The CFTR gene is conserved in Zebrafish and the CFTR-loss-of-function zebrafish mutants mimic CF human disease. In a previous project (FFC#16/2016) researchers isolated and mixed in cocktail different phages and demonstrated they are able to infect and kill a large number of P. aeruginosa clinical strains in vitro. The subsequent step is the use of the CF-zebrafish model to validate in vivo phage therapy against P. aeruginosa infections. To this aim Zebrafish embryos both wild-type and with reduced expression of CFTR (CFTR-loss-of-function) will be infected with P. aeruginosa by static immersion or by microinjection, and incubated in water added with the phage cocktail. The therapeutic effects of phages will be measured following embryo mortality, bacterial burden, and neutrophil migration. Two are the expected results of this project: to demonstrate that zebrafish is a new CF-model that can be used for in vivo validation of therapeutic treatments relative to CF; and to give evidence that phage therapy is effective in reducing P. aeruginosa infections in vivo.