| Crynodeb: | <p>From schools of fish to flocks of birds and herds of wildebeest, the movements of many organisms are characterised by emergent phenomena known as collective movements. These complex group-level behaviours result from simple interactions between individuals. To the naked eye, bacterial colonies appear to be static structures. It might be hard to believe then that collective movements could have any relevance for microbial life. Yet at the microscopic level, many of the same collective phenomena also emerge within microbial systems. In this thesis, I study the collective movements of the opportunistic pathogen Pseudomonas aeruginosa in growing colonies. P. aeruginosa crawls over surfaces by employing twitching motility, utilizing hair-like filaments known as Type-IV Pili to pull itself forwards. Through a combination of novel experimental and theoretical approaches, as well as a novel cell-tracking package called FAST, I demonstrate that cells at the edge of P. aeruginosa colonies form an active nematic. This class of active matter is characterised by higher-order structures known as topological defects, points where cells with differing orientations meet one another. Two types of defect exist in active nematics: comets and trefoils. </p>
<p>Characterisation of a mutant that lacks the pilH gene, a key regulator of twitching motility, reveals that it moves much more quickly than the wild-type. Intuitively, it might be expected that this faster single-cell movement would translate into an increased rate of migration into new territory by the ∆pilH cell type. However, experiments reveal that the wild-type is able to collectively migrate much more quickly than ∆pilH, allowing it to outcompete the mutant in mixed colonies. This disconnect between single-cell and collective behaviours is shown to be caused by a mechanism related to the system’s status as an active nematic. In the first stage, topological defects organise the segregation of the two populations, with wild-type cells accumulating at trefoil defects and ∆pilH cells accumulating at comet defects. Collisions between the ∆pilH-enriched comets then cause the cells inside to rotate vertically, trapping them in place and preventing their migration to the colony edge. Wild-type cells avoid this phenomenon by moving slowly and prudently, allowing them to collectively migrate at a much faster rate than ∆pilH. Together, these results demonstrate the intimate interplay between collective motility and evolutionary dynamics in bacterial communities.</p>
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