Abstract
Healthy human lungs have traditionally been considered to be a sterile organ. However, culture-independent molecular techniques have reported that large numbers of microbes coexist in the lung and airways. The lungs harbor diverse microbial composition that are undetected by previous approaches. Many studies have found significant differences in microbial composition between during health and respiratory disease. The lung microbiome is likely to not only influence susceptibility or causes of diseases but be affected by disease activities or responses to treatment. Although lung microbiome research has some limitations from study design to reporting, it can add further dimensionality to host-microbe interactions. Moreover, there is a possibility that extending understanding to the lung microbiome with new multiple omics approaches would be useful for developing both diagnostic and prognostic biomarkers for respiratory diseases in clinical settings.
Highlights
Healthy human lungs have been traditionally considered to be sterile or free from bacteria for a long time [1,2]
It is considered that the composition of the lung microbiome is determined by the balance of three factors: [27] (1) microbial immigration into the airways, (2) elimination of microbes from the airways, and (3) the relative reproduction rates of its community members found in the airways, which is determined by the regional growth conditions (Figure 1)
Studies comparing respiratory microbiome in bronchoalveolar lavage fluid (BALF) and sputum between chronic obstructive pulmonary disease (COPD) patients and healthy subjects have identified a change of microbial diversity with an increased relative abundance of Moraxella, Streptococcus, Proteobacteria, Veillonella, Eubacterium, and Prevotella sp. in disease [49,50]
Summary
Healthy human lungs have been traditionally considered to be sterile or free from bacteria for a long time [1,2]. Numerous studies using culture-independent molecular techniques have demonstrated that a diverse bacterial community is present in the lower airway of healthy condition, and have identified the main genera as Prevotella, Veillonella, and Streptococcus [2,5,8,9,10,11,12,13,14,15]. While luminal IgA levels are much higher in the GI tract [25], lungs have more extraluminal interactions between bacteria and host alveolar macrophages [26] These differences of environmental conditions between the GI tract and the lungs results in divergent microbial communities
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