Determine the inducible resistance phenotype in methicillin resistance staphylococcus aureus and coagulase negative staphylococci

Macrolide, lincosamide and streptogramin B (MLSB) antimicrobial agents are widely used in the treatment of staphylococcal infection. Clindamycin is the medicine of choice for some staphylococcal infections, particularly skin and soft tissues infections. Erythromycin and clindamycin are two distinct classes of antimicrobial agents which inhibit protein synthesis in bacterial cells. Inducible resistance to clindamycin is not diagnosed using conventional antibiotic susceptibility testing and most of the physicians do not prescribe clindamycin in cases where isolates show resistance to erythromycin. However,not all of the erythromycin resistant strains are resistant to clindamycin. To detect co-resistance to both antibiotics, the inducible test should be performed to determine the potency of clindamycin for treatment.The aim of this study was to detect inducible clindamycin resistant isolates of methicillin resistant isolates of Staphylococcus aureus and coagulase negative staphylococci. The inducible test was performed by disk diffusion, placing an erythromycin disk adjacent to a clindamycin disk on Muller Hinton agar plate. If the isolates were resistant to erythromycin and this resistance was induced to clindamycin an inhibitiom zone shaped like the letter D was produced. In this study all methicillin resistance Staphylococcus aureus and coagulase negative staphylococci were tested forinduced resistance. Of 128 isolates of Staphylococci, 6 were D and 1 was D+. The inducible test correctly identified the inducible resistance to clindamycin caused by erythromycin. The resistance to clindamycin was not induced by erythromycin in the majority of isolates of staphylococci in our collection of isolates. We recommend the test routinely be used for correct determination of resistance to clindamycin.