Which of the following features best distinguishes Neisseria meningitidis from Neisseria gonorrhoeae?

Explanation

## Distinguishing Features of N. meningitidis vs N. gonorrhoeae **Key Point:** Fermentation of **both glucose AND maltose** is the classic biochemical feature that best distinguishes *N. meningitidis* from *N. gonorrhoeae*, which ferments glucose only. ### Comparative Table | Feature | N. meningitidis | N. gonorrhoeae | | --- | --- | --- | | **Glucose fermentation** | Yes | Yes | | **Maltose fermentation** | **Yes** | **No** | | **Lactose fermentation** | No | No | | **Capsule** | Present (polysaccharide) | Absent | | **Oxidase test** | Positive | Positive | | **Gram stain** | Gram-negative diplococcus | Gram-negative diplococcus | | **Preferred growth medium** | Chocolate agar / Blood agar | Thayer-Martin (modified) | ### Why Option A is Correct *N. meningitidis* ferments **both glucose and maltose** (producing acid without gas), whereas *N. gonorrhoeae* ferments **glucose only**. This sugar fermentation pattern is the standard laboratory test used to differentiate the two species and is explicitly highlighted in Ananthanarayan & Paniker's *Textbook of Microbiology* and Jawetz *Medical Microbiology*. ### Why the Other Options Are Wrong - **B (Oxidase-positive gram-negative diplococcus):** Both species share this property — it does NOT distinguish them. - **C (Polysaccharide capsule):** While the capsule is a major virulence factor of *N. meningitidis* and is absent in *N. gonorrhoeae*, it is **not** a routine laboratory discriminator. The question asks for the feature that **best distinguishes** the two, and in standard microbiology practice, sugar fermentation (maltose) is the definitive biochemical test used for this purpose. - **D (Thayer-Martin medium):** Both species can grow on Thayer-Martin medium; it is not exclusive to either. Chocolate agar is preferred for *N. meningitidis*, but TM is not a distinguishing feature. **High-Yield:** The mnemonic **"M-M: Meningitidis-Maltose"** — *N. meningitidis* ferments **M**altose and causes **M**eningitis; *N. gonorrhoeae* does neither. **Clinical Pearl:** In the clinical laboratory, when a gram-negative diplococcus is isolated from CSF or blood, maltose fermentation is the key test to confirm *N. meningitidis* over *N. gonorrhoeae* (Jawetz, Melnick & Adelberg's *Medical Microbiology*, 28th ed.).