The social evolution of siderophore production in Pseudomonas aeruginosa is environmentally determined

Bacteria secrete various exoproducts whose benefits can be shared by all cells in the vicinity. The potential importance of these “public goods” in bacterial evolutionary ecology has been extensively studied. Cheating by siderophore-null mutants of the opportunistic pathogen Pseudomonas aeruginosa has received particular attention. The potential of siderophore mutants to attenuate virulence, and the possibility of exploiting this for clinical ends, have generated a wealth of publications. However, the possibility that genotype · environment interactions govern the evolutionary consequences of siderophore loss has been almost entirely ignored. A review of the available literature revealed (i) widespread use of an undefined mutant as a siderophore cheat; and (ii) a reliance on experiments conducted in iron-limited minimal medium. Whole genome sequencing of the undefined mutant revealed a range of mutations affecting phenotypes other than siderophore production. We then conducted cheating assays using defined deletion mutants, grown in conditions designed to model infected fluids and tissue in CF lung infection and non-healing wounds. Depending on the environment, we found that siderophore loss could lead to cheating, simple fitness defects, or no fitness effect at all. It is therefore crucial to develop appropriate in vitro growth conditions in order to better predict the social evolution of traits in vivo.