Similar weapons play different roles in bacterial competition

Many bacteria are aggressive organisms that inhibit and kill their competitors using a diverse range of molecular weapon systems. Strikingly, strains commonly carry multiple similar weapons that appear to perform the same function. Here we investigate the evolutionary benefits of this apparent functional duplication using the heavily-armed opportunistic pathogen, Pseudomonas aeruginosa as a model species. We first use genomics to ask whether carrying multiple similar bacteriocins (S pyocins) increases the probability of inhibiting other strains. However, simply carrying a single uncommon S pyocin is typically as effective as carrying multiple pyocins. We then combine genome-editing with competition experiments to study the effects of S pyocins alone and in combinations. S pyocins sometimes function better together than alone, which helps to explain their co-carriage. However, we also find evidence of specialisation: pyocin S5 is much more effective than S2 in iron limited conditions, and vice-versa when iron is available. We test the generality of our findings using a second class of weapon carried by P. aeruginosa, contact-dependent inhibition, which again reveals differing effectiveness as conditions change. Our work shows how similar weapons are specialised for combat under different environmental conditions. We argue that functional specialisation is key to understanding why bacteria carry so many weapons.