The D-zone test is a microbiological assay used to detect inducible clindamycin resistance in certain bacteria, particularly Staphylococcus aureus and other staphylococcal species.
This test is particularly important for identifying cases where an organism appears susceptible to clindamycin in routine testing but may actually be resistant due to inducible mechanisms.
How to perform D-test?
A standard agar plate (usually Mueller-Hinton agar) is inoculated with a bacterial isolate.
Two antibiotic discs are placed on the plate:
Erythromycin (15 µg).
Clindamycin (2 µg).
The discs are placed approximately 15-20 mm apart from each other on the agar.
The plate is incubated, usually at 35-37°C for 18-24 hours.
D-test result Interpretation
If the bacteria have inducible clindamycin resistance, the zone of inhibition around the clindamycin disc will be flattened on the side closest to the erythromycin disc. This creates a “D” shape around the clindamycin disc, indicating positive inducible resistance.
A circular zone of inhibition around the clindamycin disc suggests that there is no inducible resistance, and clindamycin is likely effective.
If the bacteria are resistant to both erythromycin and clindamycin, it will show no zone of inhibition around either disc.
What is the Purpose of the D-Zone Test in microbiology?
D-Zone Test used To detect inducible clindamycin resistance, which can lead to treatment failure if clindamycin is used.
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