A double-disk diffusion test is a kind of

disk diffusion test The disk diffusion test (also known as the agar diffusion test, Kirby–Bauer test, disc-diffusion antibiotic susceptibility test, disc-diffusion antibiotic sensitivity test and KB test) is a culture-based microbiology assay used in diagnost ...

(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 The Clinical and Laboratory Standards Institute (CLSI) is a volunteer-driven, membership-supported, not-for-profit, standards development organization. CLSI promotes the development and use of voluntary laboratory consensus standards and guideline ...

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 ''Streptococcus pneumoniae'', or pneumococcus, is a Gram-positive, spherical bacteria, alpha-hemolytic (under aerobic conditions) or beta-hemolytic (under anaerobic conditions), aerotolerant anaerobic member of the genus Streptococcus. They a ...

that are 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 du ...

resistance and clindamycin susceptibility. To test for

clindamycin Clindamycin is an antibiotic medication used for the treatment of a number of bacterial infections, including osteomyelitis (bone) or joint infections, pelvic inflammatory disease, strep throat, pneumonia, acute otitis media (middle ear infe ...

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 then the isolate is reported as resistant to clindamycin. This occurs due to erythromycin inducing the bacteria's ERM (B) gene, and thus making it resistant to Clindamycin.

References

Further reading