## Gram-Negative Diplococci in Meningitis **Key Point:** Gram-negative diplococci in CSF, especially in a young adult, are pathognomonic for *Neisseria meningitidis* (meningococcus). ### Organism Identification The gram stain morphology — gram-negative, kidney bean–shaped diplococci — is diagnostic of *N. meningitidis*. This is a leading cause of bacterial meningitis in adults and adolescents. ### Drug of Choice **High-Yield:** Ceftriaxone (or cefotaxime) is the first-line agent for meningococcal meningitis. It achieves excellent CSF penetration and covers the organism reliably. | Agent | CSF Penetration | Meningococcal Coverage | Notes | | --- | --- | --- | --- | | Ceftriaxone | Excellent (inflamed meninges) | Yes, excellent | First-line; 2 g IV Q12H | | Chloramphenicol | Good | Yes | Reserved for β-lactam allergy | | Penicillin G | Moderate | Yes, but suboptimal | Lower CSF levels; not preferred | | Fluoroquinolone | Moderate | Variable | Not recommended for meningitis | **Clinical Pearl:** In meningitis, the blood–brain barrier is disrupted by inflammation, allowing better penetration of β-lactams. Ceftriaxone is preferred over penicillin G because it achieves higher CSF concentrations and covers both meningococci and *Streptococcus pneumoniae*. **Warning:** Do not use penicillin G alone in empiric meningitis — it does not reliably cover resistant pneumococci and achieves lower CSF levels than cephalosporins. [cite:Harrison 21e Ch 297]
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