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A `!double-disk diffusion test`! is a kind of `F33f`_`[disk diffusion test`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Disk_diffusion_test]`_`f (to test for the effectiveness of an antimicrobial agent a disk infused with it is placed on a cultivated agar dish of bacteria to see if the antimicrobial agent in the disk inhibits further growth of the bacteria.`:cite-ref-fong-1-0[`F5bf`_`[1`#cite-note-fong-1]`_`f])

The `!double-disc synergy test`! (`!DDST`!) utilizes two of these disks on the cultivated agar solution, either infused with a different antimicrobial solution.`:cite-ref-2[`F5bf`_`[2`#cite-note-2]`_`f]

This test was recommended the standard by the `F33f`_`[Clinical and Laboratory Standards Institute`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Clinical_and_Laboratory_Standards_Institute]`_`f in 2004 for its use against `F33f`_`[MRSA`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Methicillin-resistant_Staphylococcus_aureus]`_`f.`:cite-ref-3[`F5bf`_`[3`#cite-note-3]`_`f] Testing for inducible clindamycin resistance is typically performed in strains of `*Staphylococcus`*, β-hemolytic streptococci, and `*`F33f`_`[Streptococcus pneumoniae`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Streptococcus_pneumoniae]`_`f`* that demonstrate `F33f`_`[erythromycin`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Erythromycin]`_`f resistance and clindamycin susceptibility.`:cite-ref-4[`F5bf`_`[4`#cite-note-4]`_`f] To test for `F33f`_`[clindamycin`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Clindamycin]`_`f resistance certain strains of `*Staphylococcus aureus`* bacteria with natural resistance to `F33f`_`[erythromycin`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Erythromycin]`_`f are chosen for cultivation on the gel. The two antimicrobial disks contain erythromycin and clindamycin and are placed 25 mm apart when testing `*Staphylococcus`* and 15 mm apart for `*Streptococcus`*. This is called a `!D-zone test`!, or `!D test`!. If a 'D' shape is formed around the clindamycin disk (distinguished from a circular zone of inhibition) then the isolate is reported as resistant to clindamycin.`:cite-ref-5[`F5bf`_`[5`#cite-note-5]`_`f] This occurs due to erythromycin inducing the bacteria's `*erm`* gene, and thus making it resistant to clindamycin (MLS-B phenotype).`:cite-ref-6[`F5bf`_`[6`#cite-note-6]`_`f]`:cite-ref-7[`F5bf`_`[7`#cite-note-7]`_`f]

>>Contents

• `F0af`_`[References`#references]`_`f

• `F0af`_`[Further reading`#further-reading]`_`f

-─

>>References

`:cite-note-fong-1`!1.`! `F0af`_`[↑`#cite-ref-fong-1-0]`_`f `:citereffongdrlica2007`aFong, I. W.; Drlica, Karl (2007). `*Antimicrobial Resistance and Implications for the 21st Century`*. Springer Science & Business Media. pp. 58–59. `F33f`_`[ISBN`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=ISBN_(identifier)]`_`f 9780387724188.

`:cite-note-2`!2.`! `F0af`_`[↑`#cite-ref-2]`_`f `:citereflorian2005`aLorian, Victor (2005). `*Antibiotics in Laboratory Medicine`*. Lippincott Williams & Wilkins. p. 487. `F33f`_`[ISBN`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=ISBN_(identifier)]`_`f 9780781749831.

`:cite-note-3`!3.`! `F0af`_`[↑`#cite-ref-3]`_`f `:citerefweigelt2007`aWeigelt, John A. (2007). `*MRSA`*. CRC Press. p. 49. `F33f`_`[ISBN`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=ISBN_(identifier)]`_`f 9781420045505.

`:cite-note-4`!4.`! `F0af`_`[↑`#cite-ref-4]`_`f `:citerefclsi2020`aCLSI (2020). M100 Performance Standards for Antimicrobial Susceptibility Standards (Report) (32 ed.). Wayne PA: Clinical and Laboratory Standards Institute. p. 148.

`:cite-note-5`!5.`! `F0af`_`[↑`#cite-ref-5]`_`f `:citerefmalone2013`aMalone, Michael A. (2013). `*Infectious Disease, an Issue of Primary Care Clinics in Office Practice`*. Elsevier Health Sciences. p. 642. `F33f`_`[ISBN`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=ISBN_(identifier)]`_`f 9780323188685.

`:cite-note-6`!6.`! `F0af`_`[↑`#cite-ref-6]`_`f "Double Disk Diffusion Test (D-Test)". `*www.parn.org.pk`*. Archived from the original on 25 October 2017. Retrieved 1 November 2017.

`:cite-note-7`!7.`! `F0af`_`[↑`#cite-ref-7]`_`f "The D-test for Macrolide-Inducible Resistance to Clindamycin". `*PharmWiki`*. Tulane University School of Medicine. 18 April 2018. Retrieved 13 December 2024.

>>Further reading

• `:citerefmeggedassousweinbergschlesinger2013`aMegged O, Assous M, Weinberg G, Schlesinger Y (2013). "Inducible clindamycin resistance in β-hemolytic streptococci and Streptococcus pneumoniae". `*Isr. Med. Assoc. J`*. `!15`!: 27–30. `F33f`_`[PMID`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=PMID_(identifier)]`_`f 23484235.

• Steward C. et al. Testing for Induction of Clindamycin Resistance in Erythromycin-Resistant Isolates of Staphylococcus aureus. Journal of Clinical Microbiology. April, 2005.

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