Neisseria meningitidis
Also known as: meningococcus
Neisseria meningitidis is a gram-negative diplococcus that colonizes the human nasopharynx and can cause bacterial meningitis and life-threatening meningococcemia, classically in adolescents and young adults living in close quarters.
Neisseria meningitidis (the meningococcus) is an encapsulated, gram-negative diplococcus that lives only in humans. It commonly colonizes the nasopharynx without causing symptoms and spreads person to person through respiratory droplets — which is why outbreaks cluster in college dormitories, military barracks, and other crowded settings.
When the organism invades the bloodstream, it causes two overlapping syndromes. Meningococcal meningitis presents with fever, headache, stiff neck, and photophobia. Meningococcemia — bloodstream infection — can progress within hours to a petechial or purpuric rash, septic shock, and disseminated intravascular coagulation. Its most feared complication is Waterhouse-Friderichsen syndrome: hemorrhage into both adrenal glands producing acute adrenal insufficiency and vascular collapse.
Key organism facts distinguish it on exams. Its polysaccharide capsule is the major virulence factor, and its lipooligosaccharide endotoxin drives the septic picture. In the laboratory, N. meningitidis ferments both maltose and glucose, whereas Neisseria gonorrhoeae ferments glucose only (remember: MeninGitidis — Maltose and Glucose). People with terminal complement deficiencies (C5–C9) are markedly more susceptible, vaccines target the capsular serogroups, and close contacts of cases receive antibiotic prophylaxis.
The USMLE Step 1 tests Neisseria meningitidis as a high-yield gram-negative coccus — expect vignettes featuring a young adult in a dorm with petechiae and nuchal rigidity, the maltose fermentation distinction, complement deficiency as a risk factor, and Waterhouse-Friderichsen syndrome.
Key takeaways
- Neisseria meningitidis is an encapsulated gram-negative diplococcus spread by respiratory droplets from nasopharyngeal carriers.
- It causes bacterial meningitis and meningococcemia, classically with a petechial rash, in adolescents and young adults in close quarters.
- Waterhouse-Friderichsen syndrome — bilateral adrenal hemorrhage with shock — is its most feared complication.
- It ferments maltose and glucose, distinguishing it from N. gonorrhoeae, which ferments glucose only.
- Terminal complement deficiencies (C5–C9) greatly increase susceptibility; vaccination targets the capsular serogroups.