Powered by flagella, many bacterial species exhibit collective motion on a solid surface commonly known as swarming. As a natural example of active matter, swarming is also an essential biological phenotype associated with virulence, chemotaxis, and host pathogenesis. Physical changes like cell elongation and hyper flagellation have been shown to accompany the swarming phenotype. However, less noticeable, are the contrasts of collective motion between the swarming cells and the planktonic cells of comparable cell density. Here, we show that confining bacterial movement in designed dimensions allows distinguishing bacterial swarming from collective swimming. We found that on a soft agar plate, a novel bacterial strain Enterobacter sp. SM3 exhibited different motion patterns in swarming and planktonic states when confined to circular microwells of a specific range of sizes. When the confinement diameter was between 40 μm and 90 μm, swarming SM3 formed a single swirl motion pattern in the microwells whereas planktonic SM3 showed multiple swirls. Similar differential behavior is observed across a range of randomly selected gram-negative bacteria. We hypothesize that the "rafting behavior" of the swarming bacteria upon dilution might account for the motion pattern difference. We verified our conjectures via numerical simulations where swarming cells are modeled with lower repulsion and more substantial alignment force. The novel technical approach enabled us to observe swarming on a non-agar tissue surface for the first time. Our work provides the basis for characterizing bacterial swarming under more sophisticated environments, such as polymicrobial swarmer detection, and in vivo swarming exploration.

中文翻译：

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坐月子发现浮游生物中的细菌

在鞭毛的推动下，许多细菌物种在通常称为“蜂群”的固体表面上表现出集体运动。作为活性物质的自然实例，蜂群也是与毒力，趋化性和宿主发病机理相关的必不可少的生物表型。诸如细胞伸长和过度鞭毛的物理变化已被证明伴随着蜂群表型。但是，群细胞和浮游细胞之间具有可比细胞密度的集体运动的对比却不那么明显。在这里，我们显示出将细菌运动限制在设计尺寸内可以区分细菌群和集体游泳。我们发现，在软琼脂板上，有一种新型细菌菌株肠杆菌属。当SM3被限制在特定大小范围的圆形微孔中时，它们在群体和浮游状态下表现出不同的运动模式。当限制直径在40μm和90μm之间时，成群的SM3在微孔中形成单个漩涡运动模式，而浮游SM3显示多个漩涡。在随机选择的革兰氏阴性细菌范围内观察到相似的差异行为。我们假设稀释后的成群细菌的“漂流行为”可能解释了运动模式的差异。我们通过数值模拟验证了我们的猜想，其中以较小的排斥力和较大的对齐力对蜂群单元进行建模。新颖的技术方法使我们能够首次观察到非琼脂组织表面上的蜂群。