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


METHODS
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.


RESULTS
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.


CONCLUSIONS
This study shows that calcium-supplemented daptomycin Etests on ISA are an accurate and convenient alternative to calcium-supplemented Isotonic agar.
Johnson
Alan P
AP
Antibiotic Resistance Monitoring and Reference Laboratory, Health Protection Agency, Specialist and Reference Microbiology Division, Colindale, London NW9 5HT, UK. Alan.Johnson@hpa.org.uk
Mushtaq
Shazad
S
Warner
Marina
M
Livermore
David M
DM
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology*; D003470:Culture Media; D017576:Daptomycin / Q000494:pharmacology*; D016983:Enterococcus / Q000187:drug effects* / Q000235:genetics; D016908:Gram-Positive Bacterial Infections / Q000382:microbiology; D006801:Humans; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests / Q000379:methods*; D011934:Reagent Strips; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics; D006113:United Kingdom
D000900:Anti-Bacterial Agents; D003470:Culture Media; D011934:Reagent Strips; D017576:Daptomycin; D002118:Calcium