Siderophore-mediated cooperation and virulence in Pseudomonas aeruginosa.
Why should organisms cooperate with each other? Helping close relatives that are likely to share the same genes (kin selection) is one important explanation that is likely to apply across taxa. The production of metabolically costly extracellular iron-scavenging molecules (siderophores) by microorganisms is a cooperative behaviour because it benefits nearby conspecifics. We review experiments focusing on the production of the primary siderophore (pyoverdin) of the opportunistic bacterial pathogen, Pseudomonas aeruginosa, which test kin selection theories that seek to explain the evolution of cooperation. First, cooperation is indeed favoured when individuals interact with their close relatives and when there is competition between groups of cooperators and noncooperators, such that the benefit of cooperation can be realized. Second, the relative success of cheats and cooperators is a function of their frequencies within populations. Third, elevated mutation rates can confer a selective disadvantage under conditions when cooperation is beneficial, because high mutation rates reduce how closely bacteria are related to each other. Fourth, cooperative pyoverdin production is also shown to be favoured by kin selection in vivo (caterpillars), and results in more virulent infections. Finally, we briefly outline ongoing and future work using this experimental system.