EXPERT OPINION – For professor Geneveviève Héry-Arnaud * the knowledge about the pulmonary microbiota and its connections with the gut microbiota opens the therapeutic possibilities through the use of probiotics at different respiratory sites.
The Human Microbiome Project, launched in 2007 by the American National Institutes of Health (NIH), has studied various organs: gastrointestinal tract, urogenital tract, skin, mouth, nose. When the study of the microbiota started, the lung was not observed with interest. But the vision of the lung microbiota has recently changed, from that in medicine (& # 39; A healthy lung is a sterile lung & # 39;), to that of an organ occupied by microorganisms, most of which, in normal situation, good for health.
The airways are poorly described in comparison with other microbial sites. This is due to the difficulty of access by invasive samples (bronchoscopy or bronchoalveolar lavage). The other difficulty comes from oropharyngeal infection, which must be overcome to make the observation more reliable. But with the increase in patients with chronic respiratory disease (more than 300 million people, according to the WHO), teams are interested in lung microbiota, betting that this could be the key to progress, for diagnosis and care.
A scarce microbiota but of rare complexity
If the human gut contains around hundreds of thousands of billion bacteria, the bacterial biomass is a million times smaller (100,000 bacteria per cm3 bronchoalveolar lavage). Despite its low density, this microbiota is distinguished by its exceptional biodiversity. This is due to the different causes of bacteria. Because the lung is an ideal playground (around 75 to 100 m2 of alveolar surface in adults) for bacteria that usually come from the oral cavity, but also inhaled air (100,000 microorganisms per liter of inhaled air) and also digestive tract (by micro-aspiration). The combined action of the mucosa and lashes of the respiratory mucosa, together with local cough and antimicrobial defense, limits the bacterial colonization of the lungs, with a microbiota of incessant ballet where bacteria settle and others leave by the mucociliary escalator. Local lung diseases select the microorganisms that are best adapted to this ecosystem.
Of most bacteria, there are such bacteria Pseudomonasand commensals: Streptococcus, Haemophilus or Neisseria. It is surprising to note that on this site that focuses on oxygenation, strong anaerobic bacteria are present, and therefore cannot tolerate oxygen: Prevotella, Fusobacterium, Veillonellaor Porphyromonas, thus forming an anaerobic heart within the lung microbiota. For a complete picture it should be noted that all branches of life are represented: viruses (phages), mushrooms and even archaea.
Bacterial imbalance and respiratory diseases
Colonization of the airways starts early in life, influenced by the method of delivery. The maturation of this microbiota would be fundamental to lung health, with three roles: 1 / Barrier to pathogenic bacteria. 2 / Education of the local immune system. 3 / Modeling of pulmonary architecture. Other roles are likely to be identified, particularly in relation to the regulation of metabolic routes.
The pulmonary microbiota is the result of a dynamic equilibrium, the accumulated disturbances of which can play a role since childhood in the occurrence or worsening of diseases such as asthma, chronic obstructive pulmonary disease or genetic diseases such as cystic fibrosis, which all result in a radical change in the pulmonary microbiota and especially in a decrease in commensal anaerobes. Thus, for asthma, a hypothesis is that a modification of this microbiota during early childhood could alter the local inflammatory response, which promotes its occurrence.
The analysis of the pulmonary microbiota reveals characteristic features of various respiratory symptoms, making it a diagnostic, prognostic and follow-up tool, in phase with the precision drug that is developing. Finally, the knowledge of the pulmonary microbiota and its connections with the gut microbiota opens the therapeutic possibilities through the use of probiotics at different respiratory sites. Thus, the pulmonary and / or digestive administration of a lactobacilli cocktail before infection by the formidable bacillus pyocyanine appears protective, whether they are cystic fibrosis or nosocomial pneumopathies. Tomorrow, deciphering the lung microbiota will also make it possible to discover new generation probiotics and to tackle common diseases such as asthma from a whole new perspective.
To find out more: "Anaerobic bacteria, these unknowns of the pulmonary microbiota", Guilloux, Lamoureux and Héry-Arnaud. Medicine / Sciences2018; 34: 253-260 Thematic dossier on the pulmonary microbiota by Geneviève Héry-Arnaud in Revuedes Microbiotes n ° 12, November 2018. https://www.larevuedesmicrobiotes.fr/numeros/12.
* Assistant professor at the Faculty of Medicine of Brest (teaches bacteriology). Hospitalist in the bacteriological laboratory of the Brest University Hospital. Director of the research axis "Microbiota" focused on respiratory diseases in the Inserm UMR1078 research unit at the University of Western Brittany.