A 35-year-old man from rural Maharashtra presents with fever, productive cough, and pleuritic chest pain for 5 days. Chest X-ray shows left lower lobe consolidation. Blood culture grows a gram-positive coccus in pairs (lancet-shaped). Which feature best distinguishes this organism from Streptococcus agalactiae (Group B Streptococcus) as the causative agent?

Explanation

## Distinguishing S. pneumoniae from S. agalactiae (GBS) **Key Point:** The Quellung reaction (capsular swelling) and optochin sensitivity are the definitive tests that identify S. pneumoniae and distinguish it from S. agalactiae, which lacks a polysaccharide capsule and is optochin-resistant. ### Clinical Presentation Context This patient has community-acquired pneumonia (CAP) with lobar consolidation — a classic presentation for S. pneumoniae. The lancet-shaped gram-positive cocci in pairs is the morphological clue. ### Differential Laboratory Features | Feature | S. pneumoniae | S. agalactiae (GBS) | |---------|---------------|--------------------| | **Quellung reaction** | Positive (capsular swelling) | Negative | | **Optochin sensitivity** | Sensitive | Resistant | | **Bile solubility** | Soluble | Insoluble | | **CAMP test** | Negative | Positive | | **Growth in 6.5% NaCl** | No growth | Grows (halotolerant) | | **PYR test** | Negative | Positive | | **Hippurate hydrolysis** | Negative | Positive | | **Hemolysis** | Alpha (greenish) | Beta (clear) | | **Clinical niche** | Respiratory, CNS, invasive | Neonatal, maternal genital | **High-Yield:** The **Quellung reaction** is pathognomonic for S. pneumoniae. When specific antisera are added, the capsule swells visibly under the microscope — a phenomenon unique to pneumococci among streptococci. ### Why Optochin Sensitivity Matters **Clinical Pearl:** Optochin inhibits pneumococcal autolysins (pneumolysin-related), causing organism lysis. GBS lacks this susceptibility. In the context of CAP with lobar consolidation, a positive Quellung + optochin sensitivity confirms S. pneumoniae and guides empiric therapy (high-dose penicillin or cephalosporin for pneumonia vs. ampicillin for GBS neonatal sepsis). **Mnemonic: PNEUMO** — **P**ositive Quellung, **N**egative PYR, **E**xtracellular polysaccharide capsule, **U**nique optochin sensitivity, **M**orphology lancet-shaped, **O**ther streptococci lack these [cite:Textbook of Microbiology Ananthanarayan & Paniker 10e Ch 18]