Abstract
SummaryBehaviors 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, 2, 3, 4, 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, 7, 8, 9], which are also performed by individuals with low reproductive potential.
Highlights
Self-lysis Frequency Is Modulated by Competitor Toxin Concentrations We followed the activation of the colicin E2 operon, encoding the toxin, its cognate immunity protein, and the lysis protein (Figure 1A), using a reporter plasmid that expresses green fluorescent protein from the native colicin E2 promoter [11]
We found that self-lysis leads to PI reliably entering cells to give the fluorescent signal, but, critically, cell death caused by the DNase colicin toxin of another E. coli strain does not
For the colicin E2-producer, we found that self-lysis frequencies within 6 h of observation were on average 75.5% at the colony edge and 97.3% in cells positioned slightly further away from the edge
Summary
Self-lysis Frequency Is Modulated by Competitor Toxin Concentrations We followed the activation of the colicin E2 operon, encoding the toxin, its cognate immunity protein, and the lysis protein (Figure 1A), using a reporter plasmid that expresses green fluorescent protein (gfp) from the native colicin E2 promoter (pUA66PcolE2::gfp) [11]. Cells carrying this reporter widely upregulate GFP expression when exposed to DNA-damaging toxins made by a competitor (Figures 1B–1D). Reporting promoter activation is not sufficient to follow the full behavior, as the self-lysis process is dependent on t
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