Abstract text (incl. figure legends and references)

The recent outbreak of monkeypox and the ongoing COVID-19 pandemic emphasize the importance of studying infectious diseases as one of the major threats of human health. While molecular diagnostics is sensitive, specific and efficient in the diagnosis of viruses, immunohistochemistry and in-situ hybridization techniques allow the localization of virus protein or nucleic acids in patient samples by using special probes at the light microscopy level and its correlation with classical histopathology. However, all of these methods only detect either molecules of viruses or effects of virus infection and not the intact virus itself which is the infectious unit. Electron microscopy (EM) is the only technique which is capable to directly identify virus particles and their precursor structures in patient samples by using their typical morphological features (size, shape, special surface pattern) [1]. The reliable identification of viruses in patient samples is important to discriminate between direct and indirect effects caused by the infection and, thus, for the understanding of the pathogenesis of the disease, which, at the end, is the basis for a better treatment of patients.

Thin section EM of patient samples (e.g. from biopsies or autopsies) offer the possibility to localize virus particles in the cellular context of the affected tissue and to correlate it with (histo-) pathological findings. However, investigation of the complex ultrastructure of human tissue and identification of virus particles need experience which seems to vanish as the many mis-interpretations of putative coronavirus particles published during the last years demonstrate [2, 3]. Here, I present the strategies (e.g. sampling, sample preparation for EM, microscopy) which we have applied to study the ultrastructural pathology of coronavirus and monkeypox virus infection in patient material (see Figure) and what we have learned from the results [e.g. 4, 5].

Figure Caption:

(A) Mature monkeypox virus particles in the cytoplasm of a fibroblast in the lamina propria mucosae of the colon. Different section profiles of the complex mature virus particles are seen, e.g. a side view (arrowhead) showing the dumbbell-shaped core and a view in top-down direction (arrow) which reveals the brick-shaped form of the virus particle. (B) Part of an isolated naso-pharyngeal ciliated mucosa cell which is regularly found in extracted swab material used for PCR-testing. Coronavirus particles are seen in the extracellular space (arrows) among the base of the cilia and within membrane-bound compartments (arrowheads) of the cytoplasm. Scale bars = 200 nm.

References:

[1] Miller (1986) Journal of Electron Microscopy Technique (doi.org/1002/jemt.1060040305)

[2] Dittmayer et al. (2020) The Lancet 20 (doi.org/10.1016/ S0140­6736(20)32079­1)

[3] Krasemann et al. (2022) eBioMedicine (doi.org/10.1016/j. ebiom.2022.104193)

[4] Meinhardt et al. (2021) Nature Neuroscience (doi.org/10.1038/s41593-020-00758-5)

[5] Cortese et al. (2022) Virchows Archive (doi.org/10.1007/s00428-022-03308-5)