Center for Information Services and High Performance Computing (ZIH), Technische Universität Dresden, Zellescher Weg 12, 01069 Dresden, Germany.
Interface Focus. 2012 Dec 6;2(6):774-85. doi: 10.1098/rsfs.2012.0034. Epub 2012 Oct 3.
Formation of spatial patterns of cells is a recurring theme in biology and often depends on regulated cell motility. Motility of the rod-shaped cells of the bacterium Myxococcus xanthus depends on two motility machineries, type IV pili (giving rise to S-motility) and the gliding motility apparatus (giving rise to A-motility). Cell motility is regulated by occasional reversals. Moving M. xanthus cells can organize into spreading colonies or spore-filled fruiting bodies, depending on their nutritional status. To ultimately understand these two pattern-formation processes and the contributions by the two motility machineries, as well as the cell reversal machinery, we analyse spatial self-organization in three M. xanthus strains: (i) a mutant that moves unidirectionally without reversing by the A-motility system only, (ii) a unidirectional mutant that is also equipped with the S-motility system, and (iii) the wild-type that, in addition to the two motility systems, occasionally reverses its direction of movement. The mutant moving by means of the A-engine illustrates that collective motion in the form of large moving clusters can arise in gliding bacteria owing to steric interactions of the rod-shaped cells, without the need of invoking any biochemical signal regulation. The two-engine strain mutant reveals that the same phenomenon emerges when both motility systems are present, and as long as cells exhibit unidirectional motion only. From the study of these two strains, we conclude that unidirectional cell motion induces the formation of large moving clusters at low and intermediate densities, while it results in vortex formation at very high densities. These findings are consistent with what is known from self-propelled rod models, which strongly suggests that the combined effect of self-propulsion and volume exclusion interactions is the pattern-formation mechanism leading to the observed phenomena. On the other hand, we learn that when cells occasionally reverse their moving direction, as observed in the wild-type, cells form small but strongly elongated clusters and self-organize into a mesh-like structure at high enough densities. These results have been obtained from a careful analysis of the cluster statistics of ensembles of cells, and analysed in the light of a coagulation Smoluchowski equation with fragmentation.
细胞的空间模式形成是生物学中的一个反复出现的主题，通常依赖于细胞的调节运动。杆状细菌粘细菌的运动性依赖于两种运动机制：IV 型菌毛（产生 S 运动）和滑行运动装置（产生 A 运动）。细胞运动性通过偶尔的反转来调节。运动中的粘细菌细胞可以根据其营养状态组织成扩展菌落或充满孢子的生殖体。为了最终理解这两种模式形成过程以及两种运动机制以及细胞反转机制的贡献，我们分析了三种粘细菌菌株的空间自组织：（i）一种突变体仅通过 A 运动系统单向运动而不反转，（ii）一种也配备 S 运动系统的单向突变体，以及（iii）除了两种运动系统外偶尔反转运动方向的野生型。通过 A 引擎运动的突变体表明，由于杆状细胞的空间相互作用，滑行细菌中可以出现大的运动簇的集体运动，而无需调用任何生化信号调节。当两种运动系统都存在并且细胞仅表现出单向运动时，双引擎菌株突变体揭示了相同的现象出现。从对这两种菌株的研究中，我们得出结论，单向细胞运动在低和中等密度下诱导大的运动簇的形成，而在非常高的密度下导致涡旋形成。这些发现与自推进棒模型中已知的结果一致，这强烈表明自推进和体积排除相互作用的综合效应是导致观察到的现象的模式形成机制。另一方面，我们了解到，当细胞偶尔如在野生型中那样反转它们的运动方向时，细胞形成小但强烈拉长的簇，并在足够高的密度下自组织成网格状结构。这些结果是从对细胞集合体的簇统计的仔细分析中获得的，并根据带有碎裂的胶体 Smoluchowski 方程进行了分析。
