A 35-year-old woman from Mumbai is admitted with acute meningitis. CSF analysis shows pleocytosis with predominant polymorphonuclear cells. A Gram stain of CSF is performed and shows Gram-negative diplococci with a kidney-bean appearance. The laboratory reports that the organism is oxidase-positive. However, the resident notices that the staining is somewhat uneven, with some organisms appearing more deeply stained than others. What is the most appropriate next step in confirming the identity and guiding immediate therapy?