Evaluation of an automated system for the identification and antimicrobial susceptibility testing of enterococci

The performance of a new version of an automated system panel, the Positive Combo Panel Type 11 of MicroScan WalkAway 96 (WA96; Dade Behring) was evaluated and compared to that of reference methods for the identification and for antimicrobial susceptibility testing of the different enterococcal species. A total of 376 enterococcal isolates were tested. The MicroScan WA96 correctly identified 99.6% (266/267), 78.3% (18/23) and 68.6% (59/86) of Enterococcus faecalis, Enterococcus faecium and species other than E. faecalis and E. faecium, respectively. Although low probability of accurate identification was obtained for 37 (9.8%) strains, the system indicated that supplementary tests were necessary for precise identification of 8 (9.3%) among the 86 strains included in the non-faecalis/non-faecium group and of 3 (13.0%) among the E. faecium isolates. In comparison to the agar screening method, the percentage of agreement for detection of resistance markers by the automated system was 90.2% (37/41) for ampicillin, 90.6% (48/53) for high-level resistance to streptomycin (HLRS), 96.4% (80/83) for high-level resistance to gentamicin (HLRG), and 100% (14/14) for vancomycin. The results indicate that the MicroScan WA96 performed well for the identification of E. faecalis and typical E. faecium isolates, and for the detection of resistance to vancomycin and HLRG. However, the system still needs further improvement in order to provide reliable results for the characterization of the other enterococcal species, including atypical variants of E. faecium.