The gut microbiomes of patients with severe COVID-19 differ from those of patients with a more moderate disease course, a new study has found. A team at Harvard Medical School analyzed the metagenomes within about 240 stool samples collected from 127 individuals hospitalized with COVID-19 between April 2020 and May 2021, 79 of whom had severe disease and 48 of whom had moderate disease. As they report in Genome Medicine, the researchers uncovered 48 bacterial species linked to severe disease, including declines in Fusicatenibacter saccharivorans and Roseburia hominis —reductions in which have been linked previously to long COVID, suggesting to them that a gut microbiome disturbance could precede the development of Long COVID. The researchers also found differences in microbial pathways between COVID-19 patients with differing disease courses that indicated bile acid and short-chain fatty acid levels may be altered in severe COVID-19. "Here, we show that the gut microbiome differentiates individuals with a more severe disease course after infection with COVID-19 and offer several tractable and biologically plausible mechanisms through which gut microbial communities may influence COVID-19 disease course," the researchers write.