Department of Zoology, University of Oxford, 11a Mansfield Road, OX1 3SZ Oxford, UK; Department of Biochemistry, University of Oxford, 3 South Parks Road, OX1 3QU Oxford, UK.
Curr Biol. 2020 Jul 20;30(14):2836-2843.e3. doi: 10.1016/j.cub.2020.05.007. Epub 2020 Jun 4.
Behaviors that cause the death of an actor are typically strongly disfavored by natural selection, and yet many bacteria undergo cell lysis to release anti-competitor toxins [1-5]. This behavior is most easily explained if only a small proportion of cells die to release toxins and help their clonemates, but the frequency of cells that actually lyse during bacterial warfare is unknown. The challenge is finding a way to distinguish cells that have undergone programmed suicide from those that were simply killed by a competitor's toxin. We developed a two-color fluorescence reporter assay in Escherichia coli to overcome this problem. This revealed conditions where nearly all cells undergo programmed lysis. Specifically, adding a DNA-damaging toxin (DNase colicin) from another strain induced mass cell suicide where ∼85% of cells lysed to release their own toxins. Time-lapse 3D confocal microscopy showed that self-lysis occurs locally at even higher frequencies (∼94%) at the interface between toxin-producing colonies. By exposing E. coli that do not perform lysis to the DNase colicin, we found that mass lysis occurs when cells are going to die anyway from toxin exposure. From an evolutionary perspective, this renders the behavior cost-free as these cells have zero reproductive potential. This helps to explain how mass cell suicide can evolve, as any small benefit to surviving clonemates can lead to this retaliatory strategy being favored by natural selection. Our findings have parallels to the suicidal attacks of social insects [6-9], which are also performed by individuals with low reproductive potential.
导致个体死亡的行为通常是自然选择强烈反对的，但许多细菌会发生细胞裂解，以释放抗竞争者毒素[1-5]。如果只有一小部分细胞死亡以释放毒素并帮助其克隆体，那么这种行为最容易解释，但实际上在细菌战争中发生细胞裂解的频率是未知的。挑战在于找到一种方法来区分经历程序性自杀的细胞和那些被竞争者毒素简单杀死的细胞。我们在大肠杆菌中开发了一种双色荧光报告测定法来克服这个问题。这揭示了几乎所有细胞都经历程序性裂解的条件。具体来说，添加另一种菌株的 DNA 损伤毒素（DNase 大肠菌素）会诱导大量细胞自杀，其中约 85%的细胞裂解以释放自己的毒素。延时 3D 共聚焦显微镜显示，自我裂解以更高的频率（约 94%）在产毒菌落之间的界面局部发生。通过使不发生裂解的大肠杆菌暴露于 DNase 大肠菌素中，我们发现当细胞因毒素暴露而死亡时，大规模裂解就会发生。从进化的角度来看，由于这些细胞的繁殖潜力为零，因此这种行为是无成本的。这有助于解释为什么大规模细胞自杀可以进化，因为对幸存克隆体的任何微小益处都可能导致这种报复性策略被自然选择所青睐。我们的发现与社会性昆虫的自杀性攻击[6-9]有相似之处，社会性昆虫的自杀性攻击也是由繁殖潜力低的个体进行的。
