Quick Review Cards for Medical Laboratory Science

Tests for Identification of Alpha-Hemolytic

Streptococci

Clinical Microbiology Review 167

TEST PRINCIPLE KEY REACTIONS OTHER

Optochin disk

Bile solubility

Also known as P disk. Contains ethylhydrocupreine

hydrochloride. Placed on lawn of inoculum on SBA.

Plates incubated overnight in ↑CO 2. If zone of in-

hibition <14 mm, ID organism as S. pneumoniae

only if bile soluble.

Can be performed in broth or on colony. Pos =

clearing of broth or disappearance of colony.

Results in 30 min or less.

Zone of inhibition ≥14 mm with

6-mm disk or ≥16 mm with

10-mm disk = susceptibility to

optochin

Bile salts (e.g., sodium deoxy-

cholate) cause lysis of some

organisms

S. pneumoniaesusceptible.

Viridans strep resistant

S. pneumoniaepos