Which test differentiates Streptococcus pneumoniae from Streptococcus viridans?

Which test differentiates Streptococcus pneumoniae from Streptococcus viridans?

• A. Optochin sensitivity
• B. Oxidase activity
• C. Bacitracin sensitivity
• D. Catalase activity

Answer: (A)

Optochin sensitivity is the test that differentiates these two. The pneumococcus is inhibited by optochin, producing a zone of inhibition around the optochin disk on a blood agar plate, whereas viridans streptococci grow right up to the disk (are optochin resistant). This simple disk-diffusion difference lets you separate S. pneumoniae from S. viridans in the lab. Other notes that help context: S. pneumoniae is typically alpha-hemolytic and bile soluble, with lancet-shaped diplococci, while viridans are usually bile insoluble and not inhibited by optochin. The other tests mentioned aren’t useful for distinguishing these two: oxidase and catalase tests are not discriminatory for streptococci (they’re generally negative), and bacitracin sensitivity mainly separates S. pyogenes from viridans, not pneumococcus from viridans.

Optochin sensitivity is the test that differentiates these two. The pneumococcus is inhibited by optochin, producing a zone of inhibition around the optochin disk on a blood agar plate, whereas viridans streptococci grow right up to the disk (are optochin resistant). This simple disk-diffusion difference lets you separate S. pneumoniae from S. viridans in the lab.

Other notes that help context: S. pneumoniae is typically alpha-hemolytic and bile soluble, with lancet-shaped diplococci, while viridans are usually bile insoluble and not inhibited by optochin. The other tests mentioned aren’t useful for distinguishing these two: oxidase and catalase tests are not discriminatory for streptococci (they’re generally negative), and bacitracin sensitivity mainly separates S. pyogenes from viridans, not pneumococcus from viridans.