The role of oral microbiome in respiratory health and diseases.

The oral cavity (mouth) has various microbial habitats, including, teeth, gingival sulcus, gingiva, tongue, inner cheek, hard palate, and soft palate. The human oral cavity houses the second most diverse microbiome in the body harboring over 700 bacterial species. The fine-tuned equilibrium of the oral microbiome ecosystem maintains oral health. Oral dysbiosis caused by food habits and poor oral hygiene leads to various oral diseases such as periodontitis, caries, gingivitis, and oral cancer. Recent advances in technology have revealed the correlation between the oral microbiome and systemic diseases such as pulmonary diseases, cardiovascular diseases, rheumatoid arthritis, Alzheimer's disease, and other metabolic diseases. Since the oral cavity directly connects with the upper respiratory tract, the oral microbiome has easier access to the respiratory system compared to other organ systems. Direct aspiration of oral microflora in the respiratory system and oral dysbiosis-induced host immune reaction and inflammation are mainly responsible for various pulmonary complications. Numbers of literature have reported the correlation between oral diseases and pulmonary diseases, suggesting the possible role of the oral microbiome in respiratory diseases such as chronic obstructive pulmonary diseases, pneumonia, lung cancer, etc. This paper reviews the current evidence in establishing a link between the oral microbiome and pulmonary diseases. We also discuss future research directions focusing on the oral microbiome to unravel novel therapeutic approaches that could prevent or treat the various pulmonary complications.