The in vitro activity of daptomycin against 224 current gram-positive clinical isolates including vancomycinresistant Enterococcus faecium (VREF), methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus spp. (MRSS), and penicillin-resistant Streptococcus pneumoniae (PRSP) was evaluated. The MICs at which 90% of isolates are inhibited for daptomycin and vancomycin, respectively, were as follows: MRSA, 1 and 2 g/ml; MRSS, 1 and 4 g/ml; PRSP, 1 and 0.5 g/ml; and VREF, 2 and >64 g/ml. Daptomycin was bactericidal against 82% of 17 VREF isolates. The antibacterial activity of daptomycin was strongly dependent on the calcium concentration of the medium. Daptomycin was active against all gram-positive cocci tested. (1,3,8,9,13,14,16). The characteristics of daptomycin that distinguish it from vancomycin and teicoplanin are its concentration-dependent bactericidal activity against enterococci and staphylococci, its novel mechanism of action, and its requirement for free calcium at physiologic concentrations (4-7). Hence, careful control of the concentration of free ionized Ca 2ϩ in the test medium is needed in order for daptomycin to exert its antibacterial activity (P. Fuchs, A. Barry, and S. Brown, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 350, p. 199, 1999). The object of this study was to compare the antibacterial activity of daptomycin to that of vancomycin against recently isolated resistant gram-positive cocci and to demonstrate its bactericidal effect against VRE and MRSA.All bacterial isolates were unique, recently isolated, grampositive cocci that included 54 MRSA isolates, 27 methicillin- The MICs of the antibiotics for the staphylococci were determined by agar dilution. The MICs for S. pneumoniae, S. pyogenes, and enterococci were determined by broth microdilution with the NCCLS-recommended medium and 0.05-ml inocula. The minimum bactericidal concentrations (MBCs) were determined from microtiter plates (after the MICs were recorded) by plating 0.01 ml from all wells that showed no growth onto blood agar plates (BAPs). All methodologies followed the guidelines of the National Committee for Clinical Laboratory Standards (NCCLS) (10-12).Daptomycin was provided by Cubist Pharmaceuticals Inc., Cambridge, Mass.; vancomycin was purchased from Sigma Chemical Co., St. Louis, Mo.S. aureus ATCC 29213, S. aureus ATCC 43300 (MRSA), E. faecalis ATCC 29212, and S. pneumoniae ATCC 49619 were used as controls.The effect of the calcium concentration on the antibacterial activity was measured by time-kill studies in media with increasing concentrations of calcium. In addition, the MICs of daptomycin against 23 VREF isolates were determined by microdilution in the following media: base Mueller-Hinton (MH) broth (assumed no calcium supplementation); base MH broth supplemented with 25 mg (broth A), 50 mg (broth B), or 75 mg Ca 2ϩ (broth C) of Ca 2ϩ per liter, or MH II (labeled ϳ20 to 25 mg of Ca 2ϩ per liter; (BBL, Cockeysville, Md.) (11). To determine the exact concentrati...