Symbioses between animals and bacteria are ubiquitous. To better understand these relationships, it is essential to unravel how bacteria evolve to colonize hosts. Previously, we serially passaged the free-living bacterium, Shewanella oneidensis, through the digestive tracts of germ-free larval zebrafish (Danio rerio) to uncover the evolutionary changes involved in the initiation of a novel symbiosis with a vertebrate host. After 20 passages, we discovered an adaptive missense mutation in the mshL gene of the msh pilus operon, which improved host colonization, increased swimming motility, and reduced surface adhesion. In the present study, we determined that this mutation was a loss-of-function mutation and found that it improved zebrafish colonization by augmenting S. oneidensis representation in the water column outside larvae through a reduced association with environmental surfaces. Additionally, we found that strains containing the mshL mutation were able to immigrate into host digestive tracts at higher rates per capita. However, mutant and evolved strains exhibited no evidence of a competitive advantage after colonizing hosts. Our results demonstrate that bacterial behaviors outside the host can play a dominant role in facilitating the onset of novel host associations.

中文翻译：

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Msh Pilus突变增加了自由细菌定殖在鱼寄主中的能力。

动物与细菌之间的共生是普遍存在的。为了更好地理解这些关系，必须弄清细菌如何进化成定殖宿主。以前，我们通过无细菌幼虫斑马鱼（Danio rerio）的消化道，连续传播了自由生存的细菌Shewanella oneidensis，从而发现了与脊椎动物宿主共生新共生的进化变化。经过20代之后，我们在msh的mshL基因中发现了一个自适应错义突变菌毛操纵子，改善宿主定植，增加游泳运动，并减少表面粘附。在本研究中，我们确定该突变是功能丧失突变，并发现它通过增加与环境表面的缔合来增加幼虫外水柱中的沙门氏菌代表，从而改善了斑马鱼的定殖。此外，我们发现含有mshL突变的菌株能够以较高的人均迁移率进入宿主消化道。但是，突变株和进化株在定植宿主后没有表现出竞争优势的证据。我们的结果表明，宿主以外的细菌行为可以在促进新型宿主关联的发生中起主导作用。