Dr. Silverio Johnson Successfully Defended his Biological Physics Dissertation
Recent Research Matters Series speaker Silverio Johnson has now successfully defended his thesis, “Swimming Kinematics of Enterobacter sp. SM3.”
Below is his abstract:
Enterobacter sp. SM3, is a recently-isolated species of gut bacteria that proficiently spreads over nutritional substrates—-a motility pattern called swarming. The connection between the motility of individual bacteria and the higher-order organization observed at the level of a swarm is not well understood and is a function of both biological and physical factors. In this defense, I will characterize and describe the motility of SM3 in liquid media, drawing physical insights from my findings.
I imaged SM3, grown both as individuals and as swarmers, with two-dimensional optical microscopy. Using tracking algorithms, I extracted motility parameters including the average speed, turning time, run time, and turning angle. I found that there were distinct differences in these parameters between swimmers and swarmers—with swarmers turning less often and for smaller angles, resulting in smoother trajectories. Using a mechanical model, my results show that the main difference in motility can be physically attributed to the longer length of swarmers.
Taking advantage of the variable length of swarmers, I imaged swarmers of different length swimming in close proximity to a surface and studied their behavior. Generally, bacteria near surfaces swim in circular trajectories due to the counter rotation of their cell body and flagellar bundle, which hydrodynamically couple with the surface. I found that swarmers of longer length traced circular trajectories that were larger in radius than shorter ones. We modeled swimming bacteria as two counter rotating spheres separated by a distance and connected by a spring. The results of the simulation showed that the dependency of radius on cell length can be explained by the torque-dipole component of the fluid flow for our model swimmer.
Finally, I imaged swimming SM3 with three-dimensional digital holographic microscopy. In nature, bacteria inhabit complex, three-dimensional environments, making 3D imaging and tracking essential tools for understanding their motility. Furthermore, 3D imaging has distinct advantages over 2D imaging, such as high-throughput, minimal surface interactions, and more robust statistics. I found that the motility parameters in 3D are similar to those in 2D, but the results are more reliable and less affected by the boundary effects of a nearby solid surface.
Congratulations, Dr. Johnson!
