Overview of the Study

• Planarians are flatworms that can regenerate their entire body from a small piece.
• This study shows that a bacterium naturally living in planarians, Aquitalea sp. FJL05, produces a small chemical called indole when given extra tryptophan.
• Indole acts as a signal that changes how planarians rebuild their bodies, sometimes causing them to grow two heads instead of one.

What Was Observed?

• When planarian fragments were exposed to Aquitalea sp. FJL05 along with tryptophan:
  • A significant portion regenerated as two-headed (double-headed) animals.
  • Control groups (without tryptophan or with bacteria not making indole) regenerated normally with one head.
• Direct treatment with indole (at 100 μM) for 2 days produced a double-head formation in about 6.5% of cases, increasing to around 14% with longer exposure.
• Some regenerates, even when they ended up with one head, showed extra (ectopic) eyes and mispatterned brain structures.
• Double-headed forms remained stable even after re-amputation—if both heads were removed, the trunk still reformed with two heads.

Step-by-Step Experimental Approach (Like a Cooking Recipe)

• Preparation:
  • Planarians were maintained in controlled water conditions and fed liver paste (which naturally contains tryptophan).
  • Fragments (pre-tail pieces) were cut from the planarians.
• Inducing the Change:
  • The fragments were placed in water containing Aquitalea sp. FJL05 along with extra tryptophan to boost indole production.
  • Alternatively, some fragments were directly exposed to a solution of indole (100 μM) for 2, 6, or 10 days.
• Regeneration and Observation:
  • After treatment, fragments were washed and allowed to regenerate in plain water.
  • Researchers checked for the number of heads formed, the presence of extra eyes, and brain patterning.
  • Some fragments were re-amputated to test if the two-headed form was permanent.

Key Findings Explained Simply

• Indole as a Signal:
  • Indole works like a “secret ingredient” in a recipe; adding it changes the final outcome (a normal one-headed worm becomes a double-headed one).
  • This chemical signal alters the “instruction manual” of regeneration by changing gene activity.
• Gene Expression Changes:
  • RNA sequencing revealed that indole exposure led to many changes in gene expression.
  • Notably, genes in the Wnt signaling pathway—critical for determining head-versus-tail identity—were down-regulated.
  • Other pathways affected include those for fibroblast growth factor receptors (FGFR), Hedgehog (Hh), and bone morphogenic protein (BMP), which all help guide body patterning.
• Long-Term Effects:
  • Once a double-headed form is established, it remains stable over multiple rounds of regeneration.
  • This suggests that indole causes a lasting “reprogramming” of the body’s blueprint.
• Additional Abnormalities:
  • Even single-headed planarians sometimes grew extra eyes (ectopic eyes) and had brains that did not scale normally to head size.
  • Some worms also showed defects along other body axes (dorsal-ventral and medial-lateral), meaning the overall body plan was disrupted.

Understanding the Technical Terms

• Indole: A small molecule produced by bacteria from tryptophan. Think of it as a flavor enhancer that changes the “recipe” of regeneration.
• Tryptophan: An amino acid (a building block of proteins) that serves as the raw material for producing indole.
• Wnt Signaling: A cell communication system that acts like a GPS, guiding cells on where to form the head or tail.
• Double-Headed Regeneration: When a planarian grows two heads; imagine making a sandwich with an extra slice of bread.
• Ectopic Eyes: Extra eyes forming in unusual places, like having a third eye where you normally wouldn’t.

Conclusions and Implications

• Bacteria living inside animals can send chemical signals that alter the animal’s developmental blueprint.
• This inter-kingdom signaling (bacteria communicating with animal cells) shows that the microbiome can directly influence body shape.
• The findings have exciting implications for regenerative medicine—by manipulating such signals, it might be possible to guide tissue regeneration and repair.
• The study also suggests that small molecules like indole could be used in synthetic biology to reprogram cell behavior and tissue patterning.

Summary Analogy

• Imagine you are baking a cake with a standard recipe that always yields a single-layer cake.
• Now, if you add a special ingredient (indole), the recipe changes and you end up with a cake that has two layers (a double-headed worm).
• Even if you remove the extra layer, the next time you bake, the cake still comes out double-layered because the recipe (the cell’s patterning instructions) has been permanently altered.

中文版本：研究概述

• 涡虫是一种能够从一小块组织再生出整个身体的扁虫。
• 本研究发现，涡虫体内的一种共生细菌 Aquitalea sp. FJL05 在额外提供色氨酸（tryptophan）时，会产生一种叫做 indole 的小分子。
• Indole 作为一种信号分子，会改变涡虫再生时的体型，有时导致涡虫长出两个头。

观察到的现象

• 当涡虫切片接触到含有 Aquitalea sp. FJL05 和色氨酸的溶液时：
  • 相当一部分涡虫再生出了双头。
  • 而对照组（没有色氨酸或使用不产生 indole 的细菌）则正常再生，仅长出一个头。
• 直接用 100 μM 的 indole 溶液处理 2 天，约 6.5% 的涡虫出现双头，再延长至 6 天或 10 天，双头比例约为 14% 左右。
• 即使是长出一个头的涡虫，也会出现额外的眼睛（异位眼）和大脑结构异常的情况。
• 通过再次切割去除双头后，再生出的涡虫仍保持双头形态，说明这种变化具有持久性。

实验步骤（像做菜一样的过程）

• 准备阶段：
  • 在控制条件下饲养涡虫，并喂以含色氨酸的肝脏糊。
  • 从涡虫体上切下前端尾部以下的片段（预尾段）。
• 诱导变化：
  • 将切下的片段放入含有 Aquitalea sp. FJL05 以及额外色氨酸的水中，以促进 indole 的产生。
  • 另外，也有部分片段直接用 100 μM 的 indole 溶液处理，时间分别为 2 天、6 天或 10 天。
• 再生与观察：
  • 处理结束后，片段被冲洗并转入普通水中进行再生。
  • 观察涡虫是否长出两个头，是否出现额外眼睛以及大脑是否排列正常。
  • 部分实验中，再次切割涡虫以验证双头形态是否稳定持久。

主要发现（简明解释）

• Indole 作为信号：
  • Indole 类似于“秘密配料”，其加入改变了原本只生成一个头的再生过程，导致生成双头涡虫。
  • 这种化学信号通过改变细胞内基因表达，调整了再生的“说明书”。
• 基因表达变化：
  • RNA测序显示，indole 处理后有许多基因表达发生了变化。
  • 尤其是控制头尾分化的 Wnt 信号通路中的基因被下调。
  • 此外，纤维母细胞生长因子受体（FGFR）、Hedgehog（Hh）和骨形态发生蛋白（BMP）等其他调控通路也受到影响。
• 长期效应：
  • 一旦形成双头，再生过程中这种形态会一直保持，即使切除多次也依然出现双头。
  • 这表明 indole 对体内“蓝图”的改写具有持久性。
• 其他异常现象：
  • 一些只再生出一个头的涡虫也会长出额外的眼睛，且大脑与头部的比例不正常。
  • 部分涡虫在背腹和左右方向上的体型也出现了异常，说明整体体型规划受到了干扰。

术语解释与类比

• Indole：细菌利用色氨酸产生的一种小分子，可视作改变再生“配方”的关键成分。
• 色氨酸：构成蛋白质的氨基酸，细菌利用它来生成 indole。
• Wnt 信号通路：细胞间的通讯系统，类似于导航系统，指导细胞决定生成头或尾。
• 双头再生：涡虫再生出两个头，就像制作三明治时多加了一片面包。
• 异位眼：在不正常的位置出现的额外眼睛，就像脸上长了第三只眼。

结论与意义

• 体内共生细菌能通过产生化学信号改变宿主的再生蓝图。
• 这种跨界信号传递展示了微生物群如何直接影响动物体型的形成。
• 该发现为再生医学带来新希望：通过调控类似 indole 的信号，有可能引导组织修复与再生。
• 研究还表明，利用小分子重新编程细胞行为和组织构型，可能成为合成生物学的重要工具。

简单总结类比

• 想象你在烤蛋糕，标准配方总是做出单层蛋糕。
• 但如果加入一种特殊的配料（indole），配方就会改变，蛋糕最终变成双层（即双头涡虫）。
• 即便你把额外的层切掉，下一次烤出的蛋糕依然会呈现双层，因为“配方”已经永久改变了。