Resistance to linezolid: Characterization of mutations in rRNA and comparison of their occurrences in vancomycin-resistant enterococci

To assess the potential for emergence of resistance during the use of linez olid, we tested 10 clinical isolates of vancomycin-resistant enterococci (V RE) (four Enterococcus faecalis, five Enterococcus faecium, and one Enteroc occus gallinarum) as well as a vancomycin-susceptible control (ATCC 29212) strain of E. faecalis, The enterococci were exposed to doubling dilutions o f linezolid for 12 passes. After the final passage, the linezolid plate gro wing VRE contained a higher drug concentration with E. faecalis than with E . faecium. DNA sequencing of the 23S rRNA genes revealed that linezolid res istance in three E. faecalis isolates was associated with a guanine to urac il transversion at bp 2576, while the one E. faecium isolate for which the MIC was 16 mug/ml contained a guanine to adenine transition at bp 2505.