e) Contact distance $r_0$ and (f) value of radial distribution function at $r_0$ obtained from fit to Eq. (1).

Figure 5

(a) Time lapse of $\mathrm{wt}*$ colony fusion event. (b) Typical $v^{1/3}[\tau (f)-\tau (f_0=0.71)]$ as a function of time. Gray circles: $\mathrm{wt}*$ (Ng151); red crosses: $pil{T}_{WB1}$ (Ng171). Full lines: fits to Eq. (3). (c) Distribution of ratio between surface tension $\sigma$ and viscosity $\eta$ obtained from Eq. (3). Gray: $\mathrm{wt}*$ (Ng151); red: $pil{T}_{WB1}$ (Ng171). $N>30$ colonies.