Back in the 1970s, biologists began to observe a remarkable social phenomenon in bacteria. They had long been aware that individual bacteria could sense and seek out nutrients in their environment, a phenomenon known as chemotaxis.
What they hadn’t realized was that bacteria could communicate between themselves using signaling molecules. In this way, bacteria can sense the presence of others and regulate their behavior accordingly, such as becoming bioluminescent or forming biofilms when their density reaches a certain critical level.
And that raises some interesting questions about how sophisticated these social communication schemes have become. For example, can they be used to warn other bacteria of potential threats, just as flocking birds or schools of fish part when attacked by a predator?
Danger Signs
Now evidence has emerged that bacterial communication can help organisms sense existential danger, allowing other organisms to stay away, say Trung Phan at Johns Hopkins University in Baltimore, Shengkai Li at Princeton University and colleagues, who have been experimenting with the bacterial workhorse, Escherichia Coli.
This group observed the phenomenon using a devious experiment in which the bacterial equivalent of a “black hole” irreversibly sweeps away any organism that falls in. This black hole consists of a petri dish filled with nutrients but with a central hole beneath which flows a stream of fluid that sweeps away any bacteria that enters it.
The dish also contains funnel-like ratchets that allow bacteria to move towards the black hole but not away from it; in the same way that gravity pulls objects into astrophysical black holes. This has the effect of gradually herding the E coli towards the black hole.
The flow of fluid into the hole is relatively slow, however, and allows the diffusion of chemical signals away from the black hole. That is until the bacteria crosses the equivalent of an event horizon, where the flow becomes fast enough to sweep away communication signals too. At this point, the signals stop, and the bacteria is no longer able to communicate with its colony.
It's easy to imagine that such a black hole would eventually sweep away the entire colony. But Phan, Li and co discovered that, in practice, exactly the opposite happens — the bacteria somehow learn to stay away. “A very clear unoccupied region develops around the black hole, into which bacteria do not enter,” say the researchers.
By modeling the process of bacterial communication, Phan, Li, and co think they have worked out why. “Clearly, real bacteria both sense nutrients (they must find food to survive), and they sense the presence of other bacteria and respond to their presence,” they say.
Survival Skills
When the signal from other bacteria is strong, they stay together. But “as the bacteria disappear into the exit hole, the signaling molecule concentration decreases, in essence a warning for the other bacteria to stay away,” say Phan, Li and co. As a result, the bacteria aggregate in some other area away from the black hole.
That’s unexpected, particularly given there is no drop in the concentration of nutrients. “The surprising experimental result [is] that increased communication within a high population density easily trumps lack of food in that region,” conclude the researchers.
Clearly, E. Coli has evolved to warn each other of impending doom. “Bacteria can collectively perform complex tasks critical for the population’s survival using information exchange,” say the team.
It’ll be interesting now to see how widespread this behavior is and whether other bacterial species behave in a similar way or have evolved even more sophisticated responses.
Ref: Social Physics of Bacteria: Avoidance of an Information Black Hole : arxiv.org/abs/2401.16691
