Optochin Test – Presumptive Diagnosis Of Streptococcus Pneumoniae

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Optochin Test - Presumptive Diagnosis Of Streptococcus pneumoniae MLTGEEKS

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

Optochin Discs
Optochin Discs

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 SensitiveZone of inhibition of ≥ 12 mm diameter/clear zone around disc
Optochin ResistantNo zone of inhibition or Zone of inhibition lesser than 12 mm from the edge of the disc
Also read  Acetate differential Agar principle and interpretation of test

Note that Optochin test is to be reported as either sensitive (Susceptible) or resistant NOT positive or Negative

Left (Optochin Resistant) - Right (Optochin Sensitive)
Left (Optochin Resistant) – Right (Optochin Sensitive)

Quality Control

SusceptibleStreptococcus pneumoniae
ResistanceStreptococcus viridans
ResistanceStreptococcus pyogenes


  • Bowers E.F. and Jeffries L.R., 1995, J. Clin. Path., 8:58

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About the Author: Arthur Westmann

DEFFE ARTHUR (AMOEBAMANN) is the founder and author of MLTGEEKS and MLTEXPO.He’s from Cameroon and is currently a Final year State Medical Laboratory Technician (MLT MA). Beyond lab works, he’s a passionate internet user with a keen interest in web design and blogging. Furthermore He likes traveling, hanging around with friends and social networking to do in his spare time.

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