Some bacteria self-destruct when their colony is attacked by rivals, but the reason was unclear. Now it seems they sacrifice themselves to save their relatives, just as some insects give their lives to defend their colonies.
This kind of sacrifice is rare in nature because it usually contradicts an individual’s evolutionary drive to survive and reproduce. However, many types of bacteria self-destruct when attacked by rival bacteria.
To try to understand why this happens, Elisa Granato and Kevin Foster at the University of Oxford developed a way to visualise it. They took a strain of E. coli bacteria that can self-destruct in the presence of a competitor’s toxins and made them turn green when they are preparing to self-destruct and pink when they actually do so.
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The researchers placed a colony of the modified bacteria in a dish next to a colony of enemy bacteria – a different, unmodified strain of E. coli. Both strains can produce different toxins against closely related colonies. They then watched what happened under a microscope as the colonies battled it out.
Self-destruct mode
The modified E. coli on the front line – closest to the enemy bacteria – received a direct hit of the toxins released by their rivals and were killed straight away. Just after, the E. coli just behind the front line, which were exposed to less toxin, switched into self-destruct mode.
These bacteria spent an hour building up supplies of their own toxin, before dying off en masse – by bursting open and firing the toxin at the enemy, probably to help the rest of the colony survive.
This self-sacrificial counter-attack makes sense from an evolutionary point of view as bacteria often live in colonies of identical clones, says Granato. “It’s like they’re helping their own genes by killing themselves, because they give their clonemates that have the same genes as them a better chance of surviving,” she says.
This is a plausible explanation, although the expression of this kind of behaviour may depend on how quickly a bacterial colony is growing, what phase of growth it is in and what kind of medium it is growing in, says Dick Strugnell at the University of Melbourne. It may also be affected by factors we haven’t studied yet, he adds.
Insect colonies
Altruistic sacrifice has also been observed in some insects, including ants and bees that live in colonies of highly related individuals, when they fight off other insects that threaten their colony and die in the process. Usually older workers perform this act of self-sacrifice because they have lower reproductive potential, meaning it is less costly for them to die from an evolutionary point of view.
These behaviours probably evolved independently in bacteria and insects because of their similar lifestyles, says Granato. “That’s the beauty of evolution: it can come up with the same solution for completely different organisms,” she says.
bioRxiv
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