Optochin susceptibility test Principle, Procedure and Results interpretation

Optochin susceptibility test: Principle, Procedure and Results interpretation

Optochin susceptibility test is a key diagnostic tool used to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci, particularly Streptococcus viridans group. The test is based on the sensitivity of S. pneumoniae to the antibiotic optochin, which inhibits its growth.

Optochin Susceptibility Test

Optochin Susceptibility Test is a laboratory method used to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci, particularly those in the viridans group. This test relies on the susceptibility of Streptococcus pneumoniae to the antibiotic optochin (ethylhydrocupreine hydrochloride).

Diffusion of optochin into the agar results in a gradient of concentration. The antibiotic’s growth-inhibiting properties will prevent bacteria that are optochin-susceptible from growing in the vicinity of the disc. As a result, upon incubation, a distinct zone of inhibition forms around the disc. A zone of inhibition with a diameter of at least 14 mm is characteristic of Streptococcus pneumoniae and confirms the organism’s optochin sensitivity.

On the other hand, the lack of a clear zone or a tiny zone indicates resistance to optochin, indicating that the bacteria are alpha-hemolytic streptococci that are not pneumococcal. Because of its ease of use, speed, and accuracy in distinguishing between various bacterial species, the optochin disc test is useful in clinical microbiology and can help determine the best course of treatment for illnesses brought on by Streptococcus pneumoniae.

Optochin Susceptibility Test

Principle

Since Streptococcus pneumoniae is susceptible to optochin while other alpha-hemolytic streptococci are resistant, this observation forms the basis of the Optochin Susceptibility Test’s principle. Through cellular process disruption, specifically by interfering with ATP generation, optochin prevents Streptococcus pneumoniae from growing. An optochin disk placed on an agar plate infected with the bacteria produces a distinct zone of inhibition as a result. The viridans group of alpha-hemolytic streptococci, on the other hand, are unaffected by optochin and will continue to develop until they reach the disk’s edge.

Procedure

Materials Needed:

Optochin Susceptibility Test
  • Blood agar plate
  • Optochin disks (5 µg or 6 mm)
  • Bacterial inoculum (suspected alpha-hemolytic streptococcus)
  • Sterile swab or inoculating loop
  • Incubator set at 35-37°C

Steps:

Inoculation:

Streak the surface of a blood agar plate evenly with the bacterial inoculum using a sterile swab or inoculating loop. This creates a lawn of bacterial growth.

Placement of Optochin Disk:

Place an optochin disk (containing 5 µg of optochin) in the center of the inoculated agar plate. Press the disk gently onto the agar surface to ensure contact.

Incubation:

Incubate the plate aerobically at 35-37°C for 18-24 hours.

Interpretation:

After incubation, examine the plate for the presence of a zone of inhibition around the optochin disk.

Results Interpretation

Positive Result (Optochin Sensitive):

Positive Result (Optochin Sensitive)
  • Look: A distinct area of inhibition that surrounds the optochin disk, often measuring less than 14 mm.
  • Interpretation: The fact that the bacteria is sensitive to optochin and that its development is inhibited around the disk suggests that it is Streptococcus pneumoniae.

Negative Result (Optochin Resistant):

  • Look: The optochin disk has no zone of inhibition or a very small zone (less than 14 mm) surrounding it, with bacterial growth continuing all the way to the disk’s edge.
  • Interpretation: This suggests that the bacteria is not Streptococcus pneumoniae, but rather is most likely a member of the optochin-resistant viridans group of streptococci.

Frequently Asked Question

What is Optochin Susceptibility Test ?

Optochin Susceptibility Test is a laboratory method used to differentiate Streptococcus pneumoniae from other alpha-hemolytic streptococci, particularly those in the viridans group.

What is the principle of optochin disc test?

The optochin disc test operates on the principle of testing the susceptibility of bacteria, particularly Streptococcus pneumoniae, to the antibiotic optochin (ethylhydrocupreine hydrochloride). This test is crucial for distinguishing S. pneumoniae from other alpha-hemolytic streptococci, such as those in the viridans group, which are optochin-resistant.

What is the function of optochin?

he function of optochin, specifically ethylhydrocupreine hydrochloride, lies in its ability to inhibit the growth of Streptococcus pneumoniae, a bacterium responsible for various respiratory tract infections including pneumonia.

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