Fig. 4.

Relationship between oral cavity and respiratory diseases: “Oral bacterium–virus–host interactions” in the oral cavity–lower airway axis.

Pneumonia, COPD, influenza, and COVID-19, all of which have been shown to be associated with oral flora, are diseases in which inflammation of the lungs is the main symptom, and there is also an association between diseases. In addition to the mechanisms between chronic periodontitis and systemic chronic inflammation, those of lower airway inflammation caused by oral bacteria, their appendages (e.g., fimbriae and LPS), and their extracellular enzymes should be elucidated, and measures should be developed for their control. For this purpose, it is necessary to examine the etiology of lower airway inflammation from the viewpoint of microbial interactions, such as oral bacteria–pneumonia-causing bacteria, oral bacteria–influenza virus, and oral bacteria–SARS-CoV-2, rather than focusing only on the pathogenicity of a single microorganism.