A 42-year-old woman from Mumbai presents with fever, neck stiffness, and altered mental status for 2 days. CSF examination shows pleocytosis with 180 cells/μL (70% neutrophils), protein 120 mg/dL, and glucose 25 mg/dL (blood glucose 95 mg/dL). Gram stain of CSF shows Gram-positive diplococci. Which is the most appropriate next investigation to identify the organism and guide antimicrobial therapy?