A Gram stain of cerebrospinal fluid from a 1-year-old girl with suspected meningitis shows many white blood cells. After 24 hours, small tan colonies grow on chocolate agar incubated in CO2, but there is no growth on sheep blood agar. Which organism is most likely?

• A. Bordetella parapertussis
• B. Haemophilus influenzae
• C. Neisseria meningitidis
• D. Brucella canis

Answer: (B)

The main idea is growth requirements of a fastidious meningitis pathogen. Haemophilus influenzae is a small gram-negative coccobacillus that needs X (hemin) and V (NAD) factors to grow. Chocolate agar provides both factors by lysing red cells, so the organism can grow there, especially in a CO2-rich environment. It will not grow on ordinary sheep blood agar because that medium lacks the free X and V factors those bacteria need. In a young child with meningitis, this combination of clinical context and specific growth pattern points to Haemophilus influenzae.

Bordetella parapertussis is a respiratory pathogen typically cultured on Bordet-Gengou or Regan-Lowe media and not a classic cause of meningitis with this growth pattern. Brucella canis is a zoonotic organism not characteristic for pediatric meningitis and has different culture cues. Neisseria meningitidis can grow on chocolate agar and SBA, so the isolation pattern here—growth on chocolate but not on SBA—is more characteristic of Haemophilus influenzae.