Figure 6-16 Dark-field microscopy. Principle (A) and dark-field photomicrograph showing the tightly coiled characteristics of the spirochete Treponema pallidum (B). (From Atlas RM: Principles of microbiology, St Louis, 1995, Mosby.)
As discussed in Chapter 3, antibodies are molecules that have high specificity for interacting with microbial antigens. That is, antibodies specific for an antigen characteristic of a particular microbial spedes will only combine with that antigen. Therefore, if antibodies are conjugated (chemically linked) to a fluorescent dye, the resulting dye-antibody conjugate can be used to detect, or "tag," specific microbial agents (see Figure 6-12). When "tagged," the microorganisms become readily detectable by fluorescent microscopy. Thus, immunofluorescence combines the amplified contrast provided by fluorescence with the specifidty of antibody-antigen binding.
This method is used to directly examine patient specimens for bacteria that are difficult or slow to grow (e.g., Legionella spp., Bordetella pertussis, and Chlamydia trachomatis) or to identify organisms already grown in culture. FITC, which emits an intense, apple green fluorescence, is the fluorochrome most commonly used for conjugation to antibodies (Figure 6-15). Immunofluorescence is also used in virology (Chapter 51) and to some extent in parasitology (Chapter 49).
TWo additional types of microscopy, dark-field microscopy and electron microscopy, are not commonly used to diagnose infectious diseases. However, because of their importance in the detection and characterization of certain microorganisms, they are discussed here.
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