Disk diffusion test

Scheme of DDT with inhibition zones

The disk diffusion test is a qualitative method for determining the susceptibility of a bacterial strain to an antibiotic.

The strain is sensitive or, conversely, resistant to the size of the inhibition zone around the disc on solid soil - if the diameter of the inhibition zone is less than the specified limit, it means that administration of this antibiotic would not be effective and the strain is therefore resistant.

This method does not measure the degree of sensitivity - the strain is always either sensitive or resistant.

Method

1. Suspend 3–4 colonies of bacteria in the broth and adjust the turbidity according to the turbidity standard corresponding to the concentration (108 bacteria / ml).
2. The resulting suspension is applied to Mueller-Hinton agar.
3. We apply the discs to the soil with an applicator (standard discs with the agreed amount of antibiotics are used).
4. Incubate for about 18 hours in a thermostat.
5. After incubation, the sensitivity is manifested by a growth inhibition zone – the diameter of the inhibition zone is measured in mm.
6. We compare the result with the table values ​​and determine whether the strain is sensitive to antibiotics or resistant.
   

   DDT

The antibiotic is gradually released from the disc and diffuses into the soil. The inoculated bacteria multiply, but they stop multiplying around the disc over time, depending on their sensitivity. At a certain distance from the disc, however, the antibiotic is no longer able to stop the growth of bacteria - the edge of the inhibition zone is created here. The slower the strain multiplies, the larger the zone at the same strain sensitivity.

Links

Related articles

• E-test

Resources

• BEDNÁŘ, M, A SOUČEK and V FRAŇKOVÁ, et al. MEDICAL MICROBIOLOGY: Bacteriology, virology, parasitology. - edition. Moscow: Triton, 0000. 560 pp.  ISBN 859-4-315-0528-0 .