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"Hidden" Clindamycin Resistance in Erythromycin-Resistant Staphylococci: Presented at ICAAC
By W. A. Thomasson, PhD
CHICAGO, IL -- September 19, 2003 -- Erythromycin-resistant staphylococci should be assumed to be clindamycin-resistant as well, according to a paper presented here September 14th at the 43rd Interscience Conference on Antimicrobial Agents and Chemotherapy.
Most erythromycin resistance in Staphylococcus bacteria results from inducible ribosomal methylase. Such bacteria will appear as erythromycin-resistant but clindamycin-sensitive on standard tests, because erythromycin induces this enzyme quickly while clindamycin does so slowly.
Michael R. Jacobs, MD, PhD, Case Western Reserve University, Cleveland, Ohio, and colleagues there and at Hershey Medical Center in Hershey, Pennsylvania, studied 153 clinical isolates of S. aureus and S. epidermidis that appeared as being erythromycin-resistant and clindamycin-sensitive on standard tests. These organisms were examined using a double-disk diffusion method that gives distinct patterns for true sensitivity, slowly inducible resistance, and early resistance.
The tests revealed that 120 of the 153 strains had inducible resistance. Genetic analysis revealed that in all of the truly clindamycin-sensitive strains, the erythromycin resistance was due to an efflux pump mechanism. Testing on a limited number of strains with inducible resistance showed that, as expected, all carried the ribosomal methylase gene.
Dr. Jacobs and his colleague C. E. Good, MD, noted that their laboratory routinely reports all erythromycin-resistant staphylococci as being clindamycin-resistant. This is by no means universal, however. Elsewhere, the decision may be entirely up to the clinician.
Dr. Good noted that the clinical outcome of using clindamycin to treat inducibly clindamycin-resistant staphylococci is not clear: The few cases reported so far presented conflicting results. What is clear is that a subset of erythromycin-resistant staphylococci exists that could be safely treated with clindamycin if adequate information were available.
The researchers urge use of methods that will clearly differentiate strains with inducible resistance from those truly sensitive.
[Study title: Mechanisms of Resistance and Clinical Implications of Erythromycin. Abstract C2-87]
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