Practical Disk Diffusion Method for Detection of Inducible Clindamycin Resistance in Staphylococcus aureus and Coagulase-Negative Staphylococci

Abstract

Bacterial resistance to antimicrobial agents generally involves drug inactivation, target site modification, impermeability, or efflux mechanisms. Macrolide antibiotic resistance in Staphylococcus aureus and coagulase-negative staphylococci (CNS) may be due to an active efflux mechanism encoded by msrA (conferring resistance to macrolides and type B streptogramins only) (16, 17) or may be due to ribosomal target modification, affecting macrolides, lincosamides, and type B streptogramins (MLS B resistance). erm genes encode enzymes that confer inducible or constitutive resistance to MLS agents via methylation of the 23S rRNA, reducing binding by MLS agents to the ribosome (15). Resistance is induced by the binding of a macrolide to upstream translational attenuator sequences, leading to changes in mRNA secondary structure, exposure of the ribosomal binding site, and translation of the erm methylase. Alterations in these 5Ј upstream sequences, including deletions, duplications, and other mutations, lead to constitutive expression of the methylase gene and constitutive MLS B resistance (1,15,24). Strains with inducible MLS B resistance (MLS B i) strains demonstrate in vitro resistance to 14-and 15-member macrolides (e.g., erythromycin), while appearing susceptible to 16-member macrolides, lincosamides, and type B streptogramins; strains with constitutive MLS B resistance (MLS B c strains) show in vitro resistance to all of these agents (15).MLS antibiotics are commonly used in treatment of staphylococcal infections. Clindamycin is a frequent choice for some staphylococcal infections, particularly skin and soft-tissue infections, and as an alternative in the penicillin-allergic patient. Inducible MLS B resistance is not recognized by using standard susceptibility test methods, including standard broth-based or agar dilution susceptibility tests. Failure to identify inducible MLS B resistance may lead to clinical failure of clindamycin therapy (3). Conversely, labeling all erythromycin-resistant staphylococci as clindamycin resistant prevents the use of clindamycin in infections caused by truly clindamycin-susceptible staphylococcal isolates.Low levels of erythromycin are the most effective inducer of inducible MLS B resistance (23). To detect MLS B i strains, there are special disk approximation tests that incorporate erythromycin induction of clindamycin resistance (23). These tests involve the placement of an erythromycin disk in close proximity to a disk containing clindamycin or lincomycin. As the erythromycin diffuses through the agar, resistance to the lincosamide is induced, resulting in a flattening or blunting of the lincosamide zone of inhibition adjacent to the erythromycin disk, giving a D shape to the zone (D-zone effect). Jenssen and colleagues suggested that closer spacing than in a standard disk dispenser was necessary to discern inducible resistance, with optimal spacing of 10 to 15 mm (6); 20-mm spacing of disks and a higher concentration of erythromycin (30 g) have also been suggested (18).

Antimicrobial resistance of Staphylococcus spp. from small ruminant mastitis in Brazil

“…This corroborates a previous report showing high resistance rates for erythromycin in S. epidermidis, and a high frequency of the mrsA gene in those species (Sawant et al 2009). Possible differences in phenotypic and genotypic resistance to erythromycin may be explained by other unidentiϐied macrolide resistance mechanisms which are common in CNS isolates (Fiebelkorn et al 2003). These are ultimately related to geographic location and therapeutic drug usage (Pitkälä et al 2004, Gatermann et al 2007).…”

Macrolide and lincosamide resistance in staphylococcal clinical isolates in Nablus, Palestine

“…Isolates resistant to erythromycin but susceptible to clindamycin were examined by double disk diffusion method (D-test) according to CLSI guidelines (16). Antibiotic disks were obtained from Oxoid.…”

Evaluation of constitutive and inducible resistance to clindamycin in clinical samples of Staphylococcus aureus from a tertiary hospital

“…Clindamycin (CL) is the preferred agent for the treatment of MRSA due to its excellent pharmacokinetic properties, such as optimal tissue penetration and accumulation in abscesses 5,6 . Furthermore, CL is a frequent choice for treating staphylococcal infections because this antibiotic can be orally administered and is well tolerated 7,8 .…”