Electron Microscopy Assays
Electron Microscopy (EM), specifically Transmission Electron Microscopy (TEM) has a long history of association with virology; this technique more than any other has enabled the discovery of a number of different virus species. The use of electron microscopy in biotechnology has been extremely useful, particularly when evaluating rodent cells such as CHO or NS0 which are frequently used for the production of biologics. These cell lines contain endogenous retroviruses which are expressed and can be characterized using transmission electron microscopy. The FDA guideline details where and when these techniques should be used.
Electron Microscopy Examination of Master Cell Banks (MCB)
Electron Microscopic examination of embedded and sectioned cells is the recommended technique to show cell morphology and to detect both the intra- and extracellular contamination by bacteria, fungi and particularly virus. Transmission electron microscopy is particularly useful for the detection of retroviruses and is used in parallel with infectivity and reverse transcriptase assays to determine the retrovirus risk of a production cell line. The test for cell lines has traditionally been performed by assessing 100 or 200 cell profiles for each cell line with the morphology of each cell examined and the number of intra and extracellular virus-like particles associated with each cell quantified.
Estimation of Retroviral Load in Bulk Harvest Samples
Electron Microscopy is currently the optimal system recommended by the US FDA to enumerate retroviral load in bulk harvest samples. The BioOutsource recommended method is based on electron microscopic examination of thin sections of bulk harvest samples prepared to ensure a consistent count of virus. Our reports and Certificate of Analysis provide a numerical count of retroviral-like particles per specified sample volume.
Other uses of Electron Microscopy
The electron microscope is particularly useful in the investigation of suspected viral or microbial contamination of a cell culture sample as the method is non-specific in its detection ability.
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