Effect of quinupristin/dalfopristin alone or in combination with vancomycin on the structure of Enterococcus faecium.
Twenty strains of Enterococcus faecium susceptible to quinupristin/dalfopristin (< 2 mg/l) were DNA fingerprinted to exclude strain duplication. Ten strains were susceptible to vancomycin (minimal inhibitory concentration [MIC] < 2 mg/l) and 10 were resistant to vancomycin (MIC > 400 mg/l). Vancomycin at 1/2 MIC, quinupristin/dalfopristin at 1/4 MIC and their combination, except for a tube control, was added to 10 ml trypticase soy broth tubes which were planted with the respective 24-h trypticase soy broth cultures. The products of incubation were sampled periodically throughout 24 h for gram stain and electron microscopy. Cell size was measured on photographs at 20,000x final magnification and results were statistically analyzed. The cells of all strains of Enterococcus faecium exposed for 12 h to quinupristin/dalfopristin were comparable in size to the control, Most cells, however, showed areas of low density of ribosome in the center of the cells. The cells of Enterococcus faecium resistant to vancomycin exposed to vancomycin were larger than the controls with means of 1.96 micron -2.07 micron versus 1.16 micron (p < 0.001); these cells consisted of individual organisms connected by wide cross walls of abnormal fibrous structure. Enterococcus faecium sensitive to vancomycin exposed to vancomycin remained comparable to the control. The combination of quinupristin/dalfopristin plus vancomycin produced large cells with multiple abnormal cross walls in both vancomycin-resistant and vancomycin-sensitive Enterococcus faecium. The addition of quinupristin/dalfopristin to vancomycin appears to modify the vancomycin-susceptible strains to respond to vancomycin in the same manner as do the vancomycin-resistant organisms.