The movement behaviour of solitary and collectively twitching Pseudomonas aeruginosa cells on surfaces

The opportunistic pathogen Pseudomonas aeruginosa uses grappling hook-like appendages called pili to move across surfaces. During early stages of biofilm development, local cell density increases such that bacteria transition from solitary individuals to densely packed monolayers. This thesis resolves how individual bacterial movement behaviour impacts their ability to navigate when they are travelling individually and collectively in densely packed groups. Collective movement in rod-shaped bacteria is routinely modelled as a two-dimensional nematic system in which neighbouring cells align their orientations, but do not necessarily pull themselves in the same direction. Using massively parallel cell tracking, fluorescent fusions, and an automated analysis pipeline, we show that cells actively reverse their movement direction when travelling in a direction opposite to that of their neighbours. This previously unobserved movement behaviour contributes to a highly polarised state where neighbouring cells actively pull themselves in the same direction. By working together, rather than against one another, this behaviour is predicted to enhance the rate of colony expansion. We then investigate the role of ‘twiddles’ in solitary cells performing pili-based chemotaxis. While chemotaxis was previously thought to be solely driven by reversals in movement direction, we exper- imentally observe that cells can smoothly turn the direction of their movement via a process that we call a ‘twiddle’. To resolve the potential role of twiddles in chemotaxis, we developed an individual- based model that was parameterised using experimental data. Our results show that twiddles and reversals can each drive chemotactic response, but show strong differences in the parameter regimes where they are most effective. Our results indicate that P. aeruginosa cells use both types of ori- entation behaviours to maximise their ability to position themselves within nutrient source across different environmental conditions. Our findings highlight the unique ways that bacteria have evolved to regulate their motility on surfaces.