****Technical Abstract**** This project investigates the effect of long-range hydrodynamic interactions (LRHI) on swimming of a single bacterium and a group of bacteria in proximity of each other. At a single cell level, how the backflow influences the swimming behavior of the cell who creates it has attracted attentions from mathematicians and physicists. Important theoretical predictions were made, but they have not been rigorously tested in the laboratory. This is in part due to most studies so far used Escherichia coli, which have multiple flagella that are difficult to model mathematically and in part due to the difficulty in changing the physical parameters, such as the hydrodynamic load, over a broad range. Using marine bacteria V. alginolyticus that possess a single polar flagellum and whose cell body can differentiate into different sizes, these difficulties can be overcome. The marine bacteria also have other interesting attributes that can contribute significantly to this research, which include: (i) a tunable motor powered by the sodium ions in the fluid, (ii) a large swimming speed, and most importantly (iii) the available mutants that swim exclusively in the forward (pushers) and backward (pullers) directions. Theoretically the mutual hydrodynamic interactions between two pushers, two pullers, and a pusher and a puller are expected to be different. We use a state-of-the-art optical trapping techniques to measure these different forces quantitatively. At a macroscopic level, we examine hydrodynamic patterns formed by these different swimmers. Recently, there have been intense theoretical efforts in this area, resulting in interesting predictions for this type of "living" fluids. These include the prediction of novel rheological behaviors, nonequilibrium order/disorder phase transitions, and liquid-crystalline-like hydrodynamic structures. This research trains graduate students in the emerging field of biophysics.
It is a common experience that a mechanical disturbance in a fluid spreads to its neighbors with a well-defined speed, i.e., the sound velocity, while its intensity decreases with distance. A swimming bacterium can be considered as a local source of mechanical disturbance and fluid motion in the vicinity can influence the bacterium's own motion as well as others in its neighborhood. This is what is called hydrodynamic interaction and can be described by a mathematical theory developed by Lighthill; it appears to work reasonably well for isolated swimming bacterium. However, its application to a large group of swimming bacteria in high concentrations is not well understood. The effect of hydrodynamic interaction on swimming of individual and a large group of bacteria is significant because it influences their ability to sequester nutrients in aqueous environments and to form biofilms on surfaces; both are important for ecological and biomedical reasons. The project uses the state-of-the-art techniques of laser trapping to manipulate bacteria to determine mutual hydrodynamic interactions between pair of bacteria in different configurations. These measurements compare and refine theoretical models that make predictions about collective behaviors of large number of bacteria. Given the multidisciplinary nature of the research, students trained are in high demand in academia and in industry.
