The optochin susceptibility test with two strains of Streptococcus pneumoniae. Strains are streaked on the surface of blood agar plate and incubated overnight with optochin disks placed on the agar medium. As penicillin and penicillin based antibiotics remain the drug of the choice for pneumococcal respiratory infections, it is useful to test for susceptibility to oxacillin at the same time as for preliminary identification of PNC-resistant strains. Notice the small zone of inhibition around the optochin disk in strain I. In this case it is necessary to perform the bile solubility test to confirm identification.
Columbia sheep blood agar. Cultivation 24 hours, aerobic atmosphere, 5% carbon dioxide, 37°C.