A cerebrospinal fluid sample from an infant suspected of meningitis shows Gram stain; after 24 hours, small tan colonies grow on chocolate agar incubated in CO2, while sheep blood agar incubated in CO2 shows no growth. Which organism is most likely?

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

Answer: (C)

Growth requirements of fastidious bacteria and how media choice reveals them. Haemophilus influenzae needs both X factor (hemin) and V factor (NAD) to grow. Chocolate agar provides these factors by lysing red blood cells, so the organism can thrive there, especially in a CO2-enriched atmosphere. The small tan colonies that appear after 24 hours on chocolate agar in CO2, along with no growth on sheep blood agar, fit this pattern perfectly because blood agar lacks the free X and V factors that Haemophilus influenzae requires. In infants, Hib is a well-known cause of meningitis, so this growth pattern in the CSF context strongly points to Haemophilus influenzae.

Other organisms don’t match this combination as neatly. Neisseria meningitidis would often grow on blood agar as well, not just chocolate, and it isn’t restricted to requiring X and V factors. Bordetella parapertussis and Brucella canis don’t typically present with this media-specific growth pattern or with infant meningitis in the CSF in the same way Haemophilus influenzae does.