Neisseria

Neisseria are Gram-negative cocci that typically appear in pairs with the opposing sides
flattened, imparting a “kidney bean” appearance. 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 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 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.
Both pathogenic species possess pili, which vary in their antigenic composition, and
several classes of outer membrane proteins (OMPs), which also vary antigenically. Various
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