Lactobacillus, which was known to be significantly reduced in COVID-19 patients (Tang et al., 2020a), can produce the aryl hydrocarbon receptor (AhR) ligand-indole-3-aldehyde that promotes AhR-dependent IL22 transcription and improves resistance to mucosal inflammation (Zelante et al., 2013). Here, AHR is linked to COVID-19.