Tracking bacterial dynamics in three dimensions

As we enter an era of quantitative biology, there is a clear need for innovative quantitative experimental tools to probe cellular dynamics at the micron-scale. We have developed a novel 3D micro DPTV~(defocused particle tracking velocimetry) that is able to track multiple micron-scale particles in fluid flow simultaneously. Using this technique, we tracked multiple swimming \textit{Escherichia coli} cells simultaneously and in three dimensions~for the first time. Using the tracking data, we obtained~a wealth of information about the motion~of each individual~cell as well as its group behavior. We identified~different types of locomotion of swimming \textit{E. coli} as a function of its genetic make-up using well-characterized mutant strains.~ The diffusion coefficient of the \textit{E. coli} suspension was computed from the tracking data, and was found to be $\sim $200 times larger than that of a non-motile bacterial suspension.~ The average motor power of each bacteria is estimated to be $\sim $ 10$^{-18}$ Watts. Finally, the role of cell-cell interactions was also explored via the evaluation of a pairwise correlation function.~