Which organism is a Gram-negative diplococcus commonly associated with meningitis?

• A. Escherichia coli
• B. Neisseria meningitidis
• C. Haemophilus influenzae
• D. Streptococcus pneumoniae

Answer: (B)

Neisseria meningitidis is the Gram-negative diplococcus most classically linked to meningitis. This organism is a Gram-negative diplococcus that often causes meningitis—and can also cause meningococcemia with a characteristic petechial rash—so it’s a key pathogen to recognize in meningitis cases. On a CSF smear, you frequently see intracellular Gram-negative diplococci inside neutrophils. It spreads through respiratory droplets and has a polysaccharide capsule that drives virulence and is the target of vaccines. It grows on chocolate or Thayer-Martin media and is oxidase-positive. Clinically, it’s an emergency requiring prompt antibiotic therapy and chemoprophylaxis for close contacts. By contrast, the other organisms listed are not Gram-negative diplococci causing meningitis: E. coli is a Gram-negative rod, Haemophilus influenzae is a Gram-negative coccobacillus, and Streptococcus pneumoniae is a Gram-positive diplococcus.