Chief R&D Officer / Sheng-Yu Wu Ph.D
   There are different microbial flora in many mucosal areas of the human body, such as the intestinal tract, skin, oral cavity, respiratory tract, and vagina. With medical development, we increasingly understand the interaction of these microorganisms with each other, which can produce local or systemic immunoregulation by the host.
In fact, the pre embryonic stage of respiratory tract development is from the foregut part, so they have similar mucosa-associated lymphoid tissue which can promote a local immune response.
There is microbial flora in the entire respiratory tract like the intestinal tract, because the mucosal layer is thicker in the nasopharynx, about 105 / ml of bacteria is present. Because of the mucous layer is thinner In the airway, only about 103-4 / ml of bacteria is present, and this area is easily damaged by new microorganisms or other factors;
From the distal airway to the base of the lung does not have the mucosa, so there is no microbial flora existence. Currently, studies have shown that changes in the microbial flora of intestinal tract and airway are related to the development of respiratory tract diseases. For example, lower respiratory tract infections in early infants will easily cause allergic reactions in infants and make them Increase the risk of asthma in the childhood period; however, administration of gastrointestinal probiotics will alleviate this development.
From the animal experiments have also shown that mice are more susceptible to the influenza virus when antibiotics are administered to disrupt the intestinal flora. Therefore, there is a complex interaction between the flora of the intestine and lungs, which affects each other's balance.
Therefore, the gut-lung axis model comes with this, and the gut-lung affects each other. Many gastrointestinal disorders are often accompanied by respiratory tract disease and respiratory tract infectious disease is also followed by intestinal symptoms. For example, most IBD (Inflammatory Bowel Disease) patients suffer from the decline of pulmonary function; while many influenza usually suffer from gastrointestinal discomfort. 
In the "gut-lung axis" model, the microbial flora of the airway can directly (via bacterial interactions) or indirectly (via metabolite by-products) affect the virulence of the pathogen. Once the respiratory tract is infected by viruses, airway obstruction, antibiotics, and other factors, the fertility of bacteria exceeds the capability of the respiratory tract can remove microorganisms. It will change the respiratory tract microbial flora and interfere with the immune response, which increases the susceptibility to respiratory tract bacterial infections. It causes epithelial damage and excessive mucus secretion and lung damage.
After reintroduced the intestinal probiotic, they directly help immune development and adjustment, also affecting the immune response of the respiratory tract. The intestinal tract probiotic indirectly promotes the health of the colonic epithelium by producing short-chain fatty acids such as butyric acid, and exert a wide range of anti-inflammatory activities, and restore the healthy state through the local (lung) or distal (intestinal) microbial flora, thereby affecting lung immunity.
The existed research shows on intestinal probiotic intervention to alleviate respiratory tract diseases, such as Lactobacillus plantarum and Lactobacillus paracasei, which can help to improve influenza infection; Lactobacillus rhamnosus can improve the pneumonia symptoms of Pseudomonas aeruginosa; Lactobacilllus casei can improve the symptoms of pneumococcus pneumoniae, etc., all of which have shown their protective effects in animal models of respiratory tract virus or bacterial infection.
Therefore, intestinal probiotics have great potential to improve respiratory diseases through the "intestinal-lung axis" model mechanism.
References list: 
  1. Dumas A, Bernard L, Poquet Y, Lugo‐Villarino G, and Neyrolles O. The role of the lung microbiota and the gut–lung axis in respiratory infectious diseases. Cellular Microbiology. 2018; 20: e12966.https://doi.org/10.1111/cmi.12966.
  2. Steven L. Taylor, Steve Wesselingh and Geraint B. Rogers. Host-microbiome interactions in acute and chronic respiratory infections. Cellular Microbiology (2016) 18(5), 652–662