Characterisation of Staphylococcus aureus and Enterococcus faecalis mutants with reduced susceptibility to the investigational oxazolidinone RWJ-416457

RWJ-416457 is a novel investigational oxazolidinone with minimum inhibitory concentrations (MICs) to staphylococci and enterococci that are two- to four-fold lower than those of linezolid. Single-step and serial passage in vitro resistance selection experiments were performed for RWJ-416457 and linezolid with Staphylococcus aureus and Enterococcus faecalis laboratory and clinical isolates. RWJ-416457 selected for resistant mutants in single-step selections at a frequency of ≤1 × 10−10, similar to that of linezolid. In serial passage selection experiments, a G2576T transversion in the domain V region of the 23S rRNA gene was the predominant mutation observed for both oxazolidinones, suggesting similar 23S rRNA binding sites. The associated development of increasing oxazolidinone resistance in E. faecalis (four 23S rRNA alleles) required fewer passages than with S. aureus isolates (six 23S rRNA alleles), and resistance was generally proportionate to the number of mutated (G2576T) 23S rRNA alleles. Fold changes in MICs were similar for both compounds, and MICs for RWJ-416457 remained two- to four-fold lower than those of linezolid for mutants selected by either compound. Serial passage of linezolid with S. aureus OC 2878 yielded a novel A2572T 23S rRNA mutation, whilst the final passages of S. aureus OC 10517 with RWJ-416457 resulted in the apparent loss of a mutated (G2576T) allele 6.