Optochin Susceptibility Test

Methylhydrocupreine hydrochloride (Optochin) is an quinine derivative and a water soluble compounds. In a very low concentrations, it inhibits S. pneumoniae. Since S. pneumoniae is sensitive to this surface-active agent cells growing in vicinity of filter paper are lysed resulting into formation of clearing zone around the disc impregnated with optochin. Optochin gradually diffuses into agar medium which causes inhibition zone . The resistant alpha-haemolytic streptococci show no zone of inhibition.

Requirements

• Filter paper discs impregnated with optochin
• S. pneumoniae culture
• 5% sheep blood agar medium
• Forceps
• Incubator

Procedure

1. Prepare 5% sheep blood agar plates
2. Streak the plates with S. pneumoniae
3. With the help of sterile forceps place optochin impregnated filter discs over the streaked area.
4. Gently press the discs so as the discs may adher to agar surface.
5. Incubate the plates at 37C for 18-24 hours in a CO₂  incubator.
6.  Growth of S. pneumoniae is inhibited and a zone of inhibition of about 14 mm is formed.

Results

S. pneumoniae shows positive and S. faecalis negative results.

Reference:

Dr. R. C. Dubey – Practical Microbiology