Analysis And Simulation Of Twitching Bacteria On Surfaces

Bacteria are known to aggregate on surfaces where they cooperate to form structures known as biofilms that are resistant to antibiotics.To search out favourable conditions for growth,bacteria can expend energy to traverse surfaces using grappling-hook like appendages known as Type IV Pili(TFP).TFP are thin but rigid polymers with sticky ends that grow from the bacterial membrane until they make contact with the surface,after which they are driven to retract back inside the cell by a molecular motor that can generate forces upwards of 100 p N.The retraction of a single TFP may drive the cell across the surface by as much as several micrometers.We extensively analyse the trajectories made by bacteria moving on surfaces to better understand the extension,surface attachment and retraction cycle of TFP,and how it relates to various types of surface motion that we observe.The analysis is made more challenging by the fact that TFP are approximately 5 nm in diameter,too thin to resolve using standard imaging methods.To address this problem,we develop a mathematical model of the whole bacterium and its appendages,which we use to make predictions about bacterial surface motion.The model suggests how the Pseudomonas Aeruginosa bacteria that we study may traverse surfaces with varying degrees of local curvature,and how the bacteria may control its orientation relative to the surface.We show that it is also possible to make quantitative predictions about TFP properties,like their average retraction speed,directly from trajectory data.Statistical methods for analysing and simplifying complex models are applied and the need for modern analytical tools that can extract useful information from biological trajectory data is discussed.Finally,an effective method for decomposing trajectory data into its piecewise-linear components is developed.This piecewise-linear transformation shows that Pseudomonas Aeruginosa has a preference for generating displacements in alternating directions,which naturally leads to movements with high directional persistence.