The oxazolidinone linezolid is a new antibacterial approved for marketing in 2000 that inhibits bacterial protein synthesis (5,20,41). It represents a new structural class of antibiotics, with activity against several gram-positive organisms, including several resistant strains. Linezolid has been shown to be effective in treating nosocomial pneumonia caused by methicillinsusceptible and -resistant Staphylococcus aureus or multidrugresistant Streptococcus pneumonia and skin and soft tissue infections caused by methicillin-susceptible and -resistant Staphylococcus aureus, Streptococcus pyogenes, and Streptococcus agalactiae. It is also effective against community-acquired pneumonia caused by methicillin-susceptible S. aureus, multidrug-resistant S. pneumoniae, and vancomycin-resistant Enterococcus faecium infections (15, 31).The oxazolidinones inhibit bacterial protein synthesis, although the exact details concerning the mechanism(s) of inhibition are still emerging. Early results demonstrated that the oxazolidinone eperezolid binds to 50S but not 30S ribosomal subunits. Furthermore, binding was inhibited by chloramphenicol and lincomycin (27). Cross-linking studies have been carried out to identify the sites of oxazolidinone binding. Using ribosomes from Escherichia coli, a number of nucleotide residues in domain V of 23S rRNA were identified, as well as residue A864 of 16S rRNA (28). Colca et al. carried out crosslinking experiments using intact Staphylococcus aureus and showed that tRNA, two ribosomal proteins, and nucleotide A2602 of 23S rRNA all were labeled by the cross-linker (10).The results from mapping oxazolidinone resistance mutations agree with the cross-linking studies. Linezolid-resistant mutants of Halobacterium halobium were isolated and shown to contain single point mutations in the central loop of domain V of 23S rRNA (24). Likewise, Escherichia coli oxazolidinoneresistant mutants contained G2032A and G2447A mutations, which also are in domain V (4, 47

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