Neisseria meningitidis, a strictly human pathogen, was first identified as the causative agent of bacterial meningitis, by A. Weichselbaum, in Vienna, in 1887. Neisseria meningitidis organisms are spherical or kidney-shaped bacilli commonly seen intracellularly in pairs (diplococcus) in Gram's stain. They are non-motile, non-sporulating, usually encapsulated and pilated (presence of special structures to attach to the host's cells). They are aerobic (requiring oxygen to survive) and can be found in the nasopharynx in 5 to 11% of the healthy adult population. This is known as a carrier state.
Based on the chemical and immunological specificity of the capsular polysaccharides (the outermost structure of the bacteria), at least 13 serogroups exist, of which types A, B and C are the most prevalent. Meningococci are classified based on the immunological reactivity of their capsular polysaccharide (serogroup), their outer membrane proteins (serotype) and their outer membrane protein subtype and lipopolysaccharide (immunotype).
The outer layer of N. meningitidis is protected by a capsule made of polysaccharides that protect the bacteria against phagocytosis by the host's immune system. Polysaccharides are specific to each serogroup, and essential to pathogenicity, which means that the risk of epidemic meningococcal disease may differ considerably between serogroups.
Neisseria meningitidis is unique among major causes of bacterial meningitis for its ability to cause endemic (sporadic) and epidemic disease (outbreak or cluster).