Hydrodynamically Assisted Bacterial Chemotaxis

1066193 PI：Koch常见细菌菌种，如E. coli和B.枯草杆菌在低雷诺数下通过鞭毛束从后面推动的流体游动。 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（例如营养物或由其他细胞释放的化学信号）的存在下变得偏向。 当沿着化学梯度向上游动时，细胞翻滚的频率较低，导致平均趋化细胞速度。 这种偏置运动还导致细胞的净取向和细胞在游动时施加在流体上的主动应力的各向异性。 因此，细菌悬浮液被视为连续的活流体，具有由于化学梯度而产生各向异性应力的不寻常特征。 该项目探讨了这些化学诱导的流体动力学应力和由此产生的流体动力学流动在细菌利用趋化性寻求营养或响应细胞-细胞化学信号时帮助或阻碍细菌悬浮的方式。本文用线性稳定性分析和系综平均运动方程非线性行为的计算解来预测细菌悬浮液的宏观流体力学流动，趋化性细菌 补充实验包括荧光细菌细胞和示踪胶体珠的运动的可视化和细菌细胞浓度的测量。 微流体孔中的细菌悬浮液经受线性化学引诱剂梯度提供了具有与时间无关的基本状态的简单情况。 在该系统中，稀释的细菌悬浮液由于趋化速度的竞争和由于它们的奔跑和翻滚运动引起的扩散而形成稳定的浓度，该浓度是化学梯度方向上的位置的指数函数。 然而，化学梯度诱导的主动应力预计会诱导高于临界细菌浓度的对流。 实验测量将测试线性稳定性分析预测的临界浓度和从数值解获得的对流模式。研究了微通道中两股平行的携带细菌和化学引诱剂的流体流的细菌向化学引诱剂中的扩散。一个准稳态的稳定性分析和运动方程的动态解将被用来预测的条件，导致在细菌吸引剂的界面在存在和不存在的情况下，施加的压力驱动流的波的形成。当细菌释放吸引其他细菌的化学信号时，它们会形成高细胞浓度的环状和球状集群。 相似的解决方案已经开发的基础上的趋化性和扩散，预测发展中的浓度场的奇异性。 本项目考虑了在这种集群现象中的积极的水动力应力的作用。 更广泛的影响：集体流体动力学运动改变重要的细菌行为的可能性，包括由于化学信号而寻找营养和组装，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环境，让学生在一个有趣的生物环境中了解耦合反应-对流-扩散的非线性动力学。 将开发一个针对高中生的网站和夏季研究机会，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.