To determine if alpha-hemolytic cocci from sputum are Streptococcus pneumoniae, which test should be performed?

To determine if alpha-hemolytic cocci from sputum are Streptococcus pneumoniae, which test should be performed?

• A. Optochin susceptibility
• B. Bacitracin susceptibility
• C. Coagulase test
• D. Catalase test

Answer: (A)

Differentiating Streptococcus pneumoniae from other alpha-hemolytic (viridans) cocci relies on a test that specifically distinguishes this species. Optochin susceptibility is the classic discriminator: Streptococcus pneumoniae is inhibited by the optochin disk, producing a zone of inhibition, whereas viridans streptococci are resistant and show no inhibition. A positive optochin test thus supports identification of S. pneumoniae in sputum isolates. Additional supportive clues include bile solubility (S. pneumoniae is bile soluble), but that test isn’t always necessary. The other tests listed aren’t useful for this purpose: bacitracin susceptibility identifies certain beta-hemolytic groups (like Group A Strep), coagulase distinguishes Staphylococcus aureus from other staphylococci, and catalase differentiates Staphylococcus from Streptococcus.

Differentiating Streptococcus pneumoniae from other alpha-hemolytic (viridans) cocci relies on a test that specifically distinguishes this species. Optochin susceptibility is the classic discriminator: Streptococcus pneumoniae is inhibited by the optochin disk, producing a zone of inhibition, whereas viridans streptococci are resistant and show no inhibition. A positive optochin test thus supports identification of S. pneumoniae in sputum isolates. Additional supportive clues include bile solubility (S. pneumoniae is bile soluble), but that test isn’t always necessary. The other tests listed aren’t useful for this purpose: bacitracin susceptibility identifies certain beta-hemolytic groups (like Group A Strep), coagulase distinguishes Staphylococcus aureus from other staphylococci, and catalase differentiates Staphylococcus from Streptococcus.