A double-disk diffusion test is a kind of disk diffusion test (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.) The double-disc synergy test (DDST) utilizes two of these disks on the cultivated agar solution, either infused with a different antimicrobial solution. This test was recommended the standard by the Clinical and Laboratory Standards Institute in 2004 for its use against

MRSA Methicillin-resistant ''Staphylococcus aureus'' (MRSA) is a group of gram-positive bacteria that are genetically distinct from other strains of ''Staphylococcus aureus''. MRSA is responsible for several difficult-to-treat infections in humans. ...

. Testing for inducible clindamycin resistance is typically performed in strains of ''Staphylococcus'', β-hemolytic streptococci, and '' Streptococcus pneumoniae'' that demonstrate

erythromycin Erythromycin is an antibiotic used for the treatment of a number of bacterial infections. This includes respiratory tract infections, skin infections, chlamydia infections, pelvic inflammatory disease, and syphilis. It may also be used ...

resistance and clindamycin susceptibility. To test for clindamycin resistance certain strains of ''Staphylococcus aureus'' bacteria with natural resistance to

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. This occurs due to erythromycin inducing the bacteria's ''erm'' gene, and thus making it resistant to clindamycin (MLS-B phenotype).

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