If you’ve ever felt like the microbes making you sick are just doing it out of spite, here is some support for your suspicion. Although bacteria and viruses only harm their host for their own benefit, the same isn’t true in their relations with others of their kind, a new study indicates. When one bacterium hurts another, the victim may respond in a way that also harms itself, just to get revenge. It’s basically Game of Thrones in a petri dish.

For all the benefits of living communally, there is one great problem that applies from the smallest bacterium to the largest whale: what to do about freeloaders. Microbes' relative simplicity and short lifespans offer an excellent opportunity to study this, and researchers have reported in PLOS Computational Biology they can be like us in ways we might not expect.

Colonies of bacteria work together by producing chemicals, such as enzymes, that benefit the colony as a whole. However, it takes energy to make these, and some don’t bother, instead relying on the effort put in by those around them. Sound familiar?

Humans deal with freeloaders through laws, social pressure, or occasionally outright violence. Hardworking bacteria have fewer options, but that doesn’t mean they’re helpless.

Researchers investigated an option called quorum-sensing, which involves organisms detecting how many of their species are around and regulating which genes get expressed depending on the answer. The authors were surprised to learn quorum sensing can have serious negative consequences, but bacteria use it anyway, apparently to punish freeloaders. 

"We didn't expect to see this behavior, which you might even call 'spiteful,'" study author Dr Andrew Eckford of York University, Canada, said in a statement. "But it indicates that quorum sensing is a remarkably flexible tool for enforcing fairness."

The team was investigating an example of quorum sensing involving the production of enzymes used to break down food sources so their nutrients become accessible. If surrounded by many of their kind, each bacterium needs to produce less of the enzyme. The system breaks down, however, if some members of the colony don’t pull their weight – feasting on the sustenance made available by the enzymes produced by others.

"It's costly for a bacterium to contribute to the community, so for a selfish individual, it's best to simply take what's offered without giving anything back," said lead author Dr Alex Moffett of Northeastern University "But obviously this is bad for everyone, so the community needs a way to discourage bad behavior."

The authors found cases where populations commit “evolutionary suicide”, refusing to produce enough enzymes to feed themselves in order to take cheater-rich populations down with them.

Using mathematical modeling, the authors show that using quorum sensing to control cheating strains can be a beneficial strategy in some circumstances, extending the life of the colony. In other circumstances it backfires. The energy demands involved in producing enzymes, and whether their production is essential or merely beneficial, help determine which applies. They acknowledge they have not investigated the long-term consequences of these outcomes on a wider scale.

The work could have implications for how we fight bacterial infections where antibiotic resistance is provided by biofilms only some colony members produce.

Human society may be orders of magnitude more complex, yet many of our problems are the same. How does one deal, for example with someone who takes advantage of the herd immunity provided by those who get vaccinated, while refusing to do so themselves? Indeed, the very survival of humanity may be endangered by those who refuse to make sacrifices to reduce their carbon emissions, hoping they can freeload on those who do cut back. 

The downside of imitating bacteria and penalizing cheaters to the point of social collapse is right there in the description, but maybe there’s a reason revenge can feel so good.