The trial aims to analyse changes in the microbiome of the lower airways after smoking cessation. Microbiome analyses (upper airway swabs, bronchoalveolar lavage, transbronchial brushing) are conducted in smokers before and 6 weeks after smoking cessation. Never smokers serve as a control group and undergo the same sampling procedures once.

The trial aims to analyze changes in the microbiome of the lower airways after smoking cessation. Microbiome analyses (upper airway swabs, bronchoalveolar lavage, transbronchial brushing) are conducted in healthy (and willing to quit) smokers before and 6 weeks after smoking cessation. Never smokers serve as a control group and undergo the same sampling procedures once.

Demographic and clinical data of the study subjects are obtained, aiming for homogeneous groups. All personal data are encrypted according to the standard operating procedures of the study site. Pharyngeal and deep nasal swabs are taken.

To avoid contamination of the lower airways from the oropharynx through the bronchoscope, standardized procedures are established according to the German guidelines for bronchoscopy. Before bronchoscopy, the endoscope is flushed with sterile saline which is analyzed for 16S ribosomal RNA (rRNA) to control for residual contaminating bacterial desoxyribonucleic acid (DNA). A 300ml bronchoalveolar lavage with warm normal saline solution is taken from the middle lobe. Three covered and "microbiologically protected" bronchial brushings are obtained from the right upper lobe. This sterile brush is sealed in wax and pushed through the wax just before taking the brushing. All samples are immediately frozen and stored at -80°C until further analyses. For cultivations, samples are kept at 4°C and rapidly processed.

Inclusion Criteria:

• Cigarette consumption of at least five cigarettes per day
• Caucasian ethnicity

Exclusion Criteria:

• Continous passive smoking exposure
• Systemic steroid therapy during the previous month
• Chronic lung disease (exception COPD stage Global Initiative on Lung Disease (GOLD) 0/I/II)
• Diabetes mellitus
• Pregnancy or breast feeding
• active tuberculosis (currently or previously)
• respiratory infection in the previous month
• antibiotic therapy in the previous two months
• immunosuppression
• malignancy
• interstitial lung disease
• rheumatic disease with lung manifestation
• member of a collaborative group of the project