Enterococci: An old bug has learned new tricks

Detection of vancomycin resistance in enterococci by the Vitek AMS system

The ability of the Vitek AMS system to detect vancomycin resistance in enterococci was evaluated by comparing the results to an agar dilution screen method. Of 100 strains tested, 43 were resistant and 51 were susceptible by both tests. Two strains were intermediate by Vitek but susceptible by agar screen, one was intermediate by Vitek but resistant by agar screen, and four were susceptible by Vitek but resistant by agar screen. When the Vitek intermediate and resistant results were combined, the false resistant rate was 4.4% and false susceptible rate was 8.5%. In its current format, the Vitek system appears not to have acceptable accuracy for the detection of vancomycin resistance in enterococci.

Epidemiologic analysis and genotypic characterization of a nosocomial outbreak of vancomycin-resistant enterococci

We are reporting on a nosocomial outbreak of 213 cases of vancomycin-resistant enterococcus infection involving 2,812 enterococcal isolates from patients over a period of 36 months. In 1990, the Enterococcus faecium vancomycin susceptibility rate was found to be 85.7% (36 of 42 cases), and an incidence of 10.9% (42 of 383) was noted. The 1991 data showed E. faecium with a vancomycin susceptibility rate of 61.8% (110 of 178) and an incidence of 26.0% (178 of 684). Subsequently, in 1992, the incidence of E. faecium increased to 34.0% (599 of 1,745), with a decreased vancomycin susceptibility rate of 25.8% (155 of 599). The E. faecalis vancomycin susceptibility rate remained near 97% (1,768 of 1,823) over the 36-month period. Of 115 vancomycin-resistant enterococcus (VRE) clinical isolates identified by the MicroScan MIC Combo-6 panels (Baxter Healthcare, Sacramento, Calif.), the agar dilution method indicated the resistance rate to be 92.3% (106 of 115) (high level), 3.5% (4 of 115) midlevel, and 3.5% (4 of 115) (low level). Genotypic characterization of 32 different VRE isolates by field-inversion gel electrophoresis demonstrated 19 dissimilar restriction endonuclease patterns, with 9 patterns associated with VRE quinolone resistance. Statistical analysis of case-control data for 32 patients with VRE infections indicated a positive association with intrabdominal surgical procedures (odds ratio, 24.12), multidrug therapy (odds ratio, 37.80), preexposure to vancomycin (odds ratio, 20.21), and death (odds ratio, 17.50).