NEISSERIA: GENERAL FEATURES
Neisseria are Gram-negative cocci that typically appear in pairs with the opposing sidesflattened, imparting a “kidney bean” appearance. They are nonmotile, non-spore form-ing, and non-acid fast. Their cell walls are typical of Gram-negative bacteria, with a pep-tidoglycan layer and an outer membrane containing endotoxic glycolipid complexed with 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.
Gonococci and meningococci require an aerobic atmosphere with added carbon diox-ide 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 carbohy-drate fermentation. Reagents are also available to distinguish N. gonorrhoeae and N. meningitidis from the other Neisseria by immunologic methods such as slide aggluti-nation and immunofluorescence.
Both pathogenic species possess pili, which vary in their antigenic composition, and several classes of outer membrane proteins (OMPs), which also vary antigenically. Vari-ous classes of the pili and OMPs of gonococci and meningococci have been separately named, but the structure and functional features of some are similar to each other and to diverse pathogens such as Pseudomonas aeruginosa and Bacteroides (Table 20 – 1). The outer membrane of pathogenic Neisseria contains a variant of lipopolysaccharide (LPS) in which the side chains are shorter and lack the repeating polysaccharide units found in the LPS of most other Gram-negative bacteria. This short chain neisserial LPS is called lipooligosaccharide (LOS). The lipid A and core oligosaccharide are structurally and functionally similar to other Gram-negative LPS. The pili, OMPs, and LOS are antigenic and have been used in typing schemes.