When testing a gram-positive culture for sensitivity to clindamycin, it is common to perform a "D-test" to determine if there is a sub-population of bacteria present with the phenotype known as iMLSB. This phenotype of bacteria are resistant to the macrolide-lincosamide-streptogramin B group of antibiotics, however, the resistance mechanism is only induced by the presence of 14-membered ring macrolides, such as erythromycin. During a D-test, bacteria of the iMLSB phenotype demonstrate in vitro erythromycin-induced in vitro resistance to clindamycin. This is because of the activity of the macrolide-inducible plasmid-encoded erm gene.
