Optochin sensitivity test

• A test applied to alpha-hemolytic streptococci in order to presumptively identify Streptococcus pneumoniae, which is susceptible to optochin (although not all strains are susceptible)
• Apply an optochin or P disc to an SBA plate with freshly-streaked inoculum, then incubate at 35-37°C with 5% CO₂ for 18 to 24 hours
• Measure the zone of inhibition
  • Using a standard 6 mm disc, a zone of 14 mm or more indicates susceptibility and suggests Streptococcus pneumoniae
  • Any zone of inhibition is uninterpretable, and should be followed by a bile solubility test

Further reading

• Microbe Online. Optochin Sensitivity Test: Principle, Procedure, Results and Quality Control.