In a review article by Popov et al. from 2019, electron microscopy (EM) is described as one of the most important and most widely used methods for identifying and characterising new viruses. However, this assessment requires differentiated consideration, particularly with regard to the specificity of structural findings.
Below are two electron micrographs from this overview article.
Figure C:
Accumulation of virions of the Fako virus (diameter approx. 45 nm) in an infected C6/36 cell.
Figure D:
Virions (diameter approx. 45 nm) of the bannavirus (strain JKT-6423), genus Seadornavirus, subfamily Sedoreovirinae, in the cytoplasm of an infected C6/36 cell. Arrowheads mark a section of the cell nucleus. Scale = 100 nm.
Independent scientists from the Virology Controls Studies Project provided NEXT LEVEL with electron microscope images of Vero-E6 cell cultures to investigate the extent to which the experimental setup itself causes structural changes in the cells. The images obtained in this way were also to be used for AI-supported image analyses and training data sets. The experiment comprised a preparatory phase in which the cells were thawed and passaged several times until a stable and morphologically unremarkable state was reached. This was followed by two experimental phases that differed only in the concentration of foetal bovine serum (FBS) in the culture medium.
The following electron micrograph is from a Vero E6 cell culture treated for five days with a growth medium containing 10 % FBS.
Irrespective of the differences between the cell lines used (C6/36 cells versus Vero-E6 cells) and possible deviations in the size of the structures observed, morphologically comparable spherical particles can be recognised in the electron micrographs. It should be emphasised that no “viral material” was added to the Vero-E6 cell cultures at any time. The observed spherical structures are therefore apparently solely the result of the experimental set-up and the cell culture conditions. The Vero-E6 cell experiment thus represents a classic and essential negative control experiment in virological research. It illustrates that virus-like structures visible under the electron microscope cannot be interpreted per se as evidence for the presence of a virus, but must always be evaluated in the context of suitable controls.
Such control experiments are not consistently carried out and documented in virological studies, or not in a sufficiently systematic form. As a result, there is a risk that morphological findings from electron microscopy are misinterpreted and structural artefacts or intrinsic cellular structures are wrongly interpreted as virus particles.
Conclusion: Do these findings lead to a questioning of previous conclusions in virology?
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