Identification of enterococci and determination of their glycopeptide resistance in German and Austrian clinical microbiology laboratories

Objectives: To determine exactly how German and Austrian routine laboratories perform tests for the identification of enterococci and determination of their glycopoptide resistance.
Methods: Six enterococcal test strains with different types of glycopeptide resistance ( Enterococcus faecium , VanA; E. faecalis , VanA; E. faecium , VanB; E. faecalis , VanB; E. gallinarum , VanC₁; E. casseliflavus , VanC₂) were sent as anonymous isolates to 73 clinical microbiology laboratories (65 in Germany; eight in Austria). The participating laboratories had to identify the strains up to the species level and to determine their antibiotic susceptibilities to the glycopeptides vancomycin and teicoplanin by the test method(s) that are used daily in the corresponding laboratories.
Results: The analysis of the results received from 62 laboratories (56 from Germany, six from Austria) demonstrated that the most used routine method in susceptibility testing was the agar diffusion test, followed by the Etest, and the microbroth dilution procedure. The majority of participants had no difficulties in susceptibility testing with the VanA-type strains. However, the agar diffusion test was often not able to recognize clearly the VanB and VanC strains; some problems also arose with VanB isolates in the Etest. With the microbroth dilution method, the corresponding type of glycopeptide resistance was correctly determined in the majority of enterococcal test strains. Difficulties also arose in identification, especially with the VanC strains ( E. gallinarum and E. casseliflavus ), which were often falsely identified as E. faecium. The reasons for these errors are obviously based on the lack of important tests (such as motility and presence of a yellow pigment) in some commercially available identification test kits.