Figure 6.

Exacerbation of lower respiratory tract inflammation by stimulation with oral bacteria viewed from the oral cavity-lower respiratory tract axis. The elderly are prone to aspirate oral bacteria along with saliva. In addition, in the current dental situation, the longer the SARS-CoV-2 epidemic continues, the less opportunity there is for professional oral hygiene management, resulting in poor oral hygiene for the subjects with oral health management needs; this is also exacerbated on cases of hospitalization due to COVID-19 attributable to a general shortage of dental professionals in large hospitals. Aspiration of periodontal pathogens and other oral bacteria in patients with COVID-19 induces the expression of ACE2 on the lower respiratory epithelia and the production of proinflammatory cytokines in the lower respiratory tract; this may lead to a “negative spiral” of proinflammatory phenomena in the lower respiratory tract caused by the collaboration between viruses and bacteria. In addition, oral bacteria may induce excessive production of mucin in the lower respiratory tract, resulting in decreased respiratory function, and they may also cause destruction of the epithelial barrier in the alveoli and bronchi. Therefore, oral health management may prevent the aggravation of COVID-19 by suppressing the abovementioned harmful phenomena in patients with COVID-19.