Dark-field microscopy is similar to phase contrast microscopy in that it involves the alteration of microscopic technique rather than the use of dyes or stains to achieve contrast. By the dark-field method,
Figure 6-17 A, Transmission electron micrograph showing Escherichia coli cells internalized by a human mast cell (arrows). B, Scanning electron micrograph of E. (»//interacting with the surface of human mast cell (arrows). (A and B Courtesy SN Abraham, Washington University School of Medicine, St Louis.)
the condenser does not allow light to pass directly through the specimen but directs the light to hit the specimen at an oblique angle (Figure 6-16, A). Only light that hits objects, such as microorganisms in the specimen, will be deflected upward into the objective lens for visualization. All other light that passes through the specimen will miss the objective, thus making the background a dark field.
This method has greatest utility for detecting certain bacteria directly in patient specimens that, because of their thin dimensions, cannot be seen by light microscopy and, because of their physiology, are difficult to grow in culture. Dark-field microscopy is used to detect spirochetes, the most notorious of which is the bacterium Treponema pallidum, the causative agent of syphilis (for more information regarding spirochetes, see Chapter 48). As shown in Figure 6-16, B, spirochetes viewed using dark-field microscopy will appear extremely bright against a black field. The use of dark-field microscopy in diagnostic microbiology has substantially decreased with the advent of reliable serologic techniques for the diagnosis of syphilis.
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