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Journal of Clinical Microbiology, June 2006, p. 2126-2129, Vol. 44, No. 6
0095-1137/06/$08.00+0     doi:10.1128/JCM.00076-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Assessment of Two Commercial Susceptibility Test Methods for Determination of Daptomycin MICs

James H. Jorgensen and Sharon A. Crawford*

Department of Pathology, The University of Texas Health Science Center, San Antonio, Texas 78229-3900

Received 12 January 2006/ Returned for modification 21 March 2006/ Accepted 12 April 2006

Daptomycin is a lipopeptide antibiotic with activity against several important gram-positive bacterial pathogens, including drug-resistant staphylococci and enterococci. Because the mechanism of action of daptomycin is calcium-dependent depolarization of the cell membrane, susceptibility testing requires medium supplemented with a physiological level of calcium. This study assessed two Food and Drug Administration-cleared commercial test devices for determination of daptomycin MICs, Etest and JustOne. A collection of 220 selected isolates, including Staphylococcus aureus, coagulase-negative staphylococci, Enterococcus faecalis, E. faecium, E. avium, E. durans, E. casseliflavus, and E. gallinarum, were tested by both methods. Included in the collection were 22 S. aureus and 14 Enterococcus sp. isolates that were recovered from patients and were nonsusceptible on the basis of the daptomycin MICs. As the reference method for comparison, all isolates were tested by the Clinical and Laboratory Standards Institute broth microdilution method incorporating cation-adjusted Mueller-Hinton broth with 50 µg/ml calcium. Daptomycin MICs agreed, within 1 twofold dilution, for 97% of the isolates by Etest and for 100% by JustOne. However, daptomycin MICs determined by Etest were 1 dilution lower than the reference MICs for 65% of the Enterococcus sp. isolates tested. This resulted in 28.5% very major (VM) errors (4/14) with enterococci (all E. faecium) but none (0/22) with staphylococci. Use of JustOne yielded MICs that were 1 dilution lower than the reference MICs for 69% of the staphylococci and 25% of the enterococci. This resulted in 13.6% VM errors (3/22) with staphylococci and 14.3% VM errors (2/14) with enterococci. The manufacturer-recommended JustOne inoculum preparation resulted in mean colony counts of only 5 x 104 to 1 x 105 CFU/ml in the wells of the strip. Increasing the inoculum to 3 x 105 to 4 x 105 CFU/ml eliminated two of five VM errors upon retesting. No major interpretive errors occurred with either device. In summary, daptomycin MICs generated by the Etest or JustOne method generally agreed within 1 dilution of the reference daptomycin MICs. However, both devices produced slightly lower MICs that resulted in some VM errors.

* Corresponding author. Mailing address: Department of Pathology University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900. Phone: (210) 567-4088. Fax: (210) 567-2367. E-mail: jorgensen{at}uthscsa.edu.

Journal of Clinical Microbiology, June 2006, p. 2126-2129, Vol. 44, No. 6
0095-1137/06/$08.00+0     doi:10.1128/JCM.00076-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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