Which organism is used to differentiate pneumococcus by optochin sensitivity?

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The test relies on optochin sensitivity to tell Streptococcus pneumoniae from the other alpha-hemolytic streptococci. Streptococcus pneumoniae is inhibited by optochin, so when an optochin disk is placed on a culture of alpha-hemolytic colonies, a zone of inhibition around the disk indicates pneumococcus. Viridans streptococci, by contrast, are resistant and show little or no zone. This makes optochin sensitivity a practical quick identifier for pneumococcus. In practice, you can corroborate with additional clues the organism is the pneumococcus—such as bile solubility, and the characteristic lancet-shaped Gram-positive diplococci that are catalase-negative and alpha-hemolytic. The other organisms listed do not display this optochin-sensitive pattern, so they aren’t differentiated this way.