JAC Advance Access published online on March 31, 2004
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkh198
© 2004 by The British Society for Antimicrobial Chemotherapy
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Original article
1 Antibiotic Resistance Monitoring and Reference Laboratory,
Health Protection Agency, Specialist and
Reference Microbiology Division, Colindale, London NW9 5HT, UK
* Corresponding author. E-mail: Alan.Johnson{at}hpa.org.uk.
Received 27 January 2004
; revised 16 February 2004
; accepted 17 February 2004
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.
Keywords: lipopeptides, enterococci, Staphylococcus
aureus, MRSA
Calcium-supplemented daptomycin Etest strips for
susceptibility testing on Iso-Sensitest agar
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