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DEFINITION

Infection with Neisseria meningitidis most commonly manifests as asymptomatic colonization in the nasopharynx of healthy adolescents and adults. Invasive disease occurs rarely, usually presenting as either bacterial meningitis or meningococcal septicemia. Patients may also present with occult bacteremia, pneumonia, septic arthritis, conjunctivitis, and chronic meningococcemia.

ETIOLOGY AND MICROBIOLOGY

N. meningitidis is a gram-negative aerobic diplococcus that colonizes humans only and that causes disease after transmission to a susceptible individual. Several related neisserial organisms have been recognized, including the pathogen N. gonorrhoeae and the commensals N. lactamica, N. flavescens, N. mucosa, N. sicca, and N. subflava. N. meningitidis is a catalase- and oxidase-positive organism that utilizes glucose and maltose to produce acid.

Meningococci associated with invasive disease are usually encapsulated with polysaccharide, and the antigenic nature of the capsule determines an organism’s capsular group (serogroup) (Table 150-1). In total, 12 capsular groups have been identified (A–C, X–Z, 29E, W, H–J, and L), but just six of these—A, B, C, X, Y, and W (formerly W135)—account for the majority of cases of invasive disease. Group D is often listed as the thirteenth capsular group but has recently been identified as an unencapsulated variant of group C. Meningococci are commonly isolated from the nasopharynx in studies of carriage; the lack of capsule often is a result of phase variation of capsule expression, but as many as 16% of isolates lack the genes for capsule synthesis and assembly. These “capsule-null” meningococci and those that express capsules other than A, B, C, X, Y, and W are only rarely associated with invasive disease and are most commonly identified in the nasopharynx of asymptomatic carriers.

TABLE 150-1Structure of the Polysaccharide Capsule of Common Disease-Causing Meningococci

Beneath the capsule, meningococci are surrounded by an outer phospholipid membrane containing lipopolysaccharide (LPS, endotoxin) and multiple outer-membrane proteins (Figs. 150-1 and 150-2). Antigenic variability in porins expressed in the outer membrane defines the serotype (PorB) and serosubtype (PorA) of the organism, and structural differences in LPS determine the immunotype. Serologic methods for typing of meningococci are restricted by the limited availability of ...

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