Symbiotic bacteria often navigate complex environments before colonizing privileged sites in their host organism. Chemical gradients are known to facilitate directional taxis of these bacteria, guiding them towards their eventual destination. However, less is known about the role of physical features in shaping the path the bacteria take and defining how they traverse a given space. The flagellated marine bacterium Vibrio fischeri,which forms a binary symbiosis with the Hawaiian bobtail squid, Euprymna scolopes, must navigate tight physical confinement, squeezing through a bottleneck constricting to ~2 μm in width on the way to its eventual home. Using microfluidic in vitro experiments, we discovered that V. fischeri cells alter their behavior upon entry into confined space, straightening their swimming paths and promoting escape from confinement. Using a computational model, we attributed this escape response to two factors: reduced directional fluctuation and a refractory period between reversals. Additional experiments in asymmetric capillary tubes confirmed that V. fischeri quickly escape from tapered ends, even when drawn into the ends by chemoattraction. This avoidance was apparent down to a limit of confinement approaching the diameter of the cell itself, resulting in a balance between chemoattraction and evasion of physical confinement. Our findings demonstrate that non-trivial distributions of swimming bacteria can emerge from simple physical gradients in the level of confinement. Tight spaces may serve as an additional, crucial cue for bacteria while they navigate complex environments to enter specific habitats.

中文翻译：

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过渡到密闭空间会影响细菌游动和逃逸反应

共生细菌通常在复杂的环境中穿行，然后才能在宿主生物中的特权位点定殖。已知化学梯度可以促进这些细菌的定向滑行，引导它们走向最终目的地。然而，人们对物理特征在塑造细菌所采取的路径和定义它们如何穿越给定空间方面的作用知之甚少。有鞭毛的海洋细菌Vibrio fischeri与夏威夷短尾乌贼Euprymna scolopes形成二元共生关系，必须在严密的物理禁闭中航行，在通往最终归宿的路上挤过一个狭窄到约 2 微米宽的瓶颈。使用微流体体外实验，我们发现V. fischeri进入密闭空间后，细胞会改变它们的行为，使它们的游泳路径变直并促进逃离禁闭。使用计算模型，我们将这种逃逸反应归因于两个因素：减少的方向波动和逆转之间的不应期。在不对称毛细管中进行的其他实验证实，费氏弧菌迅速从锥形末端脱离，即使被化学吸引拉入末端。这种避免明显降低到接近细胞本身直径的限制，导致化学吸引和物理限制的逃避之间的平衡。我们的研究结果表明，游泳细菌的非平凡分布可以从限制水平的简单物理梯度中出现。狭窄的空间可能是细菌在复杂环境中进入特定栖息地时的额外关键线索。