E. colibacteria move by rotating the flagella at their cell membranes. Changing the direction of their rotation results in two modes of cell motion: tumbling and swimming. The switching dynamics between the two modes enables bacteria to steer their motion, swim along the food gradients and in microconfinement. We model the dynamics of bacterial swimmers on two levels: a coarse-grained model for pattern formation in a suspension of bacteria and a detailed single cell model of the internal signaling pathway governing cell diffusion in various environments. We show that the patterns observed experimentally in bacterial colonies on agar form due to interplay between chemotaxis towards ambient and self-secreted attractants, and bacterial reproduction. Furthermore, we show that the diffusion of a single bacterium sensitively depends on the level of the chemotactic proteins within the cell. At low expression levels of protein CheR (an enzyme responsible for methylation of the receptor sites), the thermal noise leads to super-diffusive behaviour (Lévy walk). This represents an evolutionary advantage in environments with scarce nutrients.
In the second part, I will focus on bacterial surface motility. We studied the motility of P. aeruginossa, which swims with flagella in the planktonic mode, and moves along the surfaces in a twitching-like manner with type-IV pili – filaments that grow from the cell membrane, attach to surfaces and act as grappling hooks to pull on the cells. In recent experiments, curious slingshot events have been observed. We design a fundamental model for twitching motility and assay the minimal degree of complexity that leads to such behaviour. We find that a surprisingly small number of pili (two) are needed, and that the slingshot modes emerge when the pili anchors at the membrane are sufficiently flexible. This indicates an intriguing role of flexibility in enabling biological function and suggests that controlling the flexibility in nanotopological design of surfaces, e.g. in biomedical applications, could be a novel direction in suppressing biofilm formation.