Virus detection by transmission electron microscopy: Still useful for diagnosis and a plus for biosafety

Transmission electron microscopy (TEM) is the only imaging technique allowing the direct visualization of viruses, due to its nanometer-scale resolution. Between the 1960s and 1990s, TEM contributed to the discovery of many types of viruses and served as a diagnostic tool for identifying viruses directly in biological samples, either in suspension or in sections of tissues or mammalian cells grown in vitro in contact with clinical samples. The diagnosis of viral infections improved considerably during the 1990s, with the advent of highly sensitive techniques, such as enzyme-linked immunosorbent assay (ELISA) and PCR, rendering TEM obsolete for this purpose. However, the last 20 years have demonstrated the utility of this technique in particular situations, due to its catch-all nature, making diagnosis possible through visualization of the virus, without the need of prior assumptions about the infectious agent sought. Thus, in several major outbreaks in which molecular techniques failed to identify the infectious agent, TEM provided the answer. TEM is also still occasionally used in routine diagnosis to characterize infections not diagnosed by molecular assays. It is also used to check the microbiological safety of biological products. Many biopharmaceuticals are produced in animal cells that might contain little-known, difficult-to-detect viruses. In this context, the catch-all properties of TEM make it possible to document the presence of viruses or virus-like particles in these products.