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Clindamycin Resistance in Staph and Beta-Strep

Resistance to macrolides (erythromycin, azithromycin, & clarithromycin) in staphylococci and beta-hemolytic streptococci may be due to ribosomal target modification that also affects the activity of clindamycin. This type of antimicrobial resistance is referred to as macrolide-lincosamide-streptogramin B (MLS-B) resistance and is encoded by the bacterial gene erm(A) or erm(C). Conventional antimicrobial susceptibility testing detects resistance caused by efflux pumps that affect only macrolides. Specialized testing is needed to detect MLS-B resistance, also known as inducible resistance, to clindamycin. In MLS-B bacterial strains, erythromycin induces production of a methylase enzyme, which allows clindamycin resistance to be expressed. Inducible clindamycin resistance is determined by means of an antibiotic disk diffusion test, called the D-test, which requires overnight incubation of the organism being tested. The clindamycin induction test can be performed on staphylococci and beta-hemolytic streptococci that test resistant to erythromycin and susceptible to clindamycin by routine methods. Isolates that have a positive D-test are reported as resistant to erythromycin and clindamycin. Isolates with a negative D-test are reported as resistant to erythromycin, but susceptible to clindamycin.

A recent report indicates that inducible resistance to clindamycin is found in 7-20% of both methicillin-susceptible Staphylococcus aureus (MSSA), and methicillin-resistant Staphylococcus aureus (MRSA). Community-acquired MRSA isolates are somewhat more susceptible than hospital-acquired strains. D-testing is of particular importance in community-acquired MRSA infections since oral clindamycin is widely used in therapy.

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