FIG 6.

Infectious particles are retained at the surface of MmBr-ACE2 cells. A549-ACE2 and MmBr-ACE2 cells were left uninfected (Mock) or were infected for 24 h at an MOI of 1 or 0.04, respectively. (A) The percentages of cells that contained SARS-CoV-2 S proteins were determined by flow cytometric analysis. (B) The presence of extracellular infectious viruses in the culture medium of the cells was determined by TCID₅₀ assays performed on Vero E6 cells. Supernatants were either clarified or clarified and purified by ultracentrifugation. Alternatively, cell-associated infectious virions were titrated on Vero E6 cells from whole-cell lysates. Data points represent three independent experiments. Statistical test: Dunnett’s multiple-comparison test on a two-way ANOVA (ns, not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001). (C to E) At 16 h postinfection, numerous viral particles were observed on the cell surfaces of MmBr-ACE2 cells, attached to the plasma membrane (C and D). Boxed areas in the low-magnification photographs in panels C and D are shown at higher magnification in the right panels. (E) Uninfected cells showed no viral particles on their surfaces. The scale bars are 1 mm in panels C and D, 0.5 mm in panel E, and 200 and 100 nm in the high-magnification panels extracted from images C and D, respectively.