"Quantifying Spatiotemporal Patterns in the Expansion of Twitching Bacterial Colonies"
Type IV pili (T4P) are very thin (5-8 nm in diameter) protein filaments that can be extended and retracted by certain classes of Gram-negative bacteria including P. aeruginosa. These bacteria use T4P to move across viscous interfaces, referred to twitching motility. Twitching can occur for isolated cells or in a collective manner. We have developed experimental and data analysis techniques to quantify the expansion of the bacterial colony at the agar-glass interface. Using a custom-built, temperature and humidity controlled environmental chamber, we have studied the transition from individual to collective motion. By using particle image velocimetry (PIV) and Fourier analysis techniques, we have characterized the evolution of the advancing front of expanding colonies for a range of concentrations (and therefore stiffnesses) of the agar gel. We find that the advancing front consists of finger-like protrusions consisting of many bacteria, with the cells within the expanding colony arranged in a lattice-like pattern. We have characterized the average speed, average width and average bacterial orientation within the fingers as a function of agar concentration/stiffness. We find that the average finger width increases approximately linearly with agar concentration, whereas the average speed of the fingers is essentially independent of agar concentration. This latter result is coupled with our observation of a transition from monolayer coverage to multilayer coverage within the fingers as the agar stiffness is increased, and suggests that a critical number of T4P are required for the bacteria to break through the pristine agar-glass interface. Within the line profiles of fingers traveling in straight trajectories, we find that the maximum finger speed occurred in the direction of the finger motion, and that the bacteria cells were highly aligned along the finger axis. We have also performed high temporal and spatial resolution studies of twitching colonies that have allowed us to observe bacteria moving at speeds that are considerably larger than the pilus retraction rate measured in previous studies.
Dr. Jacek Lipkowski, Chair (Department of Mathematics and Statistics)
Dr. John Dutcher, Advisory Committee (Department of Physics)
Dr. Leonid Brown, Examinationy Committee (Deparment of Physics)