For certain infectious diseases, this process may also include measuring the patient's immune response to the infectious agent.
Microscopy is the most common method used both for the detection of microorganisms directly in clinical specimens and for the characterization of organisms grown in culture (Box 6-1). Microscopy is defined as the use of a microscope to magnify (i.e., visually enlarge) objects too small to be visualized with the naked eye so that their characteristics are readily observable. Because most infectious agents cannot be detected with the unaided eye, microscopy plays a pivotal role in the laboratory. Microscopes and microscopic methods vary, but only those of primary use in diagnostic microbiology are discussed.
The method used to process patient specimens is dictated by the type and body source of specimen (see Part VH). Regardless of the method used, some portion of the specimen usually is reserved for microscopic examination. Specific stains or dyes applied to the specimens, combined with particular methods of microscopy, can detect etiologic agents in a rapid, relatively inexpensive, and productive way. Microscopy also plays a key role in the characterization of organisms that have been cultivated in the laboratory (for more information regarding cultivation of bacteria see Chapter 7).
The types of microorganisms to be detected, identified, and characterized determine the most appropriate types of microscopy to use. Table 6-1 outlines the four types of microscopy used in diagnostic microbiology and their relative utility for each of the four major microorganism groups. Bright-field microscopy (also known as light microscopy) and fluorescence microscopy have the widest use and application. Which microorganisms can be detected or identified by each microscopic method also depends on the methods used to highlight the microorganisms and their key characteristics. This enhancement is usually achieved using various dyes or stains.
BRIGHT-FIELD (LIGHT) MICROSCOPY
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