Plaque assay

Huh7 and HeLa cells were used for coronavirus and poliovirus, respectively (Tang et al. 2005; Burrill et al. 2013); 4.5 × 10⁵ of cells were plated in each well of 12-well plates a day before the experiment. In the day of the experiment 70 µl of virus inoculum that contained 16.2*10⁶ of plaque-forming units (PFU)/ml of coronavirus and 4.4*10⁴ PFU/ml of poliovirus were suspended in 430 µl of DMEM and irradiated for either 0 s, 0.2 s, 0.5 s, 1 s or 2 s; 250 µl from serial dilutions of 10^–3 to 10^–7 for coronavirus and of 10^–2 to 10^–4 for poliovirus were added to cell monolayers and incubated at 37 °C, 7% CO₂ for an hour. The cell monolayer was then overlaid with 0.5% agarose in growth medium and incubated for 5 days at 37 °C, 7% CO₂. Then, cells were fixed in 2% paraformaldehyde (PFA) for 10 min, the agarose overlay was removed, and the cells were stained with 0.05% crystal violet in 20% Ethanol for 10 min. Following two washings with water, plaques were pictured and counted in the well with highest virus dilution in which plaques were detected. Titer (PFU/ml) = well plaque count / 0.25 ml * dilution factor. GraphPad Prism 9.1.2 (GraphPad Software, San Diego, California USA) was used to analyze the data.