| dc.description.abstract | Bacterial colonization is the first step of infection and invasion. Agar plate is a model system to study bacterial colonization. After inoculating few cells on the two dimensional agar surface, bacteria will start to grow and then expand by their swarm motility. This is a highly dynamical process in a reduced dimension system. However, very little is known regarding the physical environments of swarm colony. Three major characteristic features among different swarming cells are cell length elongation, hyperflagellation and swarming lag. To study the dynamics of bacterial colony development, we measured colony profiles, contact angles, and effective viscosity to clearly the hydrodynamical mechanisms of these three major swarming cell features.
1. Contact angle and surface profile and swarming lag:
The surface is super-hydrophilic and the contact line is moving as the colony develops. We use the strain of Vibrio alginolyticus, YM19, which only have lateral filament. YM19 have great surface swarm motility on agar plate. During the YM19 colony development, the contact line of the development process will change with the colony expansion. We design a confocal surface profile measurement method on thin film system to reconstruct the three dimensional structures from the laser scanning images. Thus we can measure the high resolution surface profiles near contact lines during the colony development. Unlike thermal equilibrium droplet on the surface, bacterial colony changes dynamically with time. The contact angle shows a hysteresis during the colony devilment indicating a pinning effect of the contact line.
2. Effective viscosity and hyperflagellation:
The colony is a thin layer of liquid above agar surface. The rheological condition is within a top air-liquid interface and a bottom liquid-agar interface. The effective viscosity in the colony varies as time increases. We measure the local effective viscosity by measuring diffusion of micron size polystyrene beads within the simulated colony. The result shows an increscent of effective viscosity near the agar surface. Therefore we suspect the swarming-cell hyperflagellation is the results of increasing thrusting force near the surface.
3. Hydrodynamical advantage of cell length elongation.
We compare the drag coefficient of long rods and the thrusting force of long rods with uniform flagella density. We found that the elongation is hydrodynamically beneficial for swarming cells to have up to several times thrusting force ratio.
The aim of this thesis is to understand the physical origin of the three major swarm cells characteristic features. All of them can be explained by hydrodynamical mechanism. Combining physical parameter measurements, rheological measurements and computation simulation, we are able to build up a complete physical model to understand the dynamical development of swarming colony such as Vibrio alginolyticus.
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