This post is written by UI postdoc Eric Bruger (twitter: @elbruger13)
We are used to thinking of ourselves as helpful beings, and humans are comparatively more cooperative in relation to many other species. The ability to cooperate is a major reason humans have been able to colonize much of the globe and form the complex societies we live in today. But the occurrence of cooperative behavior is far from a foregone conclusion: evolutionary theory tells us as much , and anyone who has tried to get a two-year-old child out the door can probably relate to the following recent exchange with my daughter:
“Ella, can you get your shoes on?”
“We have to go to the store.”
“We need to get in the car so we can go to the store.”
“Don’t you want to go outside?”
“No go outside.”
(wanders off and starts playing with stuffed Cookie Monster instead)
As with winning that negotiation with a stubborn child (when that is even possible), the incentives to cooperate must be correctly aligned, whether that be directed more by benefits for cooperating or costs for opting to not cooperate. What may not be as apparent is that there are countless examples of cooperative interactions within and between other species in nature. This even extends to microscopic organisms that we generally consider to be non-cognitive (although see [2 & 3] for further discussion). Well known among microbiologists, but interesting nonetheless, you do not need to have a brain to cooperate with others!
During my doctoral research in the Waters Lab at Michigan State University, a central question that interested me was how cooperation among bacteria might be preserved and how it was impacted by their ability to communicate. My favorite system to examine these questions was the bacterium Vibrio harveyi, a well-studied organism known for its ability to carry out a form of communication termed “quorum sensing” (QS) and also the ability to produce light, or bioluminesce. V. harveyi cells produce the enzyme luciferase, which catalyzes a reaction between its substrate and oxygen, and emits a resulting bluish hue of light. The light emitted by one cell alone is not visible to the naked eye, but it is visible when cells are present in dense clusters or in cultures where billions of cells are present, forming a detectable phenotype. The light response is regulated by QS and is only fully activated after levels of bacterially-produced chemical signals exceed a critical concentration – this is affected by prevailing environmental conditions and often driven by local cell densities.
Because QS controls the expression of a large number of V. harveyi‘s genes, and up-regulates many more than it down-regulates, activating QS is a cost burden on participating bacterial cells. So what’s the upside of having QS, and why wouldn’t these bacteria just evolve to save on this cost by not not quorum sensing? This phenomena of defectors has actually been observed in many experimental, clinical, and natural samples, lending support to the idea that defecting is increasing the fitness of these cells. However, defecting can come at other costs. The inability to sense and respond to signal levels could have negative effects when there are important correlated traits like resistance to toxic chemicals, in varying environments, in conditions where interactions between members of the same type – cooperator or defector – more often than with other types, or in the case that QS-regulated traits provide large fitness benefits.
The traits turned on by QS includes the production of extracellular proteases at high cell densities. These enzymes are cooperative public goods, as the benefits generated by them can be shared amongst the entire surrounding community. Growth on complex growth substrates like casein, which requires breakdown for cells to fully access, is aided by the production of these proteases. Some of our work  examined competitive outcomes between cooperator and defector strains in casein media. The results provided a demonstration that, while unregulated cooperation was susceptible to exploitation by defectors, QS-regulation provided a degree of protection against this and allowed cooperators to persist in the presence of defectors. This was true even in large well-mixed populations, where theory predicts they should be most susceptible to such exploits.
To test whether or not QS could stabilize cooperative behaviors over longer timescales, we conducted a 2,000 generation experimental evolution with replicate populations of Vibrio harveyi. Replicate populations of either the wild type (WT) strain that possesses a functional QS system and cooperates depending on signal levels, or a mutant strain that unconditionally cooperates regardless of external inputs like cell density, were passaged in a casein media. We found that non-QS defectors evolved from both strain backgrounds, but the resulting dynamics of the defectors were very different depending on the strain from which they evolve. From the unconditional cooperator strain, defectors rapidly evolved and uniformly swept those populations. These defectors exhibit a nonluminescent phenotype resulting from no QS activation of luciferase production. Alternatively, in nearly all WT populations, bioluminescent clones persisted at significant levels for the entirety of the experiment. Ongoing sequencing work is pursuing the molecular bases of these changes to determine whether parallel or diverse evolutionary paths are followed in our experimental populations.
Together, the competition and experimental evolution results we have observed show that bacterial chemical communication, in the form of QS, allows V. harveyi to maintain greater levels of cooperation within mixed populations and may be required to allow cooperation to persist . QS appears to be playing a critical role that may be shifting the balance of costs and benefits in cooperation’s favor. So keep up the conversation, and good things might just happen – just don’t count on that two-year-old to agree with you!
- West, S. A., Griffin, A. S., Gardner, A., & Diggle, S. P. (2006). Social evolution theory for microorganisms. Nature Reviews Microbiology, 4(8), 597-607.
- Lyon, P. (2015). The cognitive cell: bacterial behavior reconsidered. Frontiers in microbiology, 6, 264.
- Shapiro, J. A. (2007). Bacteria are small but not stupid: cognition, natural genetic engineering and socio-bacteriology. Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences, 38(4), 807-819.
- Bruger, E. L., & Waters, C. M. (2016). Bacterial quorum sensing stabilizes cooperation by optimizing growth strategies. Applied and Environmental Microbiology, 82(22), 6498-6506.
- Czárán, T., & Hoekstra, R. F. (2009). Microbial communication, cooperation and cheating: quorum sensing drives the evolution of cooperation in bacteria.PloS One, 4(8), e6655.