Streptococcus pneumoniae is usually very susceptible to optochin.
Optochin = ethyl hydrocupreine hydrochloride (a quinine derivative)
Optochin sensitivity is associated with an H+ ATPase.
Testing media: trypticase soy (TSA) sheep blood agar
Optochin on impregnated disc: 5 micrograms
Incubation: 18-24 hours at 35°C
Zone of growth inhibition associated with Streptococcus pneumoniae: >= 14 mm around a 6 mm disc or >= 16 mm around a 10 mm disc
If the zone of inhibition is too small then additional testing is required (bile solubility, other).
Causes of false negative test:
(1) optochin resistant Streptococcus pneumoniae
(2) optochin degradation on the disc
(3) failure to place the optochin disc properly to ensure diffusion into agar
(4) use of Columbia or Mueller Hinton agar
Causes of false positive test:
(1) optochin sensitive strain of viridans Streptococcus