Literature research using the terms “SARS-CoV-2 & gut-lung axis”, “gut bacteria & COVID-19”, “Coronavirus & gut-lung axis & ACE2”, “probiotics & cytokine storm & COVID-19”, “oral bacteriotherapy”, “COVID-19 & immunobiotics”, and their combination in PubMed revealed eight publications in the English language (accessed on 30 May 2022). Here, ACE2 is linked to COVID-19.