Hydrodynamically Assisted Bacterial Chemotaxis

1066193圆周率：科赫常见的细菌物种，如大肠杆菌和枯草杆菌，在鞭毛束的推动下以低雷诺数从后面的液体中游动。鞭毛束有时会解体，导致细胞翻滚。这种移动和翻滚运动在存在化学吸引剂梯度的情况下变得偏向，例如营养物质或其他细胞释放的化学信号。当细胞沿着化学梯度向上移动时，细胞翻滚的频率较低，从而导致细胞趋化的平均速度。这种有偏差的运动还导致细胞的净取向和细胞在游泳时对流体施加的主动应力的各向异性。因此，细菌悬浮液被视为一种连续的活体流体，具有由于化学梯度而产生各向异性应力的不同寻常的特征。这个项目探索了这些化学诱导的流体动力应力和由此产生的流体动力学流动在细菌利用趋化性来寻找养分或响应细胞-细胞化学信号时帮助或阻碍细菌悬浮的方式。智力优势：细菌悬浮的整体平均运动方程的非线性行为的线性稳定性分析和计算解被用来预测趋化细菌诱导的宏观流体动力学流动。补充实验包括可视化荧光细菌细胞和示踪胶体珠的运动，以及测量细菌细胞浓度。微流控井中的细菌悬浮液受到线性化学吸引剂梯度的影响，提供了一种具有与时间无关的基态的简单情况。在这个系统中，稀释的细菌悬浮液由于趋化速度的竞争和由于它们的跑动和翻滚运动而扩散，在化学梯度方向上形成一个稳定的浓度，该浓度是位置的指数函数。然而，化学梯度诱导的活动应力预计会在临界细菌浓度以上诱导对流。实验测量将检验由线性稳定性分析预测的临界浓度和由数值解得到的对流模式。研究了微通道中细菌和引诱剂在两股平行流道中的扩散行为。准稳态稳定性分析和运动方程的动态解将被用来预测在存在和不存在外加压力驱动流的情况下在细菌-引诱剂界面上形成波的条件。当细菌释放出吸引其他细菌的化学信号时，它们会形成环状和球状的图案，细胞密度很高。基于趋化性和扩散的相似解已被开发出来，用于预测浓度场中奇点的发展。该项目考虑了主动水动力应力在这种聚集现象中的作用。更广泛的影响：由于化学信号，集体水动力运动可能改变重要的细菌行为，包括寻找营养和组装，这可能在微生物学领域产生广泛的影响。本主题还提供了一个很好的环境，向学生介绍在一个有趣的生物学环境中耦合反应-对流-扩散的非线性动力学。针对高中生的网站和暑期研究机会将被开发，以探索由于细菌之间的化学信号而形成的模式。

1066193 PI: KochCommon bacterial species such as E. coli and B. subtilis swim through fluids at low Reynolds numbers propelled from behind by a flagella bundle. The flagella bundle unravels at times leading to cell tumbling. This run and tumble motion becomes biased in the presence of a chemical attractant gradient such as a nutrient or a chemical signal released by other cells. The cells tumble less frequently when swimming up the chemical gradient leading to a mean chemotactic cell velocity. This biased motion also leads to a net orientation of the cells and an anisotropy of the active stresses the cells exert on the fluid as they swim. Thus, a bacterial suspension viewed as a continuum living fluid has the unusual feature of developing anisotropic stresses due to chemical gradients. This project explores the ways in which these chemically induced hydrodynamic stresses and the resulting hydrodynamic flows aid or hinder suspensions of bacteria as they use chemotaxis to seek nutrients or respond to cell-cell chemical signaling.Intellectual Merit: Linear stability analyses and computational solutions of the nonlinear behavior of ensemble averaged equations of motion for bacteria suspensions are used to predict the macroscopic hydrodynamic flows induced by chemotactic bacteria. Complementary experiments include visualization of the motion of fluorescent bacterial cells and tracer colloidal beads and measurement of the bacteria cell concentration. A bacteria suspension in a microfluidic well subjected to a linear chemo-attractant gradient provides a simple case with a time-independent base state. In this system a dilute bacteria suspension develops a steady concentration that is an exponential function of position in the chemo-gradient direction due to the competition of the chemotactic velocity and the diffusion due to their run-and-tumble motion. However, the chemical-gradient induced active stresses are expected to induce convection above a critical bacteria concentration. Experimental measurements will test the critical concentration predicted by linear stability analysis and the convective patterns obtained from numerical solutions. The dispersal of bacteria into a chemical attractant is studied for two parallel fluid streams which carry bacteria and attractant in a microchannel. A quasi-steady stability analysis and a dynamic solution of the equations of motion will be used to predict the conditions leading to formation of waves at the bacteria-attractant interface in both the presence and absence of an imposed pressure-driven flow. When bacteria release chemical signals that attract other bacteria, they form patterns including rings and spherical clusters with high cell concentrations. Similarity solutions have been developed based on chemotaxis and diffusion that predict the development of singularities in the concentration field. This project considers the role of active hydrodynamic stresses in this clustering phenomenon. Broader Impacts: The possibility that collective hydrodynamic motions alter important bacteria behaviors involving search for nutrients and assembly due to chemical signals could have a broad impact in the field of microbiology. This topic also provides a good setting in which to introduce students to the nonlinear dynamics of coupled reaction-convection-diffusion in an interesting biological setting. A web site and summer research opportunity targeted toward high school students will be developed to explore pattern formation due to chemical signaling among bacteria.