Paper I
Question
Enumerate at least four differences between streptococcus pneumoniae and streptococcus viridians.
Answer
| Feature | Streptococcus pneumoniae | Streptococcus viridans (viridans group) |
|---|---|---|
| Haemolysis on blood agar | Alpha-haemolytic (green discolouration), same as viridans — distinguished by further tests below | Alpha-haemolytic (green discolouration) |
| Colony morphology | Colonies often show central “nipple”/umbonation (autolysis) with a raised, then depressed centre | Small, smooth, glistening colonies, no characteristic umbonation |
| Optochin sensitivity | Sensitive (zone of inhibition around optochin disc) | Resistant (no zone of inhibition) — key distinguishing test |
| Bile solubility test | Positive (colonies lyse/dissolve in the presence of bile salts, due to autolysin activation) | Negative (not lysed by bile) |
| Capsule | Prominent polysaccharide capsule (basis of serotyping and the pneumococcal vaccine) | Generally non-capsulated or minimal capsule |
| Quellung reaction | Positive (capsular swelling with type-specific antiserum) | Negative |
| Clinical significance | Major cause of community-acquired pneumonia, meningitis, otitis media, sinusitis | Normal oral/dental commensal flora; important cause of subacute infective endocarditis (especially after dental procedures in patients with pre-existing valvular disease) |
| Morphology on Gram stain | Lancet-shaped (flame-shaped) diplococci | Cocci in chains |
The two most decisive routine laboratory tests distinguishing them are optochin sensitivity (pneumococcus sensitive, viridans resistant) and the bile solubility test (pneumococcus positive, viridans negative).