A clinical microbiology laboratory receives a sample of cerebrospinal fluid (CSF) from a 6-year-old child suspected of bacterial meningitis. The sample is slightly turbid and needs to be cultured immediately. Which of the following is the investigation of choice for primary isolation and identification of the causative organism?

Question

Accepted Answer

C. Chocolate agar plate incubated in 5–10% CO₂ at 37°C. ## Investigation of Choice for CSF Culture in Bacterial Meningitis

### Why Chocolate Agar in CO₂ is Optimal

**Key Point:** Chocolate agar (heated blood agar) is the medium of choice for CSF culture because it supports growth of the most common bacterial meningitis pathogens — *Neisseria meningitidis*, *Streptococcus pneumoniae*, and *Haemophilus influenzae*.

**High-Yield:** The three critical features:
1. **Chocolate agar** — provides X factor (hemin) and V factor (NAD), essential for fastidious organisms like *Haemophilus*
2. **5–10% CO₂ atmosphere** — required for optimal growth of *Neisseria meningitidis* and *S. pneumoniae*
3. **37°C incubation** — body temperature; supports pathogenic bacteria

### Why Each Medium Fails for CSF

| Medium | Limitation | Why Unsuitable |
|--------|-----------|----------------|
| Blood agar (plain) | Lacks X and V factors | *Haemophilus* will not grow; *Neisseria* grows poorly |
| MacConkey agar | Selective for Gram-negative enteric bacteria | Misses *S. pneumoniae* and *N. meningitidis*; inhibits Gram-positive cocci |
| Sabouraud dextrose agar | Designed for fungi | Bacteria grow poorly; not used for bacterial meningitis |

**Clinical Pearl:** CSF samples are precious and limited. A single chocolate agar plate in CO₂ is sufficient for primary isolation; blood agar may be added as a backup, but chocolate agar is the non-negotiable first choice.

**Mnemonic:** **CHO-CO** = **CHOcolate agar + CO₂** for CSF culture.

Answer

A. Sabouraud dextrose agar incubated at 25°C

Answer

B. MacConkey agar plate incubated aerobically at 37°C

Answer

D. Blood agar plate incubated in CO₂ at 37°C

Explanation

Investigation of Choice for CSF Culture in Bacterial Meningitis

Why Chocolate Agar in CO₂ is Optimal

Why Each Medium Fails for CSF

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Medium	Limitation	Why Unsuitable
Blood agar (plain)	Lacks X and V factors	Haemophilus will not grow; Neisseria grows poorly
MacConkey agar	Selective for Gram-negative enteric bacteria	Misses S. pneumoniae and N. meningitidis; inhibits Gram-positive cocci
Sabouraud dextrose agar	Designed for fungi	Bacteria grow poorly; not used for bacterial meningitis