The Lung Microbiome: Challenging the "Sterile Lung" Dogma

Historical view: Lungs were considered sterile. Current science: Healthy lungs harbor a low-biomass, distinct microbiome—part of the gut-lung axis.

Key Findings

• Not sterile: 16S rRNA sequencing detects bacteria in healthy lower airways (bronchoscopically sampled).
• Low biomass: 10³–10⁴ CFU/mL vs. 10¹¹ in gut. Contamination control critical.
• Composition: Prevotella, Veillonella, Streptococcus, Haemophilus, Neisseria (oral-like). Differs from upper airway.
• Origin: Microaspiration of oropharyngeal secretions + inhaled microbes. Not colonization in gut sense.
• Dynamic: Altered in COPD, asthma, CF, IPF, lung cancer, post-transplant.

Functional Significance

• Immune education: T_reg induction, Th17 balance
• Barrier integrity: Mucin production, antimicrobial peptides
• Metabolic crosstalk: SCFAs, neurotransmitters
• Dysbiosis → inflammation → disease progression (bidirectional)

Clinical Implications

• Antibiotic stewardship: Broad-spectrum ABX disrupt lung microbiome → dysbiosis → exacerbations.
• Probiotics (oral/inhaled): Early trials for asthma, COPD, VAP prevention.
• Biomarkers: Microbial signatures for endotyping, treatment response.

Methodological Caveats

• Contamination from bronchoscope, oral cavity, reagents
• Low biomass → high false-positive risk
• Viable vs. dead DNA (PMA treatment helps)
• Spatial heterogeneity (central vs. peripheral airways)

The "sterile lung" paradigm has shifted. The lung microbiome is a low-abundance, high-significance ecosystem influencing respiratory health and disease.