Erythromycin-induced resistance to clindamycin in Staphylococcus aureus

To describe the incidence of erythromycin-induced resistance to clindamycin in a sample of Staphylococcus aureus isolates.

100 erythromycin-resistant and clindamycin-sensitive S. aureus were collected as a convenience sample from February to August 2003. Inducible clindamycin resistance was identified using the D-zone disc method.

Of the 100 Staphylococcus aureus isolates, 64 were methicillin sensitive (MSSA) and 36 were methicillin resistant (MRSA). Of the 64 MSSA isolates, 22 (34%) had inducible resistance. Of the 36 MRSA isolates, 4 (11%) had inducible resistance. Overall, 26% of these clindamycin sensitive S. aureus isolates, exhibited inducible resistance to clindamycin.

In this sample, MSSA isolates were almost three times more likely to have inducible MLS resistance compared to MRSA isolates. Inducible resistance may compromise the efficacy of clindamycin. The frequency of inducible resistance in this series of "clindamycin sensitive" S. aureus isolates is 26%. It is likely that the true percentage of clindamycin resistance is being underestimated since testing for inducible resistance is not routinely performed.