Staphylococcus aureus is not only one of the first pathogens infecting the airways of cystic fibrosis (CF) patients, but also a highly prevalent microorganism (>60% of all CF patients; European and American CF registries; (4,25), which often persists for several years in the respiratory tract of CF patients.

The purpose of this study is to dissect infection by S. aureus from colonization. Therefore, the following non-interventional prospective, longitudinal multicenter study will be conducted to develop the following hypothesis:

CF patients with high bacterial loads are more likely to be infected by S. aureus than patients with low bacterial loads.

Primary endpoint: bacterial load of sputum cultures

Secondary endpoints:

• nasal carriage
• molecular analysis of S. aureus (Monoclonal/polyclonal)
• serum: S. aureus-specific antibodies, S100A12, IL-8, TNF-alpha
• sputum: S100A12, IL-8, myeloperoxidase
• S. aureus therapy regimens
• lung function tests: FEV1, deltaFVC , deltaMEF25
• BMI development

Inclusion criteria: S. aureus cultures for more than 6 months within the last year, children (>6 years) and patients, who are able to perform lung function tests Exclusion criteria: P. aeruginosa and/or B. cepacia cultures from the specimens for more than 6 months within the last year before recruitment or during the study period In addition to microbiological investigations and clinical laboratory tests, the actual clinical situation will be evaluated and reported during the study period. The results of this observational study will be used to carefully plan a clinical interventional study. Furthermore, with the results it might be possible to characterize a subpopulation of patients, which is at greater risk for S. aureus infections.