Gut-Lung Connection - An Overview

Introduction

The human body harbors trillions of microorganisms. They play an important role in human health by modulating innate immunity. The gut houses immense microbes (microbiota), which are considered an “essential organ.” The gut microbiota helps in assimilating dietary nutrients, and they produce metabolites. These metabolites are essential for regulating gastrointestinal immunity. Any disturbance in gut microbiota has a potent impact on distant organs like the lungs, brain, mouth, liver, vagina, etc. Therefore, a high-nutrient diet is essential for maintaining gut microbiota.

What Are Gut Microbiota?

Major microbiota in the gut is the Firmicutes (e.g., Lactobacillus, Bacillus, and Clostridium) and the Bacteroidetes (e.g., Bacteroides). Proteobacteria (e.g., Escherichia) and Actinobacteria (e.g., Bifidobacterium), Cyanobacteria, and Fusobacteria are also present to some extent. Species of Fungi (Saccharomyces cerevisiae, Malassezia restricta, and Candida albicans) have also been recognized in some individuals. Yeasts (Saccharomyces boulardii) also inactivate intestinal pathogens.

What Is the Role of Gut Microbiota on the Gut?

The gut microbiota ferments dietary fibers and intestinal mucus to produce short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate.

• Butyrate destroys colon cancer cells and balances glucose and energy.

• Acetate plays a role in cholesterol metabolism and appetite regulation.

• Propionate regulates glucose production.

What Are the Lung Microbiota?

The lung harbors the same Firmicutes, Bacteroides, Proteobacteria, and Actinobacteria. Environmental fungi Ascomycota (Aspergillus, Cladosporium, Eremothecium, and Vanderwaltozyma) and Microsporidia (Systenostrema) have also been identified.

What Is the Role of the Lung Microbiota on Lungs?

Lung microbiota act with the lung immune cells (alveolar macrophages and dendritic cells) to initiate immune responses against pathogens. They also play a role in immune tolerance.

What Is a Gut-Lung Connection?

The lungs and the gut communicate through the chemical messengers produced by the microbiota. This connection is called the gut-lung axis. The gut microbiota breaks down food particles and produces metabolites (short-chain fatty acids). The SCFAs travel throughout the body via blood and lymph to influence immune cell recruitment, thereby reducing inflammation in organs, including the lungs. Microbiota also produce messengers like endotoxins that trigger immune cells to produce cytokines. Cytokines are protein molecules that promote inflammation. This shows that a healthy gut prevents inflammation, whereas an unhealthy gut provokes inflammation. The gut and the lungs host similar messengers; change in the intestinal microbiota has a significant impact on lung diseases, and change in the lung microbiota has a profound effect on the gut. The imbalance in the microbiota is called dysbiosis.

How Does Gut Microbiota Impacts Lung Diseases?

The gut microbiota resists the colonization of pathogens. The gut dysbiosis caused by antibiotics destroys some or more intestinal microbiota, increases the susceptibility to enteric infections caused by Salmonella and Escherichia coli, and impairs response to systemic and respiratory infections caused by the Influenza virus and the respiratory syncytial virus.

Dysbiosis of the normal gut microbiota induces the development of various respiratory disorders.

• Low levels of gut microbiota (Akkermansia muciniphila and Faecalibacterium prausnitzii) are associated with low levels of metabolites that are essential for protection against environmental immunity. This dysbiosis makes the individual more susceptible to allergens and thereby causing asthma.

• Dietary fiber enhances the activity of gut microbiota to produce SCFAs.This modulates innate immune response and inflammation, maintains epithelial integrity, and prevents microbial invasion. Dysbiosis in this mechanism induces COPD.

• In tuberculosis, the synthesis of butyrate and propionate was found to be decreased. Gut dysbiosis reduces the activation of the immune response against Mycobacterium tuberculosis and results in tuberculosis infection.

• Segmented filamentous bacteria in the gut stimulate inflammatory neutrophils and offer protection against pneumonia.

• Bacteroides species reduce inflammation. Reduction in this influences inflammation in cystic fibrosis.

• Increased levels of Enterococcus species and decreased levels of Bifidobacterium are associated with lung cancer.

What Is the Impact of Lung Microbiota on the Gut?

Dysbiosis in the lung microbiota moves the lung bacteria into the intestine via blood, increases the bacterial load in the intestine, and disturbs the gut microbiota.

• Influenza virus infection increases Enterobacteriaceae and reduces Lactobacilli and Lactococci in the gut microbiota.

• Pneumonia caused by Pseudomonas aeruginosa induces intestinal injury.

What Is the Role of Diet in the Gut-Lung Axis?

A fiber-rich diet is essential for producing significant metabolites in maintaining human health. The gut microbiota breakdown of dietary fibers increases the production of SCFAs and lowers the gut pH, thereby reducing the colonization of pathogens.

• Clostridium orbiscindens breaks down plant flavonoids to produce desaminotyrosine. This travels to the lungs via the blood and influences the immune system to protect from influenza infection.

• Fermentation of inulin fiber by gut bacteria offers protection against influenza infection.

• Fermentation of pectin fiber protects against Respiratory syncytial virus.

Nutrients are utilized by gut microbiota to produce metabolites to regulate the immune system. Therefore, diet directly influences intestinal microbes in maintaining human health.

What Is the Role of Probiotics?

Probiotics are live good microbes present in yogurt, fermented foods, and dietary supplements.

• The administration of probiotic bacteria like Lactobacillus casei Shirota or rhamnosus reduces symptoms of cystic fibrosis.

• Lactobacillus rhamnosus and Bifidobacterium breve reduce inflammation caused by cigarette smoke, thereby reducing diseases like COPD.

• Probiotics enhance intestinal barrier function against lipopolysaccharides and improve inflammatory conditions like inflammatory bowel disease, allergy, COPD, asthma, etc.

• Enterococcus hirae and Barnesiella intestine hominins are administered to cancer patients to improve the efficacy of cancer treatments.

What Is the Role of Prebiotics?

Prebiotics are plant fibers that are essential for the growth of good bacteria in the gut. Fibrous fruits and vegetables contain prebiotics. These complex fibers are indigestible; they pass through the digestive tract and become food for the bacteria. So prebiotics is important to promote the growth of good bacteria.

Conclusion

The gut and the lung are two distant organs in the human body. They communicate through the microbial community. Good gut health is essential for maintaining the vital functions of all organs. In addition, the gut microbiota is significant in the defense mechanism. Consumption of prebiotics and probiotics through a fiber-rich diet helps in the preventive and therapeutic interventions against pathogens.