Recently, the investigators described a new mucoid phenotype of Staphylococcus aureus cultured from the airways of cystic fibrosis (CF) patients.In this observational study, the investigators plan to determine the prevalence of mucoid S. aureus in respiratory specimens of CF patients and a possible impact of mucoid S. aureus on lung disease severity.

S. aureus is one of the first isolated pathogens recovered from the airways of CF patients. In many patients, S. aureus persists for decades in spite of antibiotic treatment and host defence. During persistence in the airways, S. aureus needs to adapt to this hostile niche. Just recently, the investigators described mucoid S. aureus isolates as a so far not reported phenotype recovered from the airways of CF patients. These mucoid isolates carried a 5bp-deletion in a part of the "intercellular adhesion operon (ica) leading to hyper-expression of biofilm. So far, neither the prevalence of mucoid S. aureus nor the impact on lung disease in CF patients is known. The investigators plan to perform a clinical study to determine the prevalence of mucoid S. aureus isolates in a cross-sectional study and the impact of identified mucoid S. aureus on the clinical course of the lung disease by observing patients with mucoid S. aureus for 12 months in comparison with a group of age-, gender- and P. aeruginosa-positive/negative matched group of S. aureus-positive patients without mucoid isolates.

Inclusion Criteria:

• CF patients with chronic S. aureus in respiratory specimens (50% positive S. aureus cultures within one or two years before recruitment)

Exclusion Criteria:

• none

CF patients with chronic S. aureus in respiratory specimens