The multifunctional roles of Type IV pili in bacterial surface motility

In \textit{Pseudomonas aeruginosa}, a commonly-studied model for biofilm formation, type IV pili mediate two different mechanisms of surface motility: a ``crawling'' mechanism, in which the bacterium moves parallel to the surface with high directional persistence; and a ``walking'' mechanism, in which a vertically-oriented bacterium moves rapidly but with low directional persistence. To characterize these mechanisms with high spatial and temporal resolution, we develop a novel tracking method called `two focus tracking' which allows us to quantitatively estimate the tilt angle between a single bacterium and the surface. By analyzing the motion of $\Delta $\textit{fliM}, a flagella-deficient isogenic knockout mutant of \textit{P. aeruginosa}, we demonstrate that a single bacterium can switch between the ``walking'' and ``crawling'' mechanisms. To elucidate the role of pili in crawling, we show that the velocity profiles of single trajectories can be decomposed into alternating ``pulses'' and ``square waves.'' The direction of alternation ``pulses and ``square waves'' are significantly correlated. We show how these observations can be explained by cooperative deployment of multiple pili.