Inducible clindamycin resistance due to expression of erm genes in Staphylococcus aureus: Report from a tertiary care hospital Karachi, Pakistan

Objective: To assess the frequency of phenotypic expression of inducible resistance of clindamycin due to expression of erm genes, in clinical isolates of Staphylococcus aureus (S. aureus), by double disk diffusion test (D-test). Method: This was a cross sectional study conducted in the clinical laboratory of Aga Khan University Hospital, Karachi. A total of 2432, non duplicate clinical isolates of S. aureus, consisting of 1562 methicillin sensitive S. aureus (MSSA) and 870 methicillin resistant S. aureus (MRSA), were selected from February 2007 to January 2008. One hundred and thirty eight isolates of S. aureus were selected based on discordant resistance pattern (erythromycin resistant and clindamycin sensitive) on Kirby Bauer Disk Diffusion test and phenotypic expression of inducible resistance was assessed using D-test. Result: Analysis of 2432 isolates showed that 64% (n=1553) were susceptible to both clindamycin and erythromycin by disc diffusion method, while 30% (n=741) showed constitutive resistance (in vitro resistance to both drugs). 6% (n=138) isolates showed clindamycin-erythromycin discordance on disc diffusion (in vitro sensitive to clindamycin and resistant to erythromycin). Among the discordant isolates 72% (n=99) had inducible resistance phenotype detected by D-test and of these 85 isolates (62%) were MRSA. Conclusion: Inducible resistance is common in our clinical isolates; D-test (a simple phenotypic test) should be performed on all S. aureus isolates showing clindamycin-erythromycin discordance on disc diffusion, to avoid erroneous reporting resulting in treatment failure.