A 22-year-old female college student presents with acute meningitis. CSF shows pleocytosis with neutrophil predominance, elevated protein, and low glucose. Gram stain reveals gram-negative diplococci. All of the following are characteristic features of Neisseria meningitidis EXCEPT:

Explanation

## Neisseria meningitidis: Identification and Virulence ### Laboratory Identification **Key Point:** N. meningitidis is oxidase-positive and ferments both glucose AND maltose — this fermentation pattern is the key biochemical test that distinguishes it from N. gonorrhoeae (glucose only). | Feature | N. meningitidis | N. gonorrhoeae | |---|---|---| | **Oxidase** | Positive | Positive | | **Glucose fermentation** | Yes | Yes | | **Maltose fermentation** | **Yes** | **No** | | **Capsule** | Polysaccharide (serogroups A, B, C, W, Y) | Present but poorly immunogenic | | **LPS in outer membrane** | **Yes, present** | Yes, present | | **IgA protease** | Yes | Yes | | **Gram stain morphology** | Gram-negative diplococci (kidney/coffee bean shape) | Gram-negative diplococci | ### Virulence Factors of N. meningitidis **High-Yield:** N. meningitidis possesses **both LPS endotoxin AND a polysaccharide capsule**. The capsule is the primary virulence factor and is the basis for all meningococcal vaccines (MenACWY, MenB). **Mnemonic:** **CLAP** — **C**apsule (primary virulence), **L**PS (endotoxin), **A**dhesins (pili), **P**rotease (IgA) = virulence factors of meningitidis. ### Why Option 4 Is Wrong **Warning:** This is a classic **trap answer**. N. meningitidis **DOES possess LPS** in its outer membrane. The question states "lacks lipopolysaccharide" — this is FALSE. Both Neisseria species are gram-negative and contain LPS, which is responsible for the inflammatory cascade in meningitis. **Clinical Pearl:** The LPS of N. meningitidis triggers massive TNF-α and IL-1β release, leading to: - Blood-brain barrier disruption - Cerebral edema - Seizures - Shock (in fulminant meningococcemia) The polysaccharide capsule provides serum resistance and is poorly immunogenic in serogroup B (hence the need for protein-conjugate vaccines). ### Correct Features of N. meningitidis 1. **Oxidase-positive** — purple colonies on oxidase reagent 2. **Mucoid colonies** — due to polysaccharide capsule 3. **Ferments glucose AND maltose** — distinguishes from gonorrhoeae 4. **Capsule is primary virulence factor** — target of conjugate vaccines (MenACWY, MenB) 5. **Produces IgA protease** — immune evasion 6. **Possesses LPS endotoxin** — triggers inflammatory cascade [cite:Jawetz, Melnick & Adelberg's Medical Microbiology 28e Ch 18; Harrison 21e Ch 137]