The electron microscope uses electrons instead of light to visualize small objects and, instead of lenses, the electrons are focused by electromagnetic fields and form an image on a fluorescent screen, like a television screen. Because of the substantially increased resolution this technology allows, magnifications in excess of 100,000* compared with the lOOOx magnification provided by light microscopy are achieved.
Electron microscopes are of two general types: the transmission electron microscope (TEM) and the scanning electron microscope (SEM). TEM passes the electron beam through objects and allows visualization of internal structures. SEM uses electron beams to scan the surface of objects and provides three-dimensional views of surface structures (Figure 6-17). These microscopes are powerful research tools, and many new morphologic features of bacteria, bacterial components, fungi, viruses, and parasites have been discovered using electron microscopy. However, because an electron microscope is a major capital investment and is not needed for the laboratory diagnosis of most infectious diseases (except for certain viruses and microsporidian parasites), few laboratories employ this method.
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