Which bacterium causes meningococcal meningitis?

The bacterium that causes meningococcal meningitis is Neisseria meningitidis.

Often called meningococcus, N. meningitidis is a Gram-negative diplococcus: its cells are round and commonly appear in pairs. It lives only in humans and may colonize the nose and throat without causing illness. In some people, it crosses into the bloodstream and reaches the meninges, the membranes surrounding the brain and spinal cord.

Meningococcal disease can produce meningitis, bloodstream infection, or both. It spreads mainly through close contact with saliva and respiratory secretions, including coughing, kissing, and sharing items that contact the mouth. Several capsular groups cause human disease; A, B, C, W, X, and Y account for most cases worldwide.

A common mix-up is assuming that N. meningitidis causes every case of bacterial meningitis. Streptococcus pneumoniae is also a major cause, while Haemophilus influenzae type b historically caused many childhood cases before widespread vaccination. The name “meningococcal” specifically identifies disease caused by N. meningitidis.