Prevalence of Constitutive and Inducible Clindamycin Resistance among Methicillin-Resistant Staphylococcus aureus Isolates in a Tertiary Care Hospital, Kashmir Valley

The increased frequency of Methicillin-resistant Staphylococcus aureus infections has led to renewed interest in the macrolide-lincosamide streptogramin B (MLS) group of antibiotics. Resistance to these antibiotics may be constitutive or inducible. Isolates resistant to erythromycin may show false in vitro susceptibility to clindamycin, leading to therapeutic failures. This study investigated the utility of the D-Test for detecting inducible clindamycin resistance in methicillin-resistant S. aureus isolates and determining the prevalence of various phenotypes in our region.

For detecting inducible clindamycin resistance, a D-test using erythromycin and clindamycin as per CLSI guidelines was performed, and four different phenotypes were interpreted as methicillin-sensitive (MS) phenotype (D-test negative), inducible MLSB (iMLSB) phenotype (D-test positive), constitutive MLSB phenotype and sensitive to both.

Of the 987 isolates tested, 400 (40.53%) were MRSA. The prevalence of iMLSB, cMLSB phenotype, MS phenotype and sensitive phenotype in MRSA isolates was 42.5%, 10.5%, 28% and 19%, respectively. The iMLSB and cMLSB phenotypes were higher in males (24.75%, 6.25%) than females (P-value = 0.137). The majority of MRSA isolates originated from pus (83%). All S. aureus isolates showed 100% sensitivity to vancomycin and linezolid. 

This study emphasizes the prevalence of inducible clindamycin resistance in MRSA in our setup. Incorporating the D-test into the routine Kirby–Bauer disk diffusion method in clinical microbiology laboratories will help clinicians make judicious use of clindamycin, minimizing treatment failure.