Which test differentiates Streptococcus pneumoniae from other alpha-hemolytic streptococci and is commonly used in identification?

• A. Catalase test
• B. Oxidase test
• C. Bile solubility
• D. Coagulase test

Answer: (C)

Differentiating Streptococcus pneumoniae from other alpha-hemolytic streptococci relies on a distinctive property: bile solubility. Streptococcus pneumoniae is dissolved by bile (deoxycholate) because bile salts trigger autolysis of its cells, so colonies disappear or clear in a bile-containing environment. In contrast, viridans streptococci and other alpha-hemolytic cousins resist this lysis, so no clearing occurs. This clear difference makes the bile solubility test a practical, widely used tool in identification workflows to confirm pneumococcus.

As additional context, the catalase test separates catalase-positive organisms (like staphylococci) from streptococci, but both pneumococcus and viridans are catalase-negative, so it doesn’t differentiate them. The oxidase test targets oxidase-producing organisms (often some Gram-negative bacteria), not useful for distinguishing these streptococci. The coagulase test distinguishes Staphylococcus aureus from other staphylococci, not relevant for differentiating alpha-hemolytic streptococci.