The effect of vancomycin on the structure of vancomycin-susceptible and -resistant Enterococcus faecium strains

Williamson et al. previously showed that a strain of Enterococcus faecium with low-level resistance to vancomycin grown in the presence of one-half the MIC of vancomycin showed bizarre cell forms with abnormal septa without apparent cell division. In the current work, 10 strains of E. faecium susceptible to vancomycin (MIC, 400 mu g /ml) which were also resistant to teicoplanin (MIC, >40 mu g/ml) were DNA fingerprinted to exclude the possibility of strain duplication. Vancomycin at one-half the MIC (0.3 to 0.6 mu g/ml for susceptible and 200 to 250 mu g/ml for resistant strains) was added to 10-ml volumes of Trypticase soy broth (TSB) which were inoculated with 100 mu l of a 1/10 dilution of the respective 24-h TSB cultures. The products of incubation were sampled periodically throughout the 24-h period of incubation for Gram staining. Two strains resistant to vancomycin, grown with vancomycin for 12 h, were washed and reincubated in TSB, and Gram stains were prepared hourly for 6 h. The vancomycin-susceptible and three vancomycin-resistant strains, from the above-described strains, were grown for 12 h with or without vancomycin and examined by electron microscopy by a technique previously described. Cell size was determined from photographs at final magnifications of x20,000. The means and standard deviations were calculated and statistically analyzed.