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. 2021 Dec 22;8:785496. doi: 10.3389/fmed.2021.785496

Figure 1.

Figure 1

Schematic diagram of coronavirus disease 2019 (COVID-19) infection and its relationship to the gut-lung axis and microbiome disorders. 1. The destruction of the intestinal barrier integrity by microbial imbalance may lead to the migration of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from the lungs through the circulatory and lymphatic systems to the intestinal cavity. Conversely, bacterial disorders can lead to the rupture of the intestinal barrier and the migration of SARS-CoV-2 viruses from the intestines to the lungs. 2. ACE2 is widely expressed in intestinal epithelial cells. SARS-CoV-2 can use a variety of host proteases to modify the s protein and ACE2 to promote the binding of SARS-CoV-2 to the receptor. After COVID-19 infection, the protective function of ACE2 is lost, which leads to damage of the RAS signal, aggravation of the inflammatory phenotype, and further aggravation of the systemic “cytokine storm” and tissue damage. The permeability of the intestinal barrier changes and intestinal leakage even occurs. 3. In addition, reactive oxygen species (ROS) produced by mitochondria are involved in the innate immune response and inflammation as targets of pathogens. The excessive production of mitochondrial ROS may affect the ROS signal transduction induced by the flora and regulate intestinal barrier function.