PURPOSE: In the cystic fibrosis (CF) airways, Pseudomonas aeruginosa undergoes diverse physiological changes in response to inflammation, antibiotic pressure, oxidative stress and a dynamic bioavailable nutrient pool. These include loss-of-function mutations that result in reduced virulence, altered metabolism and other phenotypes that are thought to confer a selective advantage for long-term persistence. Recently, clinical isolates of P. aeruginosa that hyperproduce agmatine (decarboxylated arginine) were cultured from individuals with CF. Sputum concentrations of this metabolite were also shown to correlate with disease severity. This raised the question of whether agmatine accumulation might also confer a selective advantage for P. aeruginosa during chronic colonization of the lung. METHODOLOGY AND RESULTS: We screened a library of P. aeruginosa CF clinical isolates and found that ~5 % of subjects harboured isolates with an agmatine hyperproducing phenotype. Agmatine accumulation was a direct result of mutations in aguA, encoding the arginine deiminase that catalyses the conversion of agmatine into various polyamines. We also found that agmatine hyperproducing isolates (aguA-) had increased tolerance to the cationic antibiotics gentamicin, tobramycin and colistin relative to their chromosomally complemented strains (aguA+). Finally, we revealed that agmatine diminishes IL-8 production by airway epithelial cells in response to bacterial infection, with a consequent decrease in neutrophil recruitment to the murine airways in an acute pneumonia model. CONCLUSION: These data highlight a potential new role for bacterial-derived agmatine that may have important consequences for the long-term persistence of P. aeruginosa in the CF airways.