Session X43: Invited Session: Bacterial Swimming and Chemotaxis

Bacterial Swimming and Accumulation at the Fluid Boundaries

Micro-organisms often reside and thrive at the fluid boundaries. The tendency of accumulation is particularly strong for flagellated bacteria such as \textit{Escherichia coli}, \textit{Vibro alginolyticus}, and \textit{Caulobacter crescentus}. We measured the distribution of a forward swimming strain of \textit{Caulobacter crescentus} near a solid surface using a three-dimensional tracking technique based on darkfield microscopy and found that the swimming bacteria accumulate heavily within micrometers from the surface, even though individual swimmers are not trapped long enough to display circular trajectories. We attributed this accumulation to frequent collisions of the swimming cells with the surface, causing them to align parallel to the surface as they continually move forward. The extent of accumulation at the steady state is accounted for by balancing alignment caused by these collisions with the rotational Brownian motion of the micrometer-sized bacteria. We performed simulations based on this model, which reproduces the measured results. Additional simulations demonstrate the dependence of accumulation on swimming speed and cell size, showing that longer and faster cells accumulate more near a surface than shorter and slower ones do. Our ongoing experimental effort also includes observation of similar phenomena at the interfaces of either water-oil or water-air, noting even stronger trapping of the swimming bacteria than near a solid surface. These studies reveal a rich range of fluid physics for further analysis.   

Periplasmal Physics: The Rotational Dynamics of Spirochetal Flagella

Spirochetes are distinguished by the location of their flagella, which reside within the periplasm: the tiny space between the bacterial cell wall and the outer membrane. In {\sl Borrelia burgdorferi\/} (the causative agent of Lyme Disease), rotation of the flagella leads to cellular undulations that drive swimming. Exactly how these shape changes arise due to the forces and torques acting between the flagella and the cell body is unknown. By applying low-Reynolds number hydrodynamic theory to the motion of an elastic flagellum rotating in the periplasm, we show that the flagella are most likely separated from the bacterial cell wall by a lubricating layer of fluid. We obtain analytical solutions for the force and torque on the rotating flagellum through lubrication analysis, as well as through scaling analysis, and find results are in close agreement numerical simulations. (Joint work with J. Yang and C.W. Wolgemuth.)   

Microfluidics for bacterial chemotaxis

The emerging microfluidic technology opens up new opportunities for bacterial chemotaxis studies. In this talk, I will present our efforts in correlating molecular level events with cellular phenotypes in bacterial chemotaxis using microfabricated device. I will present results of bacterial chemotaxis in both single and dual chemical gradients. In single gradient experiments, we demonstrated that bacteria sense the chemical concentration at a logrithmic scale, similar to sensory system in higher organism. In dual gradient experiments, we showed that the number ratio of the two different types of receptor plays a critical role in bacteria's chemotactic decision making process. Experimental results based on single cell analysis will be presented. This work is supported by the National Science Foundation and the Cornell Nanobiotechnology Center.   

Directional swimming in bacteria: active and passive gradient responses

The ability to swim directionally is paramount for bacteria, in their quest for nutrients and favorable microhabitats. This ability depends on both active and passive responses to gradients. Here we bring an example from each case, based on novel microfluidic experiments that quantify the swimming behavior of bacteria. First, we describe their active response to oxygen gradients - or aerotaxis - and show the unexpected consequences of competing oxygen gradients with nutrient gradients. Then, we present the first observations of directional swimming by bacteria in response to fluid velocity gradients - or rheotaxis. Combining experiments with mathematical modeling we demonstrate that, unlike in larger organisms such as fish, rheotaxis in bacteria is passive, resulting from a previously undetected torque that originates from the chirality of the bacterial flagellum.