Inducible clindamycin resistance among Staphylococcus aureus isolates

Introduction: Clindamycin is considered an useful alternate drug in penicillin-allergic patients in the treatment of skin & soft tissue infections caused by Staphylococcus aureus. Staphylococcus spp. can be resistant to erythromycin through either erm or msr A genes. Strains with erm-mediated erythromycin resistance may possess inducible clindamycin resistance but may appear susceptible to clindamycin by disc diffusion test. The objective of the present study was to know the prevalence of erythromycin-induced clindamycin resistance among clinical isolates of S. aureus. Methods: A total of 250 S. aureus isolates from various clinical samples submitted in the Dept. of Microbiology at our tertiary care hospital were studied. Methicillin resistant S. aureus strains were identified by Cefoxitin disc diffusion method. Inducible clindamycin resistance was detected by erythromycin and clindamycin disc approximation test (D-zone test) as per CLSI guidelines. Result: Among the 250 S. aureus isolates, 107 strains (42.8%) were detected as MRSA of which 26(24.3%) strains showed inducible clindamycin resistance (D-test positive). Only 7(5.0%) isolates of MSSA were D-test positive. 156(62.4%) isolates of S. aureus were sensitive to both erythromycin & clindamycin. Conclusion: High prevalence of strains with inducible clindamycin resistance particularly among MRSA indicates that inducible clindamycin resistance testing (D-test) should be included as a part of routine antibiotic susceptibility. These isolates may be missed in routine antibiotic testing by disk diffusion method.