The complementation assays showed that the identified point Isolinderalactone mutations were responsible for the phenotypes. In the signalbinding domain, particular interesting are the mutations causing a Tyr56 to Cys exchange and a Thr75 to Lys exchange as both Tyr56 and Thr75 have been shown to be important for the binding of signal molecules to LasR. In addition, Pro74, Ala105 and Gly113 were all amino acids that have been described as important for the multimerization and function of LasR. Several of the mutations described in this study have been found by other investigators in lasR mutants of P. aeruginosa obtained under in vitro evolution experiments. This reflects a level of similarity between the in vitro and in vivo bacterial evolution and suggest a possible selective advantage of these kinds of mutations in vivo. We propose that mutations in either of the QS regulatory genes can occur in isolates with non- or weak mutator phenotype, followed in time by the occurrence of strong mutators with increased accumulation of mutations which disables the entire QS system. This sequence of events in the CF lung might be explained by an impaired protection of the QS deficient strains against the mutagenic effects of reactive-oxygen species liberated by activated PMNs due to their decreased production of catalase and superoxide-dismutase. This capability is maintained till late in the chronic infection, in particular in mucoid isolates. Our results show that the mucoid isolates are protected from the antimicrobial activity of PMNs in the CF lung by both alginate which act as a ROS scavenger but also by their ability to produce rhamnolipids which provide a shield against cellular components of the innate immune response. This also suggest that a treatment with drugs 2-O-galloylhyperin interfering with QS and in particular the lower hierarchy of QS regulated virulence factors such as rhamnolipids might be useful not only in the early stages of the infection but also in the treatment of chronic infections with QS producing strains. Sputum samples obtained by expectoration or endolaryngeal suction were Gram-stained and examined under the microscope to confirm the origin from the lower airways with the exception of the samples from Norway.