Which organism is typically identified by a positive optochin sensitivity test?

Which organism is typically identified by a positive optochin sensitivity test?

• A. Enterococcus faecalis
• B. Streptococcus pneumoniae
• C. Staphylococcus aureus
• D. Streptococcus pyogenes

Answer: (B)

Optochin sensitivity testing distinguishes Streptococcus pneumoniae from other alpha-hemolytic streptococci by showing inhibition of growth when exposed to the optochin compound. Streptococcus pneumoniae is inhibited by optochin, so on a blood agar plate read after incubation, you see a clear zone of no growth around the optochin disk. This positive result is the hallmark that helps identify this organism in the lab. The other organisms listed—Enterococcus faecalis, Staphylococcus aureus, and Streptococcus pyogenes—do not show this inhibition with optochin under standard testing conditions, so they typically do not produce a positive optochin test. In practice, a positive result is often paired with other characteristics (such as bile solubility and colony morphology) to confirm Streptococcus pneumoniae, since rare strains may behave differently.

Optochin sensitivity testing distinguishes Streptococcus pneumoniae from other alpha-hemolytic streptococci by showing inhibition of growth when exposed to the optochin compound. Streptococcus pneumoniae is inhibited by optochin, so on a blood agar plate read after incubation, you see a clear zone of no growth around the optochin disk. This positive result is the hallmark that helps identify this organism in the lab. The other organisms listed—Enterococcus faecalis, Staphylococcus aureus, and Streptococcus pyogenes—do not show this inhibition with optochin under standard testing conditions, so they typically do not produce a positive optochin test. In practice, a positive result is often paired with other characteristics (such as bile solubility and colony morphology) to confirm Streptococcus pneumoniae, since rare strains may behave differently.