Neisseria species are gram-negative diplococci. There are two pathogenic and many commensal species. The pathogenic species are N. meningitidis (meningococcus) and N. gonorrhoeae (gonococcus). The organisms are gram-negative, bean-shaped diplococci that typically appear in pairs with the opposing sides flattened. They are readily phagocytosed by neutrophils. They are nonmotile and non-acid fast. Capsules and pili may be demonstrated.
They require an aerobic atmosphere with added carbon dioxide and enriched media for optimal growth. Gonococcus grows more slowly and is more fastidious than meningococcus, which can grow on routine sheep blood agar. Both are sensitive to cold and are killed by refrigeration. Specimens and unincubated cultures should never be refrigerated.
They are oxidase positive and species can be identified by biochemical tests. Often, identification of presumptive colonies is made simply by a combination positive oxidase reaction and serologic tests such as slide agglutination, coagglutination, precipitin reactions, or immunofluorescence staining.
Neisseria meningitidis grows on most nonselective media and after overnight incubation produces 1.5-mm diameter colonies on blood agar. Carbon dioxide enhances growth but is not required.
Most isolates produce a small capsule not visible by routine microscopy. Meningococci are divided into nine serogroups on the basis of polysaccharide capsules. The most prevalent serogroups are A, B, C, W135, and Y.
Meningococci are exclusively human parasites. They can exist as normal flora or produce acute disease. Presence of the carrier state in the upper respiratory tract is 5-15% in healthy adults. Group specific antibodies are developed by the carrier conveying immunity. Before antibodies are developed the meningococci can leave the nasopharynx and produce bacteremia, endotoxemia and meningitis. The meningococci are selective for the nonciliated columnar epithelium and damage to the ciliated cells is possibly by direct action of the meningococcal lipopolysaccharide.
Diseases caused by this organism include meningitis, meningococcemia and rash (thrombocytopenia and evolving DIC), and Waterhouse-Friderichsen Syndrome of bilateral hemorrhagic destruction of adrenal glands.
Recommended antimicrobial treatment is with penicillin, ceftriaxone or chloramphenicol. Questions often arise as to whom should receive prophylaxis. Family members and hospital employees in contact with respiratory secretions should receive prophylaxis. Attack rates for household contacts is 0.3-1%, 300-1000 times the rate in the general population (1,6). Causual contacts usually should not be treated. Rifampin must be used for sulfonamide-resistant strains or when the susceptibility is unknown. Sulfonamides should only be used if there is known susceptibility. Penicillin cannot be used because it does not penetrate uninflamed nasopharyngeal mucosa.
A quadrivalent vaccine is available that contains A, C, Y, and W135 polysaccharides. However, there is not an effective serogroup B vaccine and this remains a problem. Guidlines for vaccination are established by the CDC.
The gram stain of the skin lesion showing gram-positive cocci likely represents contamination with normal skin flora. In 1948, Hoyne and Brown reported good sensitivity with gram-staining of skin lesions on presumably profoundly ill patients (3). However, recent literature is lacking and Mandell reports variable results with this procedure (5).
The laboratory was notified after the patient had expired that the blood cultures obtained at the outside hospital were positive for N. meningitidis, serogroup Y.
Contributed by Patricia Aronica, M.D., William A. Pasculle, Sc.D., John P. Anhalt, Ph.D., M.D., and Charles A. Richert, M.D.