ACE2 and infection: We additionally infected SCFA-treated GF mice with a replication-competent chimeric VSV virus expressing GFP and the SARS-CoV-2 beta variant spike protein (designated rVSV/Spikeβ-GFP), which can use murine ACE2 to infect cells.22 Consistent with our results in hamsters, 2 d post infection, we found reduced infection in the lungs of SCFA-treated GF mice, as determined by measuring GFP+ cells by flow cytometry (Figure S2F).