OPTOCHIN TEST
TEST PRINCIPLE
The optochin test detects an organisms susceptibility to the chemical optochin ethylhydrocupreine hydrochloride). The chemical tests the fragility of the bacterial cell membrane and causes Streptococcus pneumoniae to lyse due to changes in surface tension.
The optochin test is widely used in the form of filter paper discs, impregnated with ethylhydrocupreine hydrochloride which are applied directly to inoculated plates before incubation.
Filter paper discs impregnated with 5µg of ethylhydrocupreine hydrochloride
Bacteriological straight wire/loop (preferably nichrome) or disposable alternative
Quality Control Organisms
Positive control: Streptococcus pneumoniae
Negative control: Streptococcus viridans
METHOD/PROCEDURE AND RESULTS
Pure colony
- Streak a blood plate with the organism to be tested
- Place an optochin disc in the centre of the inoculum
- Incubate at 35-37o C for 18-24h in 5% CO2
- Examine for zones of inhibition
Specimen
- Streak the specimen onto a blood plate
- Place an optochin disc on the edge of the primary inoculum
- Incubate at 35-37oC for 18-24h in 5% CO2
- Examine for zones of inhibition
Positive result
Zone of inhibition of ≥5mm radius from the edge of the disc eg test organism is S. pneumoniae
Negative result
No zone of inhibition, or a zone <5mm radius from the edge of the disc eg test organism is not S. pneumoniae.
PRECAUTIONS/LIMITATIONS OF PROCEDURE
Some “viridans” streptococci may produce a small zone of inhibition. Equivocal results should be confirmed using the bile solubility test or by the use of a commercial kit.
False-negatives may be reported if cultures are incubated in high concentrations of CO2.
Occasional strains of optochin resistant S. pneumoniae have been reported.
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