Streptococcus pneumoniae is the commonest cause of community-acquired pneumonia. On blood agar it shows alpha-haemolysis and is bile-soluble and optochin-sensitive. In cerebrospinal fluid from bacterial meningitis, which rapid bedside test confirms pneumococcal aetiology?

• A Quellung (capsular swelling) reaction using type-specific antisera ✓
• B Gram stain showing Gram-positive diplococci
• C India ink preparation
• D Weil-Felix reaction

Explanation

The Quellung reaction (neufeld capsular swelling test) uses type-specific pneumococcal antisera: when the specific antibody binds the polysaccharide capsule, it appears swollen/refractile under phase contrast microscopy, allowing rapid typing and confirmation. This is both specific and useful for direct identification in CSF. The Gram stain showing Gram-positive lancet-shaped diplococci in CSF is also highly suggestive but less specific than Quellung. India ink is for Cryptococcus. Weil-Felix is an agglutination test for Rickettsia (cross-reaction with Proteus antigens).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.