Abstract
Collective bacterial dynamics plays a crucial role in colony development. Although many research groups have studied the behavior of fluidic swarm colonies, the detailed mechanics of its motion remains elusive. Here, we developed a visualization method using submicron fluorescent beads for investigating the flow field in a thin layer of fluid that covers a Bacillus subtilis swarm colony growing on an agar plate. The beads were initially embedded in the agar plate and subsequently distributed spontaneously at the upper surface of the expanding colony. We conducted long-term live cell imaging of the B. subtilis colony using the fluorescent tracers, and obtained high-resolution velocity maps of microscale vortices in the swarm colony using particle image velocimetry. A distinct periodic fluctuation in the average speed and vorticity of flow in swarm colony was observed at the inner region of the colony, and correlated with the switch between bacterial swarming and growth phases. At the advancing edge of the colony, both the magnitudes of velocity and vorticity of flow in swarm colony were inversely correlated with the spreading speed of the swarm edge. The advanced imaging tool developed in this study would facilitate further understanding of the effect of micro vortices in swarm colony on the collective dynamics of bacteria.
Highlights
Biosurfactants secreted via the quorum sensing mechanism play important roles in biofilm and swarm dynamics [1,2]
We studied an interplay between the spreading rate of the B. subtilis swarm edge and microscale fluid dynamics at the upper surface of the swarm through long-term time-lapse imaging of advancing swarm edge and the submicron fluorescent beads that were initially embedded in the agar plate and distributed spontaneously on the upper surface of the growing colony
As Be’er and Harshey [7] mentioned that superdiffusive behavior is caused by the interaction of the collective motion of the bacteria within swarm, the bacterial cells in the B. subtilis colony initially touch the beads or make flow stream generating shear stress and the movement of the beads is driven by the adjacent viscous flow
Summary
Biosurfactants secreted via the quorum sensing mechanism play important roles in biofilm and swarm dynamics [1,2]. Tuson et al [4] developed a technique for measuring the mechanical properties of bacteria in vivo They considered the complex structure of the cell wall and used stiffness as an effective Young’s modulus. Shaw et al [5] demonstrated that biofilm structure in bacterial colonies is not just a simple material but is complex with fluidic characteristics such as viscoelasticity This concept provides useful tools for studying the biophysics of bacteria. We measured the velocity of bacterial cells grown on agar plates of different agar concentrations between 0.5% and 2.5% using a multi-particle tracking technique, and found that bacteria in the swarm colony have the ability of periodic switching between the growth and swarming phases [8,9]. We studied an interplay between the spreading rate of the B. subtilis swarm edge and microscale fluid dynamics at the upper surface of the swarm through long-term time-lapse imaging of advancing swarm edge and the submicron fluorescent beads that were initially embedded in the agar plate and distributed spontaneously on the upper surface of the growing colony
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