Phenotypic detection of constitutive and inducible clindamycin resistance in clinical isolates of Staphylococcus aureus and coagulase negative Staphylococcus on routine susceptibility plate.

Increasing frequency of methicillin resistant Staphylococcus aureus infections and changing patterns in antimicrobial resistance have led to renewed interest in the use of macrolidelincosamide-streptogramin antibiotics. However therapy may fail either due to constitutive or inducible resistance. This study was undertaken to detect different phenotypes including inducible clindamycin resistance in clinical isolates of Staphylococcus aureus and coagulase negative Staphylococcus. Four hundred sixty five Staphylococcus aureus and 84 coagulase negative Staphylococci isolated from different clinical specimens were included in the study. On routine susceptibility testing plate clindamycin (2 microg) disk was placed at a distance of 15mm towards the centre from a peripherally placed erythromycin (15 microg) disk. Fisher exact test was used for statistical analysis. Out of 465 Staphylococcus aureus isolates, 237 (50.96%) were methicillin sensitive (MSSA) and 228 (49.03%) methicillin resistant (MLS(B)c).Over all 118 (25.37%) isolates showed constitutive resistance (MLS(B)c), 70 (15.05%) inducible clindamycin resistance, 143 (30.75%) MS(B) phenotype and 134 (28.81%) were susceptible to both erythromycin as well as clindamycin. Constitutive and inducible resistance to clindamycin were significantly higher in MRSA than MSSA (P=0.0000 and 0.0001 respectively). Out of 84 isolates of coagulase negative Staphylococci, 43 (51.19%) were methicillin sensitive (MSCNS) and 41(48.80%) methicillin resistant (MRCNS). Constitutive MLS(B) resistance was detected in 32 (38.09%), inducible clindamycin resistance 10 (11.90%), MS(B) phenotype 27 (32.14%) and 15 (17.85%) were susceptible to both erythromycin and clindamycin. Performing D test on a routine susceptibility plate saves material, manpower and time as inducible resistance can be reported simultaneously along with other susceptibility results.