REVISED APPROACH FOR IDENTIFICATION AND DETECTION OF AMPICILLIN AND VANCOMYCIN RESISTANCE IN ENTEROCOCCUS SPECIES BY USING MICROSCAN PANELS

The frequency of antimicrobial agent-resistant enterococci is increasi ng, making accurate identification and screening for susceptibility es sential, We evaluated the ability of MicroScan Positive Breakpoint Com bo Type 6 panels (Dade MicroScan Inc., West Sacramento, Calif,) to ide ntify Enterococcus species and to detect ampicillin and vancomycin res istance, A total of 398 well-characterized Enterococcus isolates from two institutions were inoculated into MicroScan panels, into conventio nal biochemical assays, and into ampicillin and vancomycin agar diluti on media, Resistance was verified by the broth macrodilution method, M icroScan panels accurately detected resistance to ampicillin in 132 of 132 enterococcal isolates, while three isolates for which the MICs we re <16 mu g/ml were classified incorrectly by MicroScan panels as resi stant, No beta-lactamase-producing enterococci were detected. All 64 i solates shelving resistance to vancomycin (MICs greater than or equal to 32 mu g/ml) were correctly classified by MicroScan panels, Seven is olates for which the vancomycin MICs were 8 and 16 mu g/ml were incorr ectly classified as susceptible by MicroScan panels, while eight isola tes for which the MICs were 4 mu g/ml were incorrectly labeled as inte rmediate, Fourteen of these 15 isolates were subsequently identified a s motile enterococci. Overall, there were three major errors in suscep tibility testing for ampicillin and 15 minor errors for vancomycin, Co nventional testing confirmed the identity of 181 Enterococcus faecalis isolates, 157 E. faecium isolates, and 60 isolates of other species; however, 56 of these 60 isolates were misidentified by the MicroScan p anels, After recognition of this problem, a reviased approach which in cluded tests for pigment, motility, and sucrose fermentation was devis ed, In combination with these additional assays, the conventional Micr oScan panels accurately identified the 56 originally misidentified iso lates, In summary, the ability of MicroScan panels to detect vancomyci n and ampicillin resistance in enterococci was confirmed, Our study fo und that the inability of MicroScan panels to identify enterococci oth er than E. faecalis and E. faecium can be compensated for by the addit ion of standard assays.