In a child with meningitis and Gram-negative diplococci in CSF, what is the most likely etiologic agent?

• A. Haemophilus influenzae
• B. Listeria monocytogenes
• C. Moraxella catarrhalis
• D. Neisseria meningitidis

Answer: (D)

When you see Gram-negative diplococci in cerebrospinal fluid from a child with meningitis, think Neisseria meningitidis. The combination of meningitis with Gram-negative diplococci is a classic clue that points to this organism, which is a common cause of acute meningitis in school-age children and adolescents. The diplococcal appearance means two connected cocci, a hallmark morphology for Neisseria species on Gram stain, helping distinguish it from other meningitis pathogens.

In contrast, Haemophilus influenzae would appear as a Gram-negative coccobacillus and often relates to a different growth requirement (factors X and V) and to Hib vaccination status; Listeria monocytogenes is a Gram-positive rod and more typical in newborns, the elderly, or immunocompromised patients; Moraxella catarrhalis is a Gram-negative diplococcus but is primarily a respiratory pathogen and a meningitis presentation is much less common. The specific finding of Gram-negative diplococci in CSF makes Neisseria meningitidis the most likely etiologic agent.