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Antimicrobial Agents and Chemotherapy, June 2001, p. 1919-1922, Vol. 45, No. 6
0066-4804/01/$04.00+0   DOI: 10.1128/AAC.45.6.1919-1922.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

In Vitro Activities of Daptomycin against 2,789 Clinical Isolates from 11 North American Medical Centers

Arthur L. Barry,* Peter C. Fuchs, and Steven D. Brown

The Clinical Microbiology Institute, Wilsonville, Oregon 97070

Received 22 December 2000/Returned for modification 9 February 2001/Accepted 15 March 2001

The in vitro activity of daptomycin is affected by the concentration of calcium cations in the test medium. Mueller-Hinton broth is currently adjusted to contain 10 to 12.5 mg of magnesium per liter and 20 to 25 mg of calcium per liter, but for testing of daptomycin, greater concentrations of calcium (50 mg/liter) are recommended to better resemble the normal concentration of ionized calcium in human serum. Two levels of calcium were used for broth microdilution tests of 2,789 recent clinical isolates of gram-positive bacterial pathogens. MICs of daptomycin were two- to fourfold lower when the broth contained additional calcium. For most species, however, the percentages of strains that were inhibited by 2.0 µg of daptomycin per ml were essentially identical with the two broth media. Enterococci were the important exception; i.e., 92% were inhibited when tested in calcium-supplemented broth but only 35% were inhibited by 2.0 µg/ml without the additional calcium. This type of information should be considered when selecting criteria for defining in vitro susceptibility to daptomycin.

* Corresponding author. Mailing address: 9725 S.W. Commerce Circle, Suite A1, Wilsonville, OR 97070. Phone: (503) 682-3232. Fax: (503) 682-2065. E-mail: cmi{at}hevanet.com.

Antimicrobial Agents and Chemotherapy, June 2001, p. 1919-1922, Vol. 45, No. 6
0066-4804/01/$04.00+0   DOI: 10.1128/AAC.45.6.1919-1922.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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