Paradigms of Lung Microbiota Functions in Health and Disease, Particularly, in Asthma

Abstract

Improvements in our knowledge of the gut microbiota have broadened our vision of the microbes associated with the intestine. These microbes are essential actors and protectors of digestive and extra-digestive health and, by extension, crucial for human physiology. Similar reconsiderations are currently underway concerning the endogenous microbes of the lungs, with a shift in focus away from their involvement in infections toward a role in physiology. The discovery of the lung microbiota was delayed by the long-held view that the lungs of healthy individuals were sterile and by sampling difficulties. The lung microbiota has a low density, and the maintenance of small numbers of bacteria seems to be a critical determinant of good health. This review aims to highlight how knowledge about the lung microbiota can change our conception of lung physiology and respiratory health. We provide support for this point of view with knowledge acquired about the gut microbiota and intestinal physiology. We describe the main characteristics of the lung microbiota and its functional impact on lung physiology, particularly in healthy individuals, after birth, but also in asthma. We describe some of the physiological features of the respiratory tract potentially favoring the installation of a dysbiotic microbiota. The gut microbiota feeds and matures the intestinal epithelium and is involved in immunity, when the principal role of the lung microbiota seems to be the orientation and balance of aspects of immune and epithelial responsiveness. This implies that the local and remote effects of bacterial communities are likely to be determinant in many respiratory diseases caused by viruses, allergens or genetic deficiency. Finally, we discuss the reciprocal connections between the gut and lungs that render these two compartments inseparable.

Microbes, metabolites, and the gut–lung axis

“…54 In humans, it is not feasible to track the lower airway microbiome longitudinally, however, the constituents of the microbiome associated with distinct steps of immune maturation indicative of ongoing crosstalk between the airway resident microbes and immune cells. 56,57…”

Respiratory microbiome and epithelial interactions shape immunity in the lungs

“…14,23 Microbial elimination is an active process that is achieved through mucociliary clearance, cough and host immune defences. 24 The environmental conditions necessary for microbial growth within the respiratory tract (e.g. pH, temperature, nutrient availability, oxygen tension and activation of host inflammatory cells) are heterogenous, and considerable regional variation can be observed in a single healthy lung.…”

The lung microbiota: role in maintaining pulmonary immune homeostasis and its implications in cancer development and therapy

“…The microbial community is continually renewed and replaced, but the majority of microbes involved in these processes belongs to four phyla: Bacteroidetes, Firmicutes, Proteobacteria, and Actinobacteria. In healthy humans, Prevotella, Streptococcus, Veilonella, Neisseria, Haemophilus, and Fusobacterium are the most abundant genera in the lungs [2][3][4].…”