Brownian motion influences bacterial swimming by randomizing displacement and direction. Here, we report that the influence of Brownian motion is amplified when it is coupled to hydrodynamic interaction. We examine swimming trajectories of the singly flagellated bacterium Caulobacter crescentus near a glass surface with total internal reflection fluorescence microscopy and observe large fluctuations over time in the distance of the cell from the solid surface caused by Brownian motion. The observation is compared with computer simulation based on analysis of relevant physical factors, including electrostatics, van der Waals force, hydrodynamics, and Brownian motion. The simulation reproduces the experimental findings and reveals contribution from fluctuations of the cell orientation beyond the resolution of present observation. Coupled with hydrodynamic interaction between the bacterium and the boundary surface, the fluctuations in distance and orientation subsequently lead to variation of the swimming speed and local radius of curvature of swimming trajectory. These results shed light on the fundamental roles of Brownian motion in microbial motility, nutrient uptake, and adhesion.Caulobacter ͉ adhesion ͉ Derjaguin-Landau-Verwey-Overbeek theory ͉ hydrodynamics B rownian motion, the random movement of microscopic objects in fluid caused by constant thermal agitation, is of fundamental significance in life science (1), particularly in the microbial world (2). The motility of microbes in aqueous environments is substantially altered by Brownian motion. In the widely read book entitled Random Walks in Biology, Howard Berg discusses the strong influence of Brownian motion on the swimming trajectory of peritrichously flagellated Escherichia coli. (2). A monotrichous bacterium, however, would not be able to vary its swimming direction and seek food efficiently without rotational Brownian motion, because it cannot tumble to change direction as an E. coli does (3). Therefore, Brownian motion may be especially important for the chemotaxis of a monotrichous bacterium that uses its single polar flagellum to swim back and forth (4). Indeed, it has been shown by computer simulations that rotational Brownian motion significantly increases the ability of singly-flagellated marine bacteria to stay with falling marine snow particles, which are rich in nutrients (5, 6).The commonly recognized influence of Brownian motion on a swimming microbe is the random deviation of its swimming trajectory from a straight path. The deviations are caused by collisions between the microbe and its surrounding water molecules in thermodynamic equilibrium. We show here that Brownian motion has an additional and even stronger influence when it is coupled to the hydrodynamic interaction between a swimming bacterium and a fluid boundary. In this situation, the hydrodynamic interaction depends sensitively on the distance of the bacterium to the boundary surface (7,8). Brownian motion causes that distance to vary randomly, and via coupling with the hydro...
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