Quorum sensing (QS) (cell-to-cell signalling to regulate the behaviour of an entire population rather than a single cell) is cool science. The idea that bacteria talk to each other via their own social networks is now well established, but just how sophisticated the behaviour of populations of cells can be is just becoming clear.
The costs and benefits of cooperation and cheating are dependent on the environment. This paper investigates how a ubiquitous selection pressure, predation, affects selection for cooperation in the bacterium Pseudomonas aeruginosa, which cooperates by secreting and responding to QS signal molecules in the surrounding environment. Once a threshold signal concentration is reached, bacteria switch on production of fitness-enhancing ‘public goods’. By regulating the production of public goods in this way, QS determines that they are released at high cell densities when they will be most beneficial. But production of QS-dependent public goods could be exploited by cheats which do not produce these costly public goods because they do not respond to QS signal molecules. The authors show that a functioning QS system confers resistance to predation by the protist, Tetrahymena pyriformis, probably through the production of more resistant cell aggregations and biofilms. Cheats are unable to exploit behaviour. This elevated resistance does not appear to result from the activation of QS-regulated public goods, suggesting that antagonistic trophic interactions can indirectly favour QS-mediated cooperation, potentially leading to lower cheat frequency in natural microbial communities.
These results show that trophic interactions among species can maintain cooperation within species, and has further implications for P. aeruginosa virulence in environmental reservoirs by potentially enriching the cooperative and highly infective strains with functional QS system.
Protist predation can favour cooperation within bacterial species. (2013) Biol. Lett. 9 (5) 20130548 doi: 10.1098/rsbl.2013.0548
We studied how protist predation affects cooperation in the opportunistic pathogen bacterium Pseudomonas aeruginosa, which uses quorum sensing (QS) cell-to-cell signalling to regulate the production of public goods. By competing wild-type bacteria with QS mutants (cheats), we show that a functioning QS system confers an elevated resistance to predation. Surprisingly, cheats were unable to exploit this resistance in the presence of cooperators, which suggests that resistance does not appear to result from activation of QS-regulated public goods. Instead, elevated resistance of wild-type bacteria was related to the ability to form more predation-resistant biofilms. This could be explained by the expression of QS-regulated resistance traits in densely populated biofilms and floating cell aggregations, or alternatively, by a pleiotropic cost of cheating where less resistant cheats are selectively removed from biofilms. These results show that trophic interactions among species can maintain cooperation within species, and have further implications for P. aeruginosa virulence in environmental reservoirs by potentially enriching the cooperative and highly infective strains with functional QS system.