Producers of a cooperative behavior such as a public good also produce an individually retained private good that is beneficial or required for survival and growth in the focal environment.Į. This mechanism may act through directed harm (left) or restraint of benefits (right).ĭ. When cells are able to assort with kin in space, particularly in the case of biofilm formation (bottom left), dispersal of cells and diffusion of public goods are limited and promote the maintenance of cooperative behavior.Ĭ. The cooperative behavior is induced only when a sufficient amount of signal has accumulated (left).ī. ![]() Mechanisms that act to maintain cooperation.Ī. QS itself is an exploitable cooperative behavior, as QS-specific cheaters that do not signal, overproduce signal, or do not respond to signal can evolve This is often seen as a transition from non-production to production of cooperative traits such as extracellular public goods. At a specific concentration of signal, receptors bind to and sense these autoinducers, allowing the bacteria to switch from a low- to high-cell density state. This is reinforced by the positive feedback of many QS system on autoinducer synthesisġ9. ![]() As the cell density of a growing culture increases, so does the concentration of autoinducers. This process relies on the secretion and detection by bacteria of small chemical signals known as autoinducers into the extracellular environment. For this reason, production of many public goods such as exoenzymes, proteases, chitinases, and siderophores are regulated by QS (Ĥ. Therefore, there must be a sufficient number of producing cells contributing to the public good. Other mechanisms, such as relatedness, are likely required in conjunction with optional participation to preserve cooperation.įor public goods to be effective, they often must exceed a threshold concentration in the extracellular environmentġ5. It is notable, however, that facultative participation only partly mediates the problem of cooperation by limiting the times when a cell must maintain it. In this way, bacteria may preserve cooperation in conditions that would otherwise favor its collapseġ7. Engaging in cooperation at limited times, particularly when the benefit is the greatest, or in environmental conditions where the cost of cooperation is low can limit or prevent cheater invasionġ6. ![]() One approach to limit cheater invasion is facultative cooperation. Cooperative behaviors in bacteria, such as the production of extracellular “public good” molecules, defined as resources that can be utilized by both the producers and the non-producers in the community, are exploitable by non-producing cheater/defector cells. In this review, we will summarize from both a conceptual and a mechanistic perspective our understanding of how cooperation is maintained in bacteria.īacteria have evolved complex regulatory circuitry to respond and effectively acclimate to different environments, so it is not surprising that this flexible regulatory circuitry can also be utilized to control cooperative traits. Because of their short generation times, large population sizes, small genomes, and asexual reproduction, bacteria are now recognized as ideal model systems to understand the factors leading to the evolution and persistence of cooperative behaviorsġ4. Explaining the evolution of cooperative tasks has long challenged evolutionary biology, as these systems appear ripe for exploitation by non-cooperating defector/cheater cells that receive the benefits of cooperation without paying the cost of productionġ1. Microbial cooperative behaviors have important impacts on our own lives, including antibiotic resistanceħ, biofilm formation in chronic infectionsġ0. With our increased understanding of bacterial sociality comes a further appreciation of the role of cooperation in many bacterial processes. Streptomyces filaments are also being elucidatedĦ. Bacterial chemical communication, including quorum sensing (QS), is ubiquitousĤ, and the molecular underpinnings of multicellular bacterial structures such as Multicellular bacterial communities termed biofilms are now considered a normal form of bacterial growth. ![]() Bacteria were once thought to be solitary individuals, but it is now clear that they lead complex social livesĢ.
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