Calcium-supplemented daptomycin Etest strips for susceptibility testing on Iso-Sensitest agar

Iso-Sensitest agar (ISA), which is recommended by the BSAC for routine susceptibility testing of staphylococci and enterococci, contains insufficient calcium for testing daptomycin. Isotonic agar supplemented with 50 mg/L calcium has been advocated, but is not routinely available in many laboratories. We evaluated a daptomycin Etest that incorporates a constant level of calcium throughout the daptomycin gradient, designed to give an appropriate concentration around the strip during testing, as an alternative for susceptibility testing on ISA.

Ninety-one isolates of Staphylococcus aureus (45 methicillin-susceptible, 46 methicillin-resistant) and 90 enterococci (47 Enterococcus faecalis, 43 Enterococcus faecium) were tested. Daptomycin Etest MICs were determined on ISA, whereas agar dilution MICs were determined in parallel on Isotonic agar supplemented with calcium to 50 mg/L as a control.

The agar dilution and Etest MIC ranges of daptomycin for S. aureus were 0.25-1 mg/L (mode 0.5 mg/L), and 0.125-2 mg/L (mode 0.25 mg/L), respectively. The corresponding MIC values for enterococci were 0.25-4 mg/L (mode, 1 mg/L) and 0.125-4 mg/L (mode, 2 mg/L). For staphylococci, 86% of the Etest MIC results were within one dilution of the agar dilution values, and for enterococci, 90% of the Etest MIC results met these criteria. When results from the two methods were not identical, there was a tendency for the Etest MIC values to be lower than the agar dilution values.

This study shows that calcium-supplemented daptomycin Etests on ISA are an accurate and convenient alternative to calcium-supplemented Isotonic agar.