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<title>Double-disk diffusion test</title>
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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Double-disk diffusion test</span></span>
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<p>A <b>double-disk diffusion test</b> is a kind of <a href="Disk_diffusion_test" title="Disk diffusion test">disk diffusion test</a> (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.<sup id="cite_ref-fong_1-0" class="reference"><a href="#cite_note-fong-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>)
</p><p>The <b>double-disc synergy test</b> (<b>DDST</b>) utilizes two of these disks on the cultivated agar solution, either infused with a different antimicrobial solution.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
</p><p>This test was recommended the standard by the <a href="Clinical_and_Laboratory_Standards_Institute" title="Clinical and Laboratory Standards Institute">Clinical and Laboratory Standards Institute</a> in 2004 for its use against <a href="Methicillin-resistant_Staphylococcus_aureus" title="Methicillin-resistant Staphylococcus aureus">MRSA</a>.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> Testing for inducible clindamycin resistance is typically performed in strains of <i>Staphylococcus</i>, β-hemolytic streptococci, and <i><a href="Streptococcus_pneumoniae" title="Streptococcus pneumoniae">Streptococcus pneumoniae</a></i> that demonstrate <a href="Erythromycin" title="Erythromycin">erythromycin</a> resistance and clindamycin susceptibility.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> To test for <a href="Clindamycin" title="Clindamycin">clindamycin</a> resistance certain strains of <i>Staphylococcus aureus</i> bacteria with natural resistance to <a href="Erythromycin" title="Erythromycin">erythromycin</a> are chosen for cultivation on the gel. The two antimicrobial disks contain erythromycin and clindamycin and are placed 25 mm apart when testing <i>Staphylococcus</i> and 15 mm apart for <i>Streptococcus</i>. This is called a <b>D-zone test</b>, or <b>D test</b>. 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.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> This occurs due to erythromycin inducing the bacteria's <i>erm</i> gene, and thus making it resistant to clindamycin (MLS-B phenotype).<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-fong-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-fong_1-0">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><cite id="CITEREFFongDrlica2007" class="citation book cs1">Fong, I. W.; Drlica, Karl (2007). <i>Antimicrobial Resistance and Implications for the 21st Century</i>. Springer Science & Business Media. pp. <span class="nowrap">58–</span>59. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>9780387724188</bdi>.</cite></span>
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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite id="CITEREFLorian2005" class="citation book cs1">Lorian, Victor (2005). <i>Antibiotics in Laboratory Medicine</i>. Lippincott Williams & Wilkins. p. 487. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>9780781749831</bdi>.</cite></span>
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<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite id="CITEREFWeigelt2007" class="citation book cs1">Weigelt, John A. (2007). <i>MRSA</i>. CRC Press. p. 49. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>9781420045505</bdi>.</cite></span>
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<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite id="CITEREFCLSI2020" class="citation report cs1">CLSI (2020). M100 Performance Standards for Antimicrobial Susceptibility Standards (Report) (32 ed.). Wayne PA: Clinical and Laboratory Standards Institute. p. 148.</cite></span>
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<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><cite id="CITEREFMalone2013" class="citation book cs1">Malone, Michael A. (2013). <i>Infectious Disease, an Issue of Primary Care Clinics in Office Practice</i>. Elsevier Health Sciences. p. 642. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>9780323188685</bdi>.</cite></span>
</li>
<li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20171025202106/http://www.parn.org.pk/index_files/D.test.html">"Double Disk Diffusion Test (D-Test)"</a>. <i>www.parn.org.pk</i>. Archived from <a rel="nofollow" class="external text" href="http://www.parn.org.pk/index_files/D.test.html">the original</a> on 25 October 2017<span class="reference-accessdate">. Retrieved <span class="nowrap">1 November</span> 2017</span>.</cite></span>
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<li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://tmedweb.tulane.edu/pharmwiki/doku.php/the_d_test">"The D-test for Macrolide-Inducible Resistance to Clindamycin"</a>. <i>PharmWiki</i>. Tulane University School of Medicine. 18 April 2018<span class="reference-accessdate">. Retrieved <span class="nowrap">13 December</span> 2024</span>.</cite></span>
</li>
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<div class="mw-heading mw-heading2"><h2 id="Further_reading">Further reading</h2></div>
<ul><li><cite id="CITEREFMeggedAssousWeinbergSchlesinger2013" class="citation journal cs1">Megged O, Assous M, Weinberg G, Schlesinger Y (2013). "Inducible clindamycin resistance in β-hemolytic streptococci and Streptococcus pneumoniae". <i>Isr. Med. Assoc. J</i>. <b>15</b>: <span class="nowrap">27–</span>30. <a href="PMID_(identifier)" class="mw-redirect" title="PMID (identifier)">PMID</a> <a rel="nofollow" class="external text" href="https://pubmed.ncbi.nlm.nih.gov/23484235">23484235</a>.</cite></li>
<li>Steward C. et al. Testing for Induction of Clindamycin Resistance in Erythromycin-Resistant Isolates of Staphylococcus aureus. Journal of Clinical Microbiology. April, 2005.</li></ul></div><!--htdig_noindex--><div><div class="zim-footer">
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