The occurrence of altruistic cooperation is one of the greatest challenges for evolutionary biology. The problem is how can an altruistic behaviour, that is costly to perform but benefits other individuals, be maintained by selection? Recently, a great variety of cooperative traits have been discovered in microbes, which offers exciting experimental possibilities.This proposal aims to investigate siderophore production (SP), a cooperative trait and virulence factor of an opportunistic human pathogen, the bacteria Pseudomonas aeruginosa. Siderophores are agents produced by bacteria in response to iron deficiency to make host-bound iron available. SP is a cooperative trait, because it is costly for the individual to produce, but provides a benefit to other individuals who can also up-take the dissolved iron. This cooperative trait is exploitable by cheats who avoid the costs of SP but are still able to up-take the iron made available by co-operators.The objectives of the proposed work are to investigate conditions that favour the evolution and maintenance of SP by testing predictions of evolutionary theory. The objectives will be addressed by carrying out series of experiments that investigate the costs and benefits of SP under different environmental condition s and compare the relative success of co-operators and cheats when alone or in mixed cultures. The empirical work will be complemented with theoretical models that aim to develop evolutionary theory in this field. Furthermore, this work will study a co-operative trait that affects virulence in a human pathogen and, therefore, presents a novel opportunity for applying our understanding of social evolution to medical problems.The proposal directly addresses several objectives of the HRM Work Programme, namely: the adding of different complementary scientific competencies to the researcher; transnational mobility; the transfer of knowledge; the building of synergies between two European centres of excellence.