Microscopy Solutions for Virology

Virology is an important field within microbiology and is concerned with the study of viruses and viral diseases. The impact of viruses on the population is enormous, as they can cause severe endemic or even pandemic diseases, such as the Spanish flu (H1N1 influenza virus), Aids (Human immunodeficiency virus HIV), dengue fever (dengue virus DENV), or most recently Covid-19 (Coronavirus SARS-CoV-2).
Studying viruses, their interaction with host cells, and the immune response to the virus helps to develop vaccines and treatments against viral pathogens. Therefore, virology is often strongly connected to immunology.

In clinical routine a multitude of laboratory methods are available to confirm virus infections. With the advent of molecular techniques and increased sensitivity of serological assays, virology has changed rapidly with a wide variety of samples used for virologic testing.
Many viruses can be grown in cell culture in the lab. To do this, the virologist mixes the virus sample with suitable host cells, a process called adsorption or inoculation, after which the cells become infected and produce more copies of the virus. Although some viruses require a certain type of cells for replication, there are cells that support growth of a large variety of viruses such as the African monkey kidney cell line (Vero cells), human lung fibroblasts (MRC-5), and human epidermoid carcinoma cells (HEp-2).

One sign of knowing whether the cells are successfully replicating the virus is to search for changes in cell morphology or for increased cell death (apoptosis) using an inverted microscope for cell culture applications. These induced morphological changes are referred to as cytopathic effect (CPE). Light microscopy is also a valuable tool to quickly and efficiently observe histopathological changes such as typical aggregates of virus inside cells in so-called cytoplasmic inclusion bodies. One prominent example are the Negri bodies that are larger pathognomonic cellular inclusions typically observed with an hematoxylin and eosin stain (HE stain) in various nerve cells to detect rabies infections with the lyssavirus.

Fluorescence microscopy becomes increasingly important in virology. Immunofluorescence is one method of diagnosing and quantifying certain viral infections. The advancement of fluorescence labeling methods and microscopic instrumentation opens further possibilities to perform more refined studies on host-virus interactions, virus spreading, and virus replication, e.g. through co-localization measurements between cellular compartments and virus. Increased sensitivity, better resolution, and higher automation of those microscope systems provide the basis for screening applications that allow researchers to get a wealth of information on virus infected cells, e.g. upon drug treatment.

Electron microscopy is often used to examine the ultrastructure and to identify certain viruses. In particular, the correlation between light and electron microscopy (CLEM) can provide unique insights into the interactions between virus and host.

Microscope Requirements

For observation and maintenance purposes in cell culture, inverted light microscopes with a small footprint, LED fluorescence option, good ergonomics and high-quality optics for reliable digital documentation are essential tools. Immunofluorescence opens up the rapid detection of viral agents with direct (DFA) or indirect fluorescence antibody (IFA) tests, including antibody test kits against herpes simplex (HSV), Influenza A, other respiratory viruses and enteroviruses.

Automated boxed microscopes with integrated calibration, environmental control and fluorescence options are ideal for lab environments with high throughput demands, enabling fully automated 2D and 3D screening of cell cultures and tissues. Confocal microscopes give the virologist the option to investigate the details of cellular invasion in greater detail and prepare the respective sample for further investigation with immune electron microscopy.

Recent developments in scanning electron microscopy (SEM) have shown to meet the resolution and image quality requirements for virus studies. The large field of view imaging mode in combination with correlative light microscopy and automated workflows saves valuable time in finding relevant viral spots and provides fast results, even in 3D.  

Applications

Intracellular and membrane-bound distribution of GFP-labeled cytomegaloviruses © P. Walther, Electron Microscopy Centre, University of Ulm, Germany; J. von Einem, Institute of Virology, University of Ulm Germany.
Intracellular and membrane-bound distribution of GFP-labeled cytomegaloviruses
The characteristic shape of the cells identifies them as Rotavirus cells
The characteristic shape of the cells identifies them as Rotavirus cells
Interactions of viruses with bacteria as it happens on the bacteria plate © M. Leppänen, University of Jyväskylä, Finland
Interactions of viruses with bacteria as it happens on the bacteria plate
Primary neuronal cultures stained for Bornavirus antigens and Tetanus toxin © C. Charlier and D. Dunia, Inserm U1043, CPTP, Toulouse, France
Primary neuronal cultures stained for Bornavirus antigens and Tetanus toxin
Human lung epithelial cell line A549 stained with MitoTracker™ Orange (mitochondria) and SIR-DNA (nuclei). Sample courtesy of A.C. Hocke, Charité, Berlin, Germany
HeLa Cells
HeLa Cells – Imaged with ZEISS Primovert

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