A positive D test indicates that the presence of macrolide-inducible resistance to clindamycin produced by an inducible methylase enzyme that alters the common ribosomal binding site for macrolides, clindamycin and the group B streptogrammins (quinupristin). This cross-resistance, called the MLS-B phenotype, has been associated with clinical reports of clindamycin failure.
The media used for the detection of phenotypic inhibition test is cation adjusted Mueller Hinton Agar (MHA). The inoculum density of bacteria (Staphylococcus aureus) was first adjusted to 0.5 Mc Farland Standard and then plated into the MHA plate. Clindamycin(2µg) and Erythromycin(15µg) disks were placed at 25 mm distance from each other and the plate was incubated at 370C in aerobic condition for 24 hours. The result shows complete Erythromycin resistance and obliteration of Clindamycin zone of inhibition on the side of Erythromycin with the formation of semilunar(D) structure or D shape known as D Zone Test.
M Sc Clinical Microbiology
Institute of Medicine