Neisseria meningitidis (meningococcus) is a gram negative diplococcus. It is carried in the nasopharynx of around 10% of the adult population of the UK.
At least 13 serogroups of N. meningitidis are known, differing in the antigenic composition of the polysaccharide capsule. Of these, types A, B, C cause most meningococcal infections. Since the introduction of the Haemophilus influenza type B and universal meningitis C vaccinations in 1992 and 1999 respectively, meningitis B has been the most common cause of bacterial meningitis in the UK.
N. meningitidis invades a susceptable host through the nasopharyngeal mucosa, initially causing respiratory inflammation and fever. Once established, it spreads to the meninges (causing meningococcal meningitis) or rapidly replicates in the bloodstream (causing meningococcal septicaemia) or varying degrees of both. Meningococci shed large amounts of gram –ve bacterial endotoxin (bacterial surface lipopolysaccharide) as infection progresses causing intense inflammation and signs of meningitis and/or septic shock.
Vascular inflammation causes small vessel rupture and bleeding into the skin which appears as the classic non-blanching petechial/purpuric rash. Gram stain of either needle aspirates or punch biopsy of petechiae can be positive for meningococcus.
The figure shows Neisseria meningitidis plated onto blood agar.