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A: Neisseria is the best-known and most intensely studied genus of the phylum Proteobacteria, which contains many gram-negative species important in medicine. Members of this genus are nonmotile, aerobic (require oxygen), gram-negative (have a cell wall composed of a thin layer of peptidoglycan) cocci (more or less spherical in shape; approximately 0.6—1.0 µm in diameter) that most often occur in pairs with adjacent sides flattened. Neisseria may have capsules and filamentous appendages (fimbriae). They inhabit the mucous membranes of mammals, and some species, such as N. gonorrhoeae and N. meningitides are human pathogens.

Neisseria gonorrhoeae causes gonorrhea, a sexually transmitted disease that originates from humans, who are the only natural hosts. It is the most common reportable communicable disease in the United States. Anatomical sites that can be infected directly by the bacteria include the urethra, rectum, conjunctiva, pharynx, and cervix. Local complications include inflammation of these sites. The "gold standard" for diagnosis of gonorrhea is the isolation of the bacteria by culture, which allows testing of isolates for antimicrobial resistance.

Neisseria meningitides is confined entirely to humans and can cause a variety of infections; bacteremia (a blood infection) and meningitis (infection of the membranes surrounding the brain) are by far the most common. Since members of the species live and originate from the nasopharynx, they are transmitted from person to person through the inhalation of droplets of infected nasal secretions and by direct or indirect oral contact. Diagnosis is made by isolating the bacteria from blood and cerebral spinal fluid and by using specific culture media and observing the media for biochemical changes.

John P. Harley