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Prevalence of inducible clindamycin resistance among Staphylococcus aureus isolates from a tertiary care hospital

Clindamycin is an excellent drug for skin and soft tissue Staphylococcus aureus infections, but resistance mediated by inducible macrolide-lincosamide-streptogramin B (iMLS)phenotype leads to in vivo therapeutic failure even though they may be in vitro susceptible in Kirby–Bauer disk diffusion method (KBDDM). Hence the study was undertaken to detect the prevalence of iMLS phenotype among Staphylococcus aureus isolates by double disk approximation test (D-test) in a tertiary care hospital. A total of 100 consecutive Staphylococcus species isolates were identified by standard microbiological methods and subjected to antimicrobial susceptibility testing by KBDDM. Clindamycin-resistance either in the form of iMLSB or cMLSB was determined through double disk diffusion method or D-test by using erythromycin (2 μg) and clindamycin (15μg) as per the CLSI guidelines. Out of 100 Staphylococcus species studied, 50(50%) were methicillin sensitive Staphylococcus aureus, 30(30%) were Methicillin resistant Staphylococcus aureus and 20 (20%) were Coagulase negative Staphylococci. Out of 80 Staphylococcus aureus studied, iMLSB, cMLSB and MS phenotype were 32.5%, 1.25%, 5% respectively. Inducible resistance and MS phenotype were found to be higher in MRSA as compared to MSSA (60%, 6.66% and 16%, 4% respectively). The study revealed 32.5% of Staphylococcus aureus isolates were inducible clindamycin resistant, which could be easily misidentified as clindamycin susceptible in Kirby–Bauer disk diffusion method. Therefore, clinical microbiology laboratory should routinely perform D-test in all clinically isolated Staphylococcus aureus to guide clinicians for the appropriate use of clindamycin.