Core concept: Neisseria meningitidis is a human-restricted, encapsulated, Gram-negative diplococcus that commonly colonizes the nasopharynx but can occasionally invade the bloodstream and CNS, producing meningitis, meningococcemia and fulminant septic shock. 1. Why meningococcus matters Think of N. meningitidis as: Carrier state → nasopharyngeal invasion → bacteremia → endothelial injury → meningitis / septicemia The two major invasive syndromes are: Meningococcal meningitis.