The optochin susceptibility test with two strains of Streptococcus pneumoniae. The optochin test enables the differentiation of pneumococci from other alpha-hemolytic streptococci and lactobacilli. In the picture you can see an avirulent form of S.pneumoniae (upper, pneumococci in R phase) and its virulent form (down, M phase). Growth of both strains is inhibited by optochin (ethylhydrocupreine hydrochloride) around the disks impregnated by the compound. A zone of inhibition of ≥ 15 mm around the disk (6 mm) is considered a susceptible result.
Columbia sheep blood agar. Cultivation 24 hours, aerobic atmosphere, 5% carbon dioxide, 37°C.