Cooperation and Cheating Among Bacteria

Michael Giberson

Ed Yong, at Not exactly rocket science, describes recent research into, uh, I guess you could describe it as the socioeconomic life of bacteria:

Bacteria may not strike you as expert co-operators but at high concentrations, they pull together to build microscopic ‘cities’ called biofilms, where millions of individuals live among a slimy framework that they themselves secrete. These communities provide protection from antibiotics, among other benefits, and they require cooperation to build.

… One individual can’t build a biofilm on its own so it pays for a colony to be able to measure its own size. To do this, they use a method ‘quorum sensing’, where individuals send out signalling molecules in the presence of their own kind.

When another bacterium receives this signal, it sends out some of its own, so that once a population reaches a certain density, it sets off a chain reaction of communication that floods the area with chemical messages.

These messages provide orders that tell the bacteria to secrete a wide range of proteins and chemicals. Some are necessary for building biofilms, others allow them to infect hosts, others make their movements easier and yet others break down potential sources of food. They tell bacteria to start behaving cooperatively and also when it’s worth doing so.

Yong mentions research which inserted “signal-negative” and “signal-blind” variants of a bacteria – mutants that can’t pass along signals or that can’t sense them at all – into a population of normal bacteria.  It turns out that all that signaling behavior needed for social coordination is costly; abstaining from social activities allows the mutant bacterium to devote more time and energy to the more self-interested pursuits of eating and reproducing.  The research found that, as summed up by Yong, “cheaters can indeed prosper and then some – they outgrew their cooperating cousins by 60 to 80 times.”

Another researcher working with the same species of bacteria found that non-cooperators occur naturally.

But if cheaters prosper, why does anyone not cheat?  Yong said, “Both studies found that as the proportion of cheaters increased, their growth rate dropped because the value of cheating diminished.”

The second study also discovered that when faced with a population crash, cheaters developed further mutations that restored cooperative abilities.

But, to me, this last finding draws into question the characterization of non-signaling bacteria as non-cooperators and cheaters.  Perhaps the non-signalers emerge naturally in a population to regulate investment in the social infrastructure.  Perhaps “they also serve who only sit and wait (and eat and reproduce).”  This reserve army of slacker bacteria is getting ready for the hard times ahead, and when hard times come they have the energy stored up to leap into action, developing mutations necessary to restore cooperative abilities.

[HT to Economics and Mechanisms, which titles its post “Bacteria that channel Elinor Ostrom,” and CogiDDo ergo sum.]

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