Optochin (P Disk) Susceptibility Test
A visual guide to the Optochin susceptibility test, key for the presumptive identification of Streptococcus pneumoniae.
Optochin plates A-B
Compare the zone of inhibition around the ethyl hydrocupreine hydrochloride (P) disk on blood agar.
Optochin-sensitive organism showing a zone of inhibition ≥14 mm. Growth is inhibited, leaving the surrounding blood agar unhemolyzed (red).
Optochin-resistant organism showing growth right up to the 6 mm disk edge. The alpha or beta hemolytic lawn extends to the disk.
Optochin-sensitive organism showing a zone of inhibition ≥14 mm. Growth is inhibited, leaving the surrounding blood agar unhemolyzed (red).
Optochin-resistant organism showing growth right up to the 6 mm disk edge. The alpha or beta hemolytic lawn extends to the disk.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Zone of Inhibition ≥14 mm | Growth is inhibited by Optochin; agar remains blood red around the disk | Susceptible (Presumptive S. pneumoniae) |
| Zone <14 mm | Questionable for S. pneumoniae; needs confirmation with a bile-solubility test | Equivocal reaction |
| No Zone of Inhibition | Organism grows up to the disk edge; growth converts blood to olive-green methemoglobin (alpha-hemolysis) | Resistant (Other viridans/alpha-strep) |
Optochin Susceptibility Interpretive Matrix
| Pattern | What to call it | How to think about it |
|---|---|---|
| Zone ≥14 mm (6 mm disk) | Susceptible | Presumptive Streptococcus pneumoniae. |
| Zone <14 mm (6 mm disk) | Equivocal | Perform bile-solubility test; some S. pneumoniae are atypical or zones are borderline. |
| No zone (growth to disk) | Resistant | Presumptive viridans Streptococcus or other alpha-hemolytic species. |
- Optochin is ethyl hydrocupreine hydrochloride, a detergent-like compound that selectively lyses pneumococci.
- A zone of inhibition ≥14 mm using a standard 6 mm disk indicates susceptibility.
- The test is specifically used to differentiate S. pneumoniae from other alpha-hemolytic viridans streptococci.
P is for Pneumoniae (and P-disk). Susceptible = Pneumoniae. Resistant = Other viridans strep.