Comparison of daptomycin MIC results by DIN, NCCLS, SFM, and SRGA methods for 297 Gram-positive organisms

Daptomycin is a novel lipopeptide antibiotic with potent in vitro antibacterial activity against Gram-positive pathogens. For daptomycin minimal inhibitory concentration (MIC) testing, National Committee for Clinical Laboratory Standards (NCCLS) recommends the use of broth containing physiological levels of calcium (50 microg/ml). The daptomycin susceptibility of 297 organisms was determined by NCCLS (Mueller-Hinton (MH) broth), Deutsches Institut für Normung (DIN; isotonic broth), Société Française de Microbiologie (SFM; three batches MH agar), and Swedish Reference Group for Antibiotics (SRGA; PDM agar). All media were supplemented to 50 microg/ml Ca(2+). There was good correlation between DIN and SFM methods (for staphylococci) with NCCLS results. Enterococci MICs using SFM methods were one to three dilutions lower and pneumococci results were one dilution higher than NCCLS. SRGA results were higher than NCCLS by one to four dilutions. Use of isotonic agar is an accepted alternative to isosensitest agar for the DIN method.