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 ( Figure 1A) 4. 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 environment 15. 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 collapse 17. 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 invasion 15, 16. 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 behaviors 12– 14. 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 production 11. Microbial cooperative behaviors have important impacts on our own lives, including antibiotic resistance 7, biofilm formation in chronic infections 8, and virulence during acute infections 9, 10. With our increased understanding of bacterial sociality comes a further appreciation of the role of cooperation in many bacterial processes. Bacterial chemical communication, including quorum sensing (QS), is ubiquitous 3, 4, and the molecular underpinnings of multicellular bacterial structures such as Myxococcus xanthus fruiting bodies and Streptomyces filaments are also being elucidated 5, 6. 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 lives 1, 2.