Pulmonary inflammation is an independent risk factor for disease progression in cystic fibrosis patients (CF). Yet, no effective treatment is known to reduce this detrimental inflammation. Dysbiosis of the gut microbiota has been linked to inflammation in several inflammatory diseases. As children with CF have different faecal microbiota from their healthy siblings, modulating gut microbiota by lactobacillus rhamnosus diet supplementation might be a strategy to target the inflammatory state in CF. Study subjects (CF or healthy control) will receive either placebo or lactobacillus rhamnosus once daily as dietary supplementation for 12 weeks. After a one-week washout phase, they will be switched for another 12 weeks to the other trial arm. Effect on in intestinal and pulmonary inflammation as well as clinical outcome will be studied.

Inclusion Criteria:

• moderate to severe CF disease

Exclusion Criteria:

• acute gastroenteritis 2 weeks prior to inclusion
• chronic disease other than CF (except CF associated disorders)
• oral or parenteral antibiotics 2 weeks prior to inclusion
• systemic steroids 4 weeks prior to inclusion
• any probiotic intake