UCLA

Bacterial biofilms are structured microbial communities which are widespread both in nature and in clinical settings. When organized into a biofilm, bacteria are extremely resistant to many forms of stress, including a greatly heightened antibiotic resistance. In the early stages of biofilm formation on an abiotic surface, many bacteria make use of their motility to explore the surface, finding areas of high nutrition or other bacteria to form microcolonies. They use motility appendages, including flagella and type IV pili (TFP), to navigate the near-surface environment and to attach to the surface. Bacterial motility has previously been studied on a large scale, describing collective motility modes involving large aggregates of cells such as swarming and twitching. This dissertation provides an in-depth look at bacterial motility at the single-cell level, focusing on Pseudomonas aeruginosa and Myxococcus xanthus, two commonly-studied organisms; in addition, it describes particle tracking algorithms and methodology used to analyze single-bacterium behaviors from flow cell microscopy video. P. aeruginosa flagella are used in swimming but also in surface-bound spinning, which is a precursor to detachment; for P. aeruginosa, flagella and TFP work synergistically in a detachment sequence. P. aeruginosa TFP drive walking, a surface-bound exploratory motility mode in which the bacterium is oriented normal to the surface, as well as crawling, in which the bacterium is oriented parallel to the surface and moves directionally. The transition between the two modes is affected by cyclic di-GMP, which P. aeruginosa uses as a global regulator of biofilm formation. M. xanthus does not swim or walk, but when stimulated to initiate social motility it exhibits a slow pili-driven crawling behavior. By imaging this motion at high frame rates, we see earthquake-like sticks and slips; these are well-described by crackling noise friction models, and are caused by the friction between the bacterial body, bacterial exopolysaccharides, and the surface. This crackling noise analysis reveals

that M. xanthus EPS is an efficient lubricant as well as a glue, and facilitates the social motility of connected cells.