A 60-year-old diabetic presents with bacterial meningitis. CSF analysis: turbid, WBC 2800 cells/µL (95% neutrophils), protein 280 mg/dL, glucose 22 mg/dL (simultaneous blood glucose 120 mg/dL). Gram stain shows gram-positive lancet-shaped diplococci. An automated blood culture system flags positive at 12 hours. Which combination correctly describes the MOST important virulence factor of this organism and the optimal antibiotic choice for penicillin-susceptible strains?

• A Capsular polysaccharide is the primary antiphagocytic virulence factor of S. pneumoniae; for penicillin-susceptible pneumococcal meningitis (MIC ≤0.06 µg/mL), high-dose IV penicillin G (or ceftriaxone) is the drug of choice; empirically vancomycin + ceftriaxone is used pending susceptibility ✓
• B Capsular polysaccharide is the primary virulence factor; vancomycin + ceftriaxone covers all strains including penicillin-susceptible
• C M-protein is the primary virulence factor preventing phagocytosis; penicillin G IV is the drug of choice for susceptible Streptococcus pneumoniae meningitis
• D Autolysin (LytA) is the primary virulence factor; meropenem monotherapy is preferred for CNS infection as it achieves higher CSF penetration than penicillin

Explanation

Streptococcus pneumoniae (gram-positive lancet diplococci) meningitis: the capsular polysaccharide is the primary virulence factor inhibiting phagocytosis. Given rising penicillin resistance (through PBP mutations), empirical therapy for bacterial meningitis must include vancomycin + ceftriaxone (± dexamethasone) until susceptibility is known. For confirmed penicillin-susceptible S. pneumoniae (MIC ≤0.06 µg/mL), high-dose penicillin G IV (or ceftriaxone) can be de-escalated. M-protein is the virulence factor of Streptococcus pyogenes (Group A), not S. pneumoniae. Meropenem is reserved for beta-lactam-resistant strains or when standard therapy fails.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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