A 32-year-old woman presents with acute purulent meningitis. CSF analysis shows pleocytosis with predominant neutrophils. Gram stain of CSF reveals gram-negative diplococci. Which is the most common causative organism in this clinical presentation?

Explanation

## Gram-Negative Diplococci in Meningitis **Key Point:** Gram-negative diplococci in CSF with acute meningitis is pathognomonic for *Neisseria meningitidis* (meningococcus). ### Morphology & Staining - Gram-negative diplococci with characteristic kidney bean or coffee bean shape - Intracellular location within neutrophils on Gram stain - Oxidase-positive (purple discoloration on oxidase paper) ### Clinical Epidemiology - Most common cause of bacterial meningitis in adults and adolescents in developed countries - Peak incidence in infants < 5 years and young adults (15–24 years) - Rapid progression with petechial/purpuric rash in 50–80% of cases - High mortality (10–15%) if untreated; requires immediate empiric therapy ### Gram Stain Interpretation Table | Organism | Gram Stain Morphology | CSF Appearance | Intracellular? | |---|---|---|---| | *N. meningitidis* | Gram-negative diplococci (kidney bean) | Clear or slightly turbid | Yes (PMNs) | | *S. pneumoniae* | Gram-positive diplococci (lancet-shaped) | Turbid | Rare | | *L. monocytogenes* | Gram-positive rod (short, tumbling motility) | Clear or slightly turbid | No | | *H. influenzae* type b | Gram-negative coccobacillus | Clear or slightly turbid | Rare | **High-Yield:** The kidney bean shape and intracellular location within neutrophils are the diagnostic hallmarks on Gram stain. **Clinical Pearl:** Empiric therapy for meningitis in this age group must include ceftriaxone or cefotaxime (covers meningococci, pneumococci, and gram-negatives) plus vancomycin (for penicillin-resistant pneumococci). **Mnemonic: NEIS** — *Neisseria*, Extracellular (intracellular in PMNs), Intracellular, Gram-negative diplococci, Kidney bean shape.