A 32-year-old woman with acute bacterial meningitis has cerebrospinal fluid (CSF) examined by Gram stain. Gram-negative diplococci are seen. Which single feature on Gram stain best distinguishes *Neisseria meningitidis* from *Haemophilus influenzae*?

## Distinguishing Gram-Negative Diplococci in CSF ### Clinical Context Both *Neisseria meningitidis* and *Haemophilus influenzae* are common causes of bacterial meningitis and appear as Gram-negative organisms in CSF. However, their morphologies on Gram stain are distinctly different. ### Key Morphological Discriminator **Key Point:** The **shape of the diplococci** is the most reliable Gram stain feature to distinguish *N. meningitidis* from *H. influenzae*. *N. meningitidis* appears as kidney bean or coffee bean-shaped diplococci, while *H. influenzae* appears as pleomorphic coccobacilli. ### Comparative Table | Feature | *Neisseria meningitidis* | *Haemophilus influenzae* | | --- | --- | --- | | **Gram stain** | Gram-negative | Gram-negative | | **Shape** | Kidney/coffee bean-shaped diplococci | Pleomorphic coccobacilli | | **Size** | 0.6–1.0 μm | 0.3–0.4 μm (variable) | | **Arrangement** | Diplococci (pairs) | Single or short chains | | **Capsule visibility** | Not visible on Gram stain | Not visible on Gram stain | | **Oxidase test** | Positive | Negative | | **Catalase test** | Positive | Positive | ### Why This Matters **High-Yield:** In meningitis, rapid Gram stain identification is critical because empiric antibiotic therapy must be started immediately. The distinctive **kidney bean shape** of *N. meningitidis* diplococci is the fastest morphological clue on Gram stain, even before culture confirmation. **Clinical Pearl:** The kidney bean appearance of *N. meningitidis* is so characteristic that it is often called the "diagnostic" morphology for meningococcal meningitis. In contrast, *H. influenzae* is smaller and more pleomorphic (variable in shape and size), making it less distinctive on Gram stain. ### Mechanism of Shape Difference 1. *Neisseria meningitidis*: Flattened diplococci with adjacent sides flattened → kidney bean appearance 2. *Haemophilus influenzae*: Small coccobacilli with variable morphology → pleomorphic appearance **Mnemonic:** **NEISSERIA = KIDNEY BEANS, HAEMOPHILUS = TINY STICKS** — Neisseria forms distinctive kidney-shaped pairs, Haemophilus appears as small, variable rod-like forms.