Hydrodynamically Assisted Bacterial Chemotaxis

常见的细菌种类如大肠杆菌和枯草芽孢杆菌在低雷诺数的液体中游动，由鞭毛束从后面推动。鞭毛束有时会散开，导致细胞翻滚。这种奔跑和翻滚运动在化学引诱剂梯度（如营养物质或其他细胞释放的化学信号）的存在下变得偏颇。当沿着化学梯度向上游动时，细胞翻滚的频率会降低，从而导致平均趋化细胞速度。这种偏置运动也导致细胞的净方向和细胞在游动时对流体施加的主动应力的各向异性。因此，作为连续流体的细菌悬浮液具有由于化学梯度而产生各向异性应力的不寻常特征。该项目探索了这些化学诱导的水动力应力和由此产生的水动力流动如何帮助或阻碍细菌悬浮液，因为它们利用趋化性来寻找营养物质或对细胞-细胞化学信号作出反应。智力优势：细菌悬浮液系综平均运动方程非线性行为的线性稳定性分析和计算解用于预测趋化细菌引起的宏观流体动力流动。辅助实验包括荧光细菌细胞和示踪胶体珠运动的可视化和细菌细胞浓度的测量。微流控井中的细菌悬浮液受到线性化学引诱剂梯度的影响，提供了一个具有时间无关基态的简单案例。在这个系统中，稀释的细菌悬浮液在化学梯度方向上形成稳定的浓度，这是由于趋化速度和由于它们的奔跑和翻滚运动而引起的扩散的竞争而形成的指数函数。然而，化学梯度诱导的主动应力有望在临界细菌浓度以上诱导对流。实验测量将测试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。研究了在微通道中携带细菌和引诱剂的两种平行流体流在化学引诱剂中的扩散。准稳态稳定性分析和运动方程的动态解将用于预测在存在和不存在施加压力驱动流的情况下导致细菌-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，它们会形成图案，包括细胞浓度高的环状和球形簇。基于趋化性和扩散的相似性解已经被开发出来，可以预测浓度场中奇点的发展。本项目考虑了主动水动力应力在这种聚类现象中的作用。更广泛的影响：集体流体动力学运动改变细菌重要行为的可能性，包括寻找营养物质和由于化学信号而组装的可能性，可能在微生物学领域产生广泛的影响。这个主题也提供了一个很好的背景，在一个有趣的生物环境中，向学生介绍耦合反应-对流-扩散的非线性动力学。一个针对高中生的网站和暑期研究机会将被开发出来，以探索细菌之间化学信号的模式形成。

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.