Inducible Clindamycin resistance in Staphylococcus aureus isolates recovered in specimen from tertiary care hospital

Objective: to find out the percentage of staphylococcus aureus having inducible clindamycin resistance in our hospital using d-test and to know the relationship between methicillin-resistant staphylococcus aureus (mrsa) and inducible clindamycin resistance. materials and methods: this was a descriptive cross-sectional study conducted on 93 staph aureus isolates in a tertiary care hospital of lahore during period of april 2012 to june 2012. susceptibility to penicillin (10μg),cefoxitin (30μg),erythromycin (15μg),clindamycin (2μg),linezolid (30μg),ciprofloxacin (15μg),gentamycin (10μg) trimethoprim/sulfmethoxazole (1.25/23.75μg) discs by kirby bauer disc diffusion method was determined as per nccls guideline. a disc containing erythromycin (15 μg) was placed 15mm from centre to centre of a clindamycin (2 μg) disc. inducible resistance to clindamycin is manifested by flattening or blunting of the clindamycin zone of inhibition adjacent to the erythromycin disc,giving a d-shape to the zone of inhibited growth. d-shaped clindamycin susceptibility patterns where considered as d-test positive. results: total 93 staph aureus was isolated from different clinical samples,42 (45%) were methicillin-resistant staphylococcus aureus (mrsa) and 51 (55%) were methicillin-sensitive staphylococcus aureus (mssa). maximum isolates of staph aureus were recovered from pus. out of 42 mrsa isolates,5 (12%) were d-test positive and out of 51 mssa only 1 (2%) isolate was d-test positive. maximum resistance was observed with penicillin (96%) followed by trimethoprim/ sulfamethoxazole (91%). all isolates were sensitive to linezolid (100%),sensitivity to clindamycin and erythromycin was 73% and 51% respectively. out of total 93 staph aureus isolates 43(46%) were sensitive to both erythromycin and clindamycin,20(22%) isolates were resistant to both antibiotics and 6 (6%) isolates were resistant to erythromycin and sensitive to clindamycin (d-test positive) 24 (26%) isolates were resistant to erythromycin and sensitive to clindamycin (d-test negative). out of 30 isolates which were resistant to erythromycin and sensitive to clindamycin,6 isolates (20%) were d-test positive and 24 isolates (80%) were d-test negative. conclusion: all laboratories should routinely evaluate staph aureus isolates that initially test as resistant to erythromycin and susceptible to clindamycin for inducible clindamycin resistance using the d- test. when inducible clindamycin resistance is present,the isolate is presumed to be resistant,and use of an alternative agent should be considered.