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The family Neisseriaceae includes the genera Neisseria, Kingella, Eikenella, and 32 other genera. The Neisseria species are Gram-negative cocci that usually occur in pairs (diplococci). Neisseria gonorrhoeae (gonococci) and Neisseria meningitidis (meningococci) are exclusively pathogenic for humans and typically are found associated with or inside polymorphonuclear cells (PMNs). Many other Neisseria species are normal inhabitants of the human respiratory tract, rarely if ever cause disease, and occur extracellularly. Members of the group are listed in Table 20-1.
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Gonococci and meningococci are closely related, with 70% DNA homology, and are differentiated by a few laboratory tests and specific characteristics. Meningococci have polysaccharide capsules but gonococci do not, and meningococci rarely have plasmids but most gonococci do. Most importantly, the two species are differentiated by the usual clinical presentations of the diseases they cause: Meningococci typically are found in the upper respiratory tract and cause meningitis, but gonococci cause genital infections. However, the clinical spectra of the diseases caused by gonococci and meningococci do overlap.
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Morphology and Identification
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The typical Neisseria is an aerobic, Gram-negative, nonmotile diplococcus, approximately 0.8 µm in diameter (Figures 20-1 and 20-2). Individual cocci are kidney bean shaped; when the organisms occur in pairs, the flat or concave sides are adjacent.
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The various pathogenic and nonpathogenic Neisseria species can be differentiated by their ability to grow on ...