剪切流中的奔跑和翻滚趋化性：时间比较和其它复杂性的影响
Run and tumble chemotaxis in a shear flow: the effect of temporal comparisons and other complications
摘要：
大肠杆菌是一种能动的细菌，它通过执行由交替运行和翻滚组成的有偏随机行走来沿着化学引诱物梯度向上移动。本文计算了大肠杆菌的趋化漂移速度vd（沿趋化梯度向上的平均速度）。大肠杆菌细胞在均匀稳定的剪切流中执行趋化性，具有与流动成直角的弱趋化剂梯度。扩展早期的模型，使用组合的分析和数值方法来评估几种复杂性的影响，即（i）细胞不能直接检测化学引诱物梯度，而是进行化学引诱物浓度的时间比较，（ii）翻滚表现出方向的持久性，这意味着翻滚之前和之后的游泳方向是相关的，（iii）细胞由于旋转布朗运动而遭受随机重新取向，以及（iv）细胞的非球形形状影响其通过剪切流旋转的方式。这些复杂性影响了vd对剪切速率γ的依赖性。当它们都包括在内时，发现（a）剪切破坏趋化性，并且超过γ = 2/秒的剪切速率导致细胞沿着化学引诱物梯度向下游动而不是向上游动，（b）就最大化漂移速度而言，方向的持续性在静止流体中是有利的，但在剪切流中是不利的，和（c）更细长的体型有利于在强剪切流中进行趋化。
英文摘要：
Escherichia coli is a motile bacterium that moves up a chemoattractant gradient by performing a biased random walk composed of alternating runs and tumbles. This paper presents calculations of the chemotactic drift velocity vd (the mean velocity up the chemoattractant gradient) of an E. coli cell performing chemotaxis in a uniform, steady shear flow, with a weak chemoattractant gradient at right angles to the flow. Extending earlier models, a combined analytic and numerical approach is used to assess the effect of several complications, namely (i) a cell cannot detect a chemoattractant gradient directly but rather makes temporal comparisons of chemoattractant concentration, (ii) the tumbles exhibit persistence of direction, meaning that the swimming directions before and after a tumble are correlated, (iii) the cell suffers random re-orientations due to rotational Brownian motion, and (iv) the non-spherical shape of the cell affects the way that it is rotated by the shear flow. These complications influence the dependence of vd on the shear rate gamma. When they are all included, it is found that (a) shear disrupts chemotaxis and shear rates beyond gamma = 2/second cause the cell to swim down the chemoattractant gradient rather than up it, (b) in terms of maximising drift velocity, persistence of direction is advantageous in a quiescent fluid but disadvantageous in a shear flow, and (c) a more elongated body shape is advantageous in performing chemotaxis in a strong shear flow.
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