Wikipedia
Optochin (or ethylhydrocupreine hydrochloride) is a derivative of hydroquinine introduced in 1911 by Morgenroth and Levy with the intention to treat pneumococci infection. In very high dilutions inhibits the growth of representatives of all four groups of pneumococci in vitro. That is the main reason it is now used in cell culture techniques for the presumptive identification of Streptococcus pneumoniae, which is optochin-sensitive, from other alpha-hemolytic streptococci such as viridans streptococci, which are resistant. A common mnemonic is "OVRPS" (Optochin - Viridans [are] Resistant, Pneumoniae [are] Sensitive). Or "VROPS" (Viridans Resistant [to] Optochin, Pneumoniae Sensitive)
The growth of bacteria that are optochin-sensitive will show a zone of inhibition around an optochin disc, while the growth of bacteria that are optochin-resistant will not be affected. In vitro, an minimum inhibitory concentration (MIC) of 1:10,000,000 will affect growth of pneumococcus, and 1:500,000 is bactericidal.