Morphometry of SARS-CoV and SARS-CoV-2 particles in ultrathin plastic sections of infected Vero cell cultures

SARS-CoV-2 is the causative of the COVID-19 disease, which has spread pandemically around the globe within a few months. It is therefore of great interest to collect fundamental information about the disease, its epidemiology and treatment, as well as about the virus itself. While the virus has been identified rapidly, detailed ultrastructural analysis of virus cell biology and architecture is still in its infancy. We therefore studied the virus morphology and morphometry of SARS-CoV-2 in comparison to SARS-CoV as it appears in Vero cell cultures by using conventional thin section electron microscopy and electron tomography. Both virus isolates, SARS-CoV Frankfurt 1 and SARS-CoV-2 Italy-INMI1, were virtually identical at the ultrastructural level and revealed a very similar particle size distribution with a median of about 140 nm. SARS-CoV showed a slightly broader size distribution with a few smaller and bigger particles than SARS-CoV-2. The peplomer density was measured using very thin sections and showed a difference between the two virus isolates. SARS-CoV displayed one-third more peplomers per segmental particle volume than SARS-CoV-2. This result complements a previous qualitative finding, using another isolate of the SARS-CoV-2, which was related to a lower productivity of SARS-CoV-2 in cell culture in comparison to SARS-CoV.