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The genus Neisseria contains the two Gram-negative cocci which are established human pathogens The genus also contains many commensal species, most of which are harmless inhabitants of the upper respiratory and alimentary tracts. The pathogenic species are Neisseria meningitidis (meningococcus), a major cause of meningitis and bacteremia, and Neisseria gonorrhoeae (gonococcus), the cause of gonorrhea.
NEISSERIA: GENERAL FEATURES
Neisseria typically appear in pairs (diplococcic) with the opposing sides flattened, imparting a “kidney bean” appearance (Figure 30–1). They are nonmotile, non–spore-forming, and non–acid-fast. Their cell walls are typical of Gram-negative bacteria, with a peptidoglycan layer and an outer membrane containing polysaccharides complexed with lipid and protein. The structural elements of N meningitidis and N gonorrhoeae are the same, except that the meningococcus has a polysaccharide capsule external to the cell wall.
Neisseria gonorrhoeae. Gram stain of urethral exudate. Note the many pairs of Gram-negative bean-shaped diplococci (arrow) collected in polymorphonuclear neutrophils (PMNs) and free in the purulent material. The morphology of N meningitidis and other Neisseria is identical. (Image contributed by Professor Shirley Lowe, University of California, San Francisco School of Medicine, with permission.)
Gram-negative diplococci are bean shaped
Gonococci and meningococci require an aerobic atmosphere with added carbon dioxide and enriched medium for optimal growth. Gonococci grow more slowly and are more fastidious than meningococci, which can grow on routine blood agar. All Neisseria are oxidase-positive. Species are defined by growth characteristics and patterns of carbohydrate fermentation. Reagents are also available to distinguish N gonorrhoeae and N meningitidis from the other Neisseria by immunologic methods such as slide agglutination and immunofluorescence.
Gonococci are more fastidious than meningococci
All Neisseria are oxidase-positive
Both pathogenic species possess pili and outer membrane proteins (OMPs), which vary in their function and antigenic composition. In the study of these meningococcal and gonococcal proteins, investigators have assigned names for molecules which appear to have similar functions in pathogenesis. Table 30–1 is an attempt to show similarities and differences. It should be understood that the assignment of the same name (eg, PorA) to a protein found in both species does not mean they are identical. It does suggest that they have similar structure and function.
TABLE 30–1Bacteriologic and Pathogenic Features of Neisseria |Favorite Table|Download (.pdf) TABLE 30–1 Bacteriologic and Pathogenic Features of Neisseria
| || || || ||ANTIGENIC STRUCTURE || || || |
| ||GROWTH || || ||OUTER MEMBRANE PROTEINS || |
|ORGANISM ||BLOOD AGAR ||ML AGARa ||CAPSULE ||PILI ||ADHERENCE ASSOCIATED ||PORINS ||BLOCKING AB ASSOCIATEDb ||TRANSMISSION ||DISEASE |
|N meningitidis ||+ ||+ ||Polysaccharide (12 serogroups)c ||Class I,d II Antigenically diverse ||Class 5 (4 variants) ||PorA, PorBe ||Class ...|