Pascal Vallotton, Quantitative Imaging CSIRO Mathematics, Informatics and Statistics Title: Size matters: Vortex formation and colony expansion in Paenibacillus vortex interstitial biofilms Bacterial motility plays an important role in infectious diseases by speeding up the spread of pathogens. We study motility in vitro, at the interface between a glass cover slip and an agar gel. In this so called interstitial biofilm system, bacteria evolve in two dimensions, enabling high resolution imaging studies. We will present new results pertaining to the species Paenibacillus vortex, famous for its ability to self-assemble on agar plates into macroscopic vortices comprising thousands of individuals. Using movies acquired under interstitial conditions and analysed by cell tracking, we present results that shed new light on the process of vortex formation and biofilm expansion.