Background: Exposure to indoor bacteria has been linked to asthma and respiratory symptoms in children.

Objective: We studied the association between indoor bacterial exposure and lung function and airway inflammation measured as Fractional exhaled Nitric Oxide (FeNO) in adults in Northern Europe.

Methods: The bacterial communities of settled airborne dust samples from the bedrooms of 1038 participants in the European Community Respiratory Health Survey (ECRHS) III from five study centres were characterised by 16S rRNA amplicon sequencing, and bacterial load by qPCR. The samples were collected concurrently with spirometry and FeNO measurements (outcomes). Adjusted linear regression stratified by sex were used to model the association between bacterial profiles and outcomes.

Results: Higher bacterial diversity and richness were associated with an increase in FVC and FEV1 Z scores in males (P < 0.05), and with elevated FeNO in females only (P < 0.05).Most bacterial genera associated with higher lung function were from the Actinobacteriota phylum. Higher relative abundance of Bacteroidia, Myxococcota, and Clostridia was associated with lower lung function, as was true also for several bacterial genera from the core oral microbiome, including Streptococcus and Veillonella. Higher FeNO levels were positively associated with the presence of Campylobacter and negatively with the presence of Cellulomonas.

Conclusion: We conclude that a higher microbial diversity is associated to higher lung function in males and increased inflammation and lower lung function in females. Further studies are needed understand the relation between exposure to specific types of bacteria and lung conditions.