Microbiota of the respiratory tract
From a sterility to an ecosystem
The mucous membranes of the respiratory tracts are constantly exposed to microorganisms in the air we breathe. However, for a long time the sub-glottal respiratory tree have been considered as a sterile site in healthy subjects, as a result of mechanical purification phenomena and phagocytosis. A belief that explains the delayed interest to the respiratory microbiota.
Now new microbial identification technologies show that this concept of sterility exists nowhere on our planet. The respiratory tract, including the lungs, is not exception and hosts a fairly developed microbiological ecosystem, essentially including Proteobacteria, Firmicutes, and Bacteroidetes1,2. The origin of this ecosystem is still unknown, even if the environment, diet and intestinal microbiota may contribute to it3.
Dynamic and balance
The configuration of the respiratory microbiota is influenced by several ethnic and environmental factors4,5. By the bidirectional movement of air, mucous and bacteria, without barrier from the nose to the lungs, it is not stationary but well dynamic.
This dynamic configuration is part of a balance game determined by migration, elimination and the reproduction of microorganisms2,5.
Link with the intestinal microbiota
We know today that the intestinal microbiota plays a key role in initiating and adapting the immune response, not only in the digestive tract but also remotely in the lungs6. In fact, a change in the intestinal microbiota may have an impact on the respiratory system promoting the appearance of several allergic or infectious pulmonary diseases.
Health and disease
The configuration of the respiratory microbiota in healthy subjects differs from that of sick people2,7. There are several arguments in favour of its involvement in inflammatory diseases of the respiratory tree3.
The role of the respiratory microbiota in normal condition is not yet elucidated.
Identified only five years ago, the respiratory microbiota will doubtless enable a better understanding of broncho-pulmonary diseases in the coming years.
Its study could also lead to therapeutic perspectives, such as the exposure to specific microorganisms to restore the balance and, consequently, to treat certain diseases3. This exposure can be done by the use of probiotics or a possible faecal transplantation.
1 Beck JM, Young VB, Huffnagle GB. The microbiome of the lung. Transl Res. 2012; 160: 258-266.
2 Dickson RP, Erb-Downward JR, Martinez FJ, Huffnagle GB. The microbiome and the respiratory tract. Annu Rev Physiol. 2016; 78: 481-504.
3 Madan JC, Koestler DC, Stanton BA, Davidson L, Moulton LA, Housman ML, Moore JH, Guill MF, Morrison HG, Sogin ML, Hampton TH, Karagas MR, Palumbo PE, Foster JA, Hibberd PL, O’Toole GA. Serial analysis of the gut and respiratory microbiome in cystic fibrosis in infancy: interaction between intestinal and respiratory tracts and impact of nutritional exposures. MBio. 2012; 3. pii: e00251-12.
4 Gollwitzer ES, Marsland BJ. Microbiota abnormalities in inflammatory airway diseases – Potential for therapy. Pharmacol Ther. 2014; 141: 32-39.
5 Dickson RP, Huffnagle GB. The lung microbiome: new principles for respiratory bacteriology in health and disease. PLoS Pathog. 2015; 11: e1004923.
6 Samuelson DR, Welsh DA, Shellito JE. Regulation of lung immunity and host defense by the intestinal microbiota. Front Microbiol. 2015; 6: 1085.
7 Fujimura KE, Lynch SV. Microbiota in allergy and asthma and the emerging relationship with the gut microbiome. Cell Host Microbe. 2015; 17: 592-602.