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The opportunistic bacteriumPseudomonas aeruginosais a major nosocomial pathogen causing both devastating acute and chronic persistent infections. During the course of an infection,P. aeruginosarapidly adapts to the specific conditions within the host. In the present study, we aimed at the identification of genes that are highly expressed during biofilm infections such as in chronically infected lungs of patients with cystic fibrosis (CF), burn wounds and subcutaneous mouse tumours. We found a common subset of differentially regulated genes in all threein vivohabitats and evaluated whether their inactivation impacts on the bacterial capability to form biofilmsin vitroand to establish biofilm-associated infections in a murine model. Additive effects on biofilm formation and host colonization were discovered by the combined inactivation of several highly expressed genes. However, even combined inactivation was not sufficient to abolish the establishment of an infection completely. These findings can be interpreted as evidence that either redundant traits encode functions that are essential forin vivosurvival and chronic biofilm infections and/or bacterial adaptation is considerably achieved independently of transcription levels. Supplemental screens, will have to be applied in order to identify the minimal set of key genes essential for the establishment of chronic infectious diseases.