Type IV pili mediated ``walking'' motility of bacteria

We develop image recognition and particle tracking algorithms to identify and track large numbers of surface-associated bacteria, up to $\sim$800 cells for up to 6 hours. To characterize the pili-dependent motility mechanisms, we image wild type (WT) and flagella-deficient (pilA) knockout strains of P. aeruginosa. In the pilA strain, we observe two motility mechanisms: a novel ``walking'' mechanism, characterized by bacteria orienting themselves normal to the surface, and a ``crawling'' mechanism, characterized by the bacteria lying flat on the surface. We find that ``crawling'' bacteria move along their long axis and maintain their orientation over time, whereas ``walking'' bacteria change direction rapidly, allowing them to sample microenvironments more efficiently. We also observe both ``walking'' and ``crawling'' in the WT strain, suggesting that flagella do not interfere with these mechanisms of Type-IV pili based motility.