Phenotypic detection of inducible clindamycin resistance among Staphylococcus aureus isolates by using the lower limit of recommended inter-disk distance

CONTEXT: Clindamycin is one of the important alternative antibiotics in the therapy of Staphylococcus aureus, particularly in methicillin-resistant S. aureus (MRSA) infections. Inducible clindamycin resistance (iMLS B--inducible Macrolide-Lincosamide-Streptogramin B resistance) is a critical factor in antimicrobial susceptibility testing. AIMS: To know the rate of inducible clindamycin resistance among clinical isolates of Staphylococcus aureus in our hospital by Disk approximation test (D-test) using the average recommended inter-disk distance and comparing the results with that of D-test using the lower limit of recommended inter-disk distance. MATERIALS AND METHODS: A total of 51 erythromycin-resistant and clindamycin-susceptible S. aureus isolates were subjected to disk approximation testing with 21 +/- 1 mm and 15 mm edge-to-edge distance between the clindamycin and erythromycin disks. STATISTICAL METHODS: Z-test levels. RESULTS: Among 51 erythromycin-resistant and clindamycin-susceptible S. aureus isolates, 25 (49%) were recorded as inducible clindamycin resistant by D-test with 21 +/- 1 mm edge-to-edge distance between the clindamycin and erythromycin disks. When we re-tested all the 51 strains by D-test with 15 mm inter-disk distance, we identified 14% more iMLS B strains previously reported as D-test negative. Z-test for MRSA indicates that 15 mm edge-to-edge distance has significant advantage. CONCLUSIONS: Since the incidence of inducible clindamycin resistance is high (63% in our study), accurate identification of inducible clindamycin resistance is important to prevent therapeutic failure in infections caused by these strains. We suggest the use of D-test with 15 mm edge-to-edge inter-disk distance for detecting iMLS B .