Induction of clindamycin resistance in erythromycin-resistant, clindamycin susceptible and methicillin-resistant clinical Staphylococcal isolates

OBJECTIVE: To demonstrate the in vitro ability of erythromycin to induce clindamycin in erythromycin resistant and clindamycin susceptible clinical isolates of Staphylococci.

METHODS: We studied 291 clinical isolates of erythromycin-resistant (ER-R) clindamycin-susceptible Staphylococci (CL-S) at Almana General Hospitals, Al-Khobar, Dammam, Saudi Arabia during the period from June 2004 to May 2005. The isolates included 70 Staphylococcus aureus, 81 Methicillin Resistant Staphylococcus aureus (MRSA) and 140 coagulase-negative Staphylococci (CNS). We examined these isolates for inducible clindamycin resistance (ICR) by erythromycin induction test using double disc susceptibility test (D-test). Strains producing ICR show flattening of the clindamycin disc zone adjacent to the erythromycin disc.

RESULTS: Of the 291 ER Staphylococci studied, 82 (28%) demonstrated constitutive clindamycin resistance [2 (2.9%) S. aureus, 43 (53%) MRSA and 37 (26%) CNS]. Inducible clindamycin resistance was demonstrated in 113 (38.8%) of Staphylococcal isolates, including 84 (28.9%) from adult patients and 29 (10%) from pediatric patients. The incidence of ICR was 49 (70%) for S. aureus, 35 (43%) for MRSA and 29 (20.7)% for CNS. Overall, 96 (33%) of the isolates remained susceptible to clindamycin and were negative for clindamycin induction [19 (27%) S. aureus, 3 (3.7%) MRSA and 74 (52.8%) CNS].

CONCLUSION: We conclude that a significant number of ER-R CL-S staphylococcal isolates studied were positive for ICR. These isolates should be reported as clindamycin resistant. Given the high rate of inducible resistance to clindamycin in the staphylococcal isolates, we recommend that microbiology laboratories perform erythromycin induction test on all ER-R CL-S staphylococcal isolates prior to reporting clindamycin susceptibility.