OPTOCHIN DISC TEST

Objective:

To test an organism's susceptibility to the chemical, optochin. Optochin susceptibility tests the fragility of the bacterial cell membrane. This test is mainly used to differentiate between Streptococcus pneumoniae (sensitive) and other Streptococcus species (resistant)

Test Procedure

1. Pick a single pure colony with a sterile swab to inoculate a SBA plate. Streak the entire blood agar plate with the swab. Turn plate 90 degrees and re-streak with the same swab. Blood agar plate must be used for optochin testing since all species of Streptococcus are fastidious organisms and require extra enrichment for growth.
2. With alcohol flamed forceps, aseptically remove an optochin disc and apply to the center of the plate. Gently apply pressure to disc so that it adheres to the surface of the plate but do not press disc down into the medium.
3. Invert plate and incubate for 48 hours at your organism's optimum growth temperature.

Interpretation

• Sensitive (S): A distinct zone of inhibition (5 to 30 mm) with a clear-cut margin around disc.
• Resistant (R): Growth not inhibited around disc.

Precautions

• Occasionally, a few scattered optochin resistant colonies of S. pneumoniae may be observed in a wide zone of inhibition.
• Occasionally an alpha-Streptococcus spp. may exhibit a very small zone (1 to 2 mm) of inhibition. S. pneumoniae exhibits a zone of inhibition at least 5 mm or greater in diameter.