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THE DEVELOPMENT OF CONCENTRATION GRADIENTS IN A SUSPENSION OF CHEMOTACTIC BACTERIA

机译:化学细菌悬浮液中浓度梯度的发展

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When a suspension of bacterial cells of the species Bacillus subtilis is placed in a chamber with its upper surface open to the atmosphere complex bioconvection patterns are observed. These arise because the cells: (1) are denser than water; and (2) usually swim upwards, so that the density of an initially uniform suspension becomes greater at the top than the bottom. When the vertical density gradient becomes large enough, an overturning instability occurs which ultimately evolves into the observed patterns. The reason that the cells swim upwards is that they are aerotactic, i.e. they swim up gradients of oxygen, and they consume oxygen. These properties are incorporated in conservation equations for the cell (N) and oxygen (C) concentrations, and these are solved in the pre-instability phase of development when N and C depend only on the vertical coordinate and time. Numerical results are obtained for both shallow- and deep-layer chambers, which are intrinsically different and require different mathematical and numerical treatments. It is found that, for both shallow and deep chambers, a thin boundary layer, densely packed with cells, forms near the surface. Beneath this layer the suspension becomes severely depleted of cells. Furthermore, in the deep chamber cases, a discontinuity in the cell concentration arises between this cell-depleted region and a cell-rich region further below, where no significant oxygen concentration gradients develop before the oxygen is fully consumed. The results obtained from the model are in good qualitative agreement with the experimental observations.
机译:当将枯草芽孢杆菌菌种的细菌细胞悬液置于其上表面通向大气的室内时,观察到复杂的生物对流模式。这些是由于细胞:(1)比水致密; (2)通常向上游泳,使得最初均匀的悬浮液的密度在顶部大于底部。当垂直密度梯度变得足够大时,会发生倾覆不稳定性,最终会演变成观察到的模式。细胞向上游动的原因是它们是有气致作用的,即它们游动了梯度的氧气,并且消耗了氧气。这些特性被合并到电池(N)和氧气(C)浓度的守恒方程中,并且当N和C仅取决于垂直坐标和时间时,可以在显影的预不稳定阶段求解这些特性。浅层和深层室均获得了数值结果,它们本质上是不同的,需要不同的数学和数值处理。已经发现,对于浅室和深室,在表面附近都形成了一个薄的边界层,密密麻麻地堆满了细胞。在该层下,悬浮液严重耗尽细胞。此外,在深腔室情况下,在该贫细胞区域和更远的下方的富细胞区域之间会出现细胞浓度的不连续性,在此区域中,在氧气被完全消耗之前,没有明显的氧气浓度梯度。从模型获得的结果与实验观察结果在质量上吻合良好。

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