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Antimicrobial Agents and Chemotherapy, November 2008, p. 3863-3867, Vol. 52, No. 11
0066-4804/08/$08.00+0     doi:10.1128/AAC.00399-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Proposed MIC and Disk Diffusion Microbiological Cutoffs and Spectrum of Activity of Retapamulin, a Novel Topical Antimicrobial Agent{triangledown}

Maria M. Traczewski* and Steven D. Brown

The Clinical Microbiology Institute, Inc., Wilsonville, Oregon

Received 24 March 2008/ Returned for modification 28 April 2008/ Accepted 12 August 2008

Retapamulin, the first pleuromutilin antimicrobial agent approved for the topical treatment of skin infections in humans, was tested against 987 clinical isolates representing 30 species and/or resistance groups. MICs were determined along with disk diffusion zone diameters using a 2-µg disk. Population distribution and MIC versus disk zone diameter scattergrams were analyzed to determine microbiological MIC cutoff values and inhibition zone correlates. Minimum bactericidal concentrations were performed on a smaller subset of key species. The retapamulin MIC90 against 234 Staphylococcus aureus isolates and 110 coagulase-negative staphylococci was 0.12 µg/ml. Retapamulin MIC90s ranged from 0.03 to 0.06 µg/ml against beta-hemolytic streptococci including 102 Streptococcus pyogenes, 103 Streptococcus agalactiae, 59 group C Streptococcus, and 71 group G Streptococcus isolates. The MIC90 against 55 viridans group streptococci was 0.25 µg/ml. Retapamulin had very little activity against 151 gram-negative bacilli and most of the Enterococcus species tested. Based on the data from this study, for staphylococci, MICs of ≤0.5, 1, and ≥2 µg/ml with corresponding disk diffusion values of ≥20 mm, 17 to 19 mm, and ≤16 mm can be proposed for susceptible, intermediate, and resistant microbiological cutoffs, respectively. For beta-hemolytic streptococci, a susceptible-only MIC of ≤0.25 µg/ml with a corresponding disk diffusion value of ≥15 mm can be proposed for susceptible-only microbiological cutoffs.

* Corresponding author. Mailing address: The Clinical Microbiology Institute, Inc., 9725 SW Commerce Circle, Suite A-1, Wilsonville, OR 97070. Phone (503) 682-3232. Fax: (503) 682-2065. E-mail: mtrac{at}clinmicroinst.com

{triangledown} Published ahead of print on 25 August 2008.

Antimicrobial Agents and Chemotherapy, November 2008, p. 3863-3867, Vol. 52, No. 11
0066-4804/08/$08.00+0     doi:10.1128/AAC.00399-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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