Fig. 2.

Replication success of ten ancestral strains of SARS-CoV-2 over ten serial passages. The replication success is defined as the development of the viral load in the course of the experiment. All strains started with viral loads of 7.6 ​× ​10⁸. Three strains did not manage to replicate and got lost within the first three passages (yellow bars), four strains maintained in culture with a decreased or nearly unchanged viral load (blue bars), three strains showed significantly higher replication success (green bars). The bars show the growth factor of the diverse strains, calculated as the percentage growth of the strain during the experiment (PFU/mL at P10 ​× ​100)/(PFU/mL at P0). The numbers above the bars represent the acquired aa substitutions during the serial passages. Numbers in parenthesis show additional temporary substitutions which did occur once in a sequence throughout the experiment but were not preserved. The first three strains, here marked with the first letter B, did not manage to replicate successfully in cell culture and were lost after at least four passages. Actual increases, thus successful adaptation to the cell cultures, were recorded in the six strains A818 to A4707, with the last three strains A7326, A6137 and A4707 achieving the highest increase in PFU/mL with growth of 4 ​× ​10³% up to 3 ​× ​10⁴%.