Inducible clindamycin resistance in staphylococcus isolates from a tertiary care hospital in Eastern India

Purpose: The increasing resistance to antimicrobial agents among Staphylococci has led to renewed interest in the usage of macrolide-lincosamide-streptogramin B (MLS B) antibiotics to treat Staphylococcus aureus (S. aureus) infections. The resistance to macrolide can be mediated by msr A gene coding for the efflux mechanism or via erm gene encoding for enzymes that confer inducible or constitutive resistance to MLS B antibiotics. In vitro routine tests for clindamycin susceptibility may fail to detect inducible clindamycin resistance due to erm genes resulting in treatment failure, thus necessitating the need to detect such resistance by a simple D test on a routine basis. Materials and Methods: One hundred and ninety-five S. aureus isolates were subjected to routine antibiotic susceptibility testing including cefoxitin (30 mu g) by a modified Kirby Bauer disk diffusion method. Inducible resistance to clindamycin in S. aureus was tested by the "D test" as per CLSI guidelines. Results: Thirty-three (16.9%) isolates showed inducible clindamycin resistance, 45 (23%) showed a constitutive resistance while remaining 33 (16.9%) showed the MS phenotype. Inducible resistance and constitutive resistance were found to be higher in MRSA compared to MSSA (22.6%, 35.5%, and 11.8%, 11.8%, respectively). Conclusion: Clindamycin is kept as a reserve drug and is usually advocated in severe MRSA infections depending upon the antimicrobial susceptibility results. This study showed that a D test should be used as a mandatory method in routine disk diffusion testing to detect inducible clindamycin resistance in Staphylococci for the optimum treatment of patients.