Detection of daptomycin-nonsusceptible strains using the Neo-Sensitab prediffusion method

Two prediffusion methods with daptomycin (DAP) Neo-Sensitabs were evaluated against a challenge set of 30 Staphylococcus aureus isolates and 30 enterococci. DAP Neo-Sensitabs were prediffused for either 8 or 20 h on Mueller-Hinton agar. Inhibition zones were plotted versus Etest MIC values determined on the same prediffused agar. A generalization to the genus level of the manufacturer's suggested Neo-Sensitabs breakpoints for staphylococci and enterococci was used to interpret the results. DAP-susceptible and DAP-nonsusceptible enterococci, Enterococcus faecium in particular, were not reliably discriminated using a 20-h prediffusion method and the manufacturer's suggested breakpoints. Further development of this testing methodology, such as changing the format to a susceptibility screen followed by a confirmatory MIC, is needed to accurately categorize the DAP susceptibility of enterococcal isolates. Prediffusion for either 8 or 20 h with DAP Neo-Sensitabs discriminated between susceptible and nonsusceptible S. aureus with minimal errors. Both prediffusion methods also detected changes in MIC values between isogenic pairs of susceptible and nonsusceptible S. aureus. These results suggest that a multisite evaluation of either prediffusion method with DAP Neo-Sensitabs against a larger collection of S. aureus is warranted.