Ultrastructural organization and virion production in Vero E6 cell line under infection with the Delta and Omicron SARS-CoV-2 genovariants

BACKGROUND: The emergence of the coronavirus (SARS-CoV-2) caused a pandemic that led to an increase in morbidity and mortality among infected individuals. The SARS-CoV-2 variants circulating during that period, including Delta, were characterized by a broad spectrum of pathological effects at both the tissue and cellular levels. In 2025, the Omicron variant predominates, producing milder forms of respiratory conditions.

Among the key research questions related to SARS-CoV-2, considerable attention has been devoted to its pathogenicity, which is characterized by high virulence and the ability to induce systemic pathological processes.

Transmission electron microscopy is used to visualize virus–cell receptor interactions and to study the destructive impact of the pathogen on host cells. The findings of such studies form the basis for understanding the course and pathogenesis of the disease and contribute to vaccine development.

AIM: The work aimed to investigate SARS-CoV-2 virion reproduction and the trends of cytopathic effects in Vero E6 cell culture infected with Delta and Omicron variants.

METHODS: Ultrastructural analysis was performed on Vero E6 cell cultures infected with the Delta and Omicron SARS-CoV-2 variants. Infected cell cultures were fixed at three time points: 6, 18, and 24 hours after viral inoculation.

RESULTS: Most samples in both groups (Delta and Omicron) retained near-normal morphology 6 hours post-inoculation and exhibited numerous transport vesicles. At 18 hours, Delta and Omicron groups showed similar morphological changes, including vacuolization of the endoplasmic reticulum and Golgi complex. The Omicron group demonstrated altered mitochondrial morphology. Marked destructive changes were observed 24 hours post-inoculation. Both groups displayed cytoplasmic condensation, extensive organelle vacuolization, and numerous vesicles containing viral particles. Analysis of virion reproduction trends revealed an increase in viral particle numbers at 18 hours, followed by a decline in viral virion replication by 24 hours.

CONCLUSION: The study suggests similar mechanisms of cytopathic pattern formation in Vero E6 cells infected with the Delta and Omicron SARS-CoV-2 variants, along with active virion reproduction in cell culture between 6 and 18 hours post-infection.