The optochin susceptibility test is used to identify Streptococcus pneumoniae and differentiate it from other alpha-hemolytic streptococci, including Streptococcus pyogenes. In this test, a zone of inhibition is observed around a paper disk containing optochin (ethylhydrocupreine hydrochloride) when the bacteria are susceptible to it. Streptococcus pyogenes is resistant to optochin, while Streptococcus pneumoniae is susceptible.