An optochin sensitivity test is used for the alpha-hemolytic streptococcus pneumoniae biochemical test. With the help of this test, we diagnose streptococcus pneumoniae another alpha-hemolytic streptococcus.

Optochin senstivity test principle

Optochin is a chemical (ethylhydrocupreine hydrochloride). It is an antibiotic. It interferes with ATP synthesizing enzyme called ATPase in bacteria. It causes the change in the fragility of the cell membrane of particular bacteria results in membrane rapture due to surface tension.

In the optochin sensitivity test, the optochin impregnated disk is placed in the middle of the test organisms on the blood agar plate. This antibiotic diffuse in the media. It inhibits the growth of susceptible organisms around itself and creates a clear zone.

Note: 14 mm clear zone is the indication of optochin-sensitive organisms.

Procedure of optochin sensitivity test

  1. Select and pick the isolated colony of alpha hemolytic organisms with the help of steril loop.
  2. Streak the organism on the sheep blood agar plate.
  3. Now pick the the optochin disk with sterile forcepe and place it on between the test organisms on the agar plate.
  4. Gently press the disk, so that it may adhre to the agar surface.
  5. Incubate the plate at 37 C for 24 hours, at 5% CO2 environments.
  6. Observe the plate after 24 hour and note the clear zone around the disk.

NOTE: If the clear area is equal to 15 mm or more than 15 mm, then the organism is optochin test sensitive. But, if the clear area is less than 15 mm then it will be an optochin-resistant organism.

Which bacterias are Optochin sensitive?

Only streptococcus Pneumonia is Optochin sensitive organism. The clear area will be more than 14 mm around the optochin disk.

Optochin sensitivity test: Principle, procedure (streptococcus pneumoniae)
Optochin senstive test

What bacteria is Optochin resistant?

Alpha hemolytic organism other than Streptococcus pneumoniae is optochin resistant. There will be a small clear zone around the optochin disk.

Optochin sensitivity test with no clear zone

When there is no clear zone around the optochin disk after 24 hours, it will be concluded that the test organism is other than Streptococcus pneumonia. Therefore other biochemical tests are necessary for further identification.

What are the limitations of the Optochin test?

  1. Optochin susptibility test is only used to identify the Alpha hemolytic arganism not beta hemolytic organisms.
  2. We can not differentiate other alpha hemolytic organisms on the base small clear zone, for this purpose we need to do more biochemical test bile solubility test, and more.
  3. For this test we need special chamber with 5-10 % CO2 for bacteria growth.

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