What is already known on this topic
The lungs, previously thought to be sterile, are now known to harbor their own microbiota and to be influenced by microbial signals from distal body sites, such as the gut.
What this research adds
Scientists reviewed the role of the lung and gut microbiota in respiratory health and disease, focusing on the main communication pathways that link the gut and the lungs.
Many studies have focused on the bacterial component of the microbiota, but little is known about the role of microorganisms such as fungi, protozoa, and viruses. Future studies that look at all the components of the microbiota could improve disease prevention and treatment.
For a long time, scientists considered the lungs to be sterile, but recent studies have shown that they harbor their own microbiota and are influenced by microbial signals coming from distal body sites, such as the gut.
A team of scientists led by Tomasz Wypych at Monash University in Melbourne reviewed the role of the lung and gut microbiota in respiratory health and disease, focusing on the main communication pathways that link the gut and the lungs. Their work is published in Nature Immunology.
Lung microbiota in health and disease
Several studies have shown that the lungs of healthy people harbor a microbiota, whose main bacterial components include Prevotella, Streptococcus, and Haemophilus. Colonization of the airways begins immediately after birth, and within 7 weeks, the lung microbiota of infants becomes similar to that of healthy adults.
But in respiratory disease, the lung microbiota appears to be altered. People with cystic fibrosis or other lung disorders have more bacteria in the lower airways than healthy individuals. The most common bacteria in people with respiratory diseases include Pseudomonas aeruginosa, Neisseiria, and Veillonella.
Similarly, a type of asthma with a low abundance of immune cells is accompanied by an increase in pro-inflammatory bacteria such as Moraxella or Neisseria. On the other hand, asthma with a high abundance of immune cells has been associated with decreased levels of bacteria in the lower airways.
The lung-gut axis
The main means of communication between gut bacteria and the lungs are microbial components, such as peptidoglycans, and metabolites, including short-chain fatty acids. These molecules are transported through the blood and regulate immune responses in the lungs. But the gut and the lungs can also communicate through the direct migration of immune cells from the intestine to the respiratory tract.
Because the effects of microbial products and metabolites are more predictable than those of the bacteria themselves, they may be used in the clinic in lieu of microbes. For a long time, Lactobacillus and Bifidobacterium bacteria have been known to protect from respiratory infections. More recently, studies in mice have shown that Acinetobacter lwoffii reduced allergic airway inflammation, and Staphylococcus sciuri and Lactococcus lactis reduced asthma.
Bacterial strains such as Lactobacillus crispatus, Staphylococcus aureus, or Lactobacillus reuteri protected mice against respiratory infection by Streptococcus pneumoniae or Klebsiella pneumoniae.
However, despite the beneficial properties of Lactobacillus and Bifidobacterium in mice, probiotic interventions in people have been disappointing. Given the promising effects of fecal microbiota transplantation in treating people with gastrointestinal disorders such as irritable bowel syndrome, researchers are now starting to explore the possible benefits of fecal transplants for respiratory illnesses. Preliminary studies in mice showed that fecal transplants reversed mortality following infection with S. pneumoniae. But whether this applies also to people remains unclear.
What’s more, researchers have tended to focused on the bacterial component of the microbiota, but little is known about the role of other microorganisms such as fungi, protozoa, and viruses. Future studies that look at all the components of the microbiota could improve disease prevention and treatment, the scientists say.