“Evil” bacteria would rather starve their colony than allow profiteers to thrive
Bacterial colonies would rather perform “evolutionary suicide” than support cheater strains that escape the colony without giving anything in return.
That’s the discovery made by a team of researchers who modeled how a colony responds to bacteria that consume more than their fair share.
Bacteria often work together as a colony to survive, producing resources that are used by other bacteria.
For example, bacteria often secrete enzymes that break down food sources into nutrients, which benefits neighboring bacteria.
Sometimes, however, they can be selfish operators who consume resources without contributing any themselves.
“It’s expensive for bacteria to contribute to the community, so for a selfish individual it’s best to just take what’s offered and don’t give anything back,” says Alex Moffett, a computational biologist at York University in Toronto and lead author of the study. new study. “But obviously it’s bad for everyone, so the community needs a way to discourage bad behavior.”
To determine the rate of biochemicals (also called ‘public goods’) as a function of population density, colonies of bacteria use a system called quorum sensing.
Quorum sensing works by regulating the expression of cooperative genes in a population of bacteria through small molecules called autoinductors.
The more bacteria there are, the more autoinductors accumulate. If the concentration of autoinducers exceeds critical levels, this triggers changes in the bacteria which, as a group, have exceeded the available resources.
Using a mathematical model, the researchers discovered that quorum sensing had the effect of punishing profiteers by reducing the availability of shared resources.
But, in some cases, punishing cheaters is detrimental to the survival of the entire colony, as the producers are also starved of resources.
“In the absence of alternative nutrients, quorum sensing by producers withholds the sole source of nutrients from the entire population, which hurts both producers and cheaters in terms of the average extinction time of the total population,” the researchers write.
“We didn’t expect to see this behavior, which you might even call ‘malicious,'” says Andrew Eckford, a computational biologist also from York University. “But it does indicate that quorum sensing is a remarkably flexible tool for enforcing fairness.”
The researchers describe this behavior as “a possible example of evolutionary demographically stochastic suicide.”
Although filled with assumptions, mathematical models are useful tools that help scientists master complex and dynamic systems. In this case, understanding the brutal trade-offs bacterial colonies make to survive could, with further research, shed light on how bacterial infections take hold and cause disease.
For example, people with cystic fibrosis often develop lung infections, with Pseudomonas aeruginosa – a drug-resistant “superbug” – being a common culprit.
In the lungs, P. aeruginosa forms small, dense biofilms of about 1,000 bacterial cells, which act as a tough protective layer, making it more difficult for bacteria to be killed and infection cleared with antibiotics
Mathematical modeling of quorum sensing can help researchers understand how bacteria can colonize the lungs so effectively, which could pave the way for new treatments, Moffett says.
This research was published in Computational Biology PLOS.