Alpha haemolytic streptococci species and Streptococcus pneumoniae cannot be easily distinguished when cultured on Blood Agar plates as pneumococci strain shows partial clearing of blood resulting to a greenish discolouration (alpha-hemolysis).
Optochin or ethylhydrocupreine is a chemical substance used in culture media for the differentiation of Streptococcus pneumoniae, which is optochin sensitive from other alpha hemolytic streptococci species such as Streptococcus viridans which is optochin resistant.Hence optochin test is a useful diagnostic aid for identification and differentiation of pneumococci and Streptococcus viridans
Principle of Optochin Test(optochin mechanism of action)
- The optochin test is based on the detection of those organisms susceptible to optochin ethylhydrocupreine hydrochloride.
- This chemical test the fragility of bacteria cell membrane which is lyse due to changes in surface tension.
- Optochin test is widely used in the form of filter papers disc which are impregnated with the chemical
Appearance of Optochin Discs
It is a filter paper discs of 6 mm diameter bearing letters
Reagents and equipments
- Filter paper discs which is impregnated with 5µg of the chemical ethylhydrocupreine hydrochloride
- Wire Loop
Optochin Test Procedure
- Inoculate a pure growth of the suspected organism on a blood agar plate or tryptone soya agar with blood added by streaking all over the surface of the medium
- Place an optochin disc on the center of the inoculum
- Incubate at a temperature of 35-37 °C for a period of 18-24 hours in 5% Carbon Dioxide
- Examine for zone of Inhibition
Result and interpretation
|Optochin Sensitive||Zone of inhibition of ≥ 12 mm diameter/clear zone around disc|
|Optochin Resistant||No zone of inhibition or Zone of inhibition lesser than 12 mm from the edge of the disc|
Note that Optochin test is to be reported as either sensitive (Susceptible) or resistant NOT positive or Negative
- Bowers E.F. and Jeffries L.R., 1995, J. Clin. Path., 8:58