Intracellular Accumulation of Linezolid and Florfenicol in OptrA-Producing Enterococcus faecalis and Staphylococcus aureus

The gene, which confers transferable resistance to oxazolidinones and phenicols, is defined as an ATP-binding cassette (ABC) transporter but lacks transmembrane domains. The resistance mechanism of and whether it involves antibiotic efflux or ribosomal protection remain unclear. In this study, we determined the MIC values of all bacterial strains by broth microdilution, and used ultra-high performance liquid chromatography-tandem quadrupole mass spectrometry to quantitatively determine the intracellular concentrations of linezolid and florfenicol in and . Linezolid and florfenicol both accumulated in susceptible strains and -carrying strains of and No significant differences were observed in the patterns of drug accumulation among JH2-2, JH2-2/pAM401, and JH2-2/pAM401+ , but also among RN4220, RN4220/pAM401, and RN4220/pAM401+ . ANOVA scores also suggested similar accumulation conditions of the two target compounds in susceptible strains and -carrying strains. Based on our findings, the mechanism of -mediated resistance to oxazolidinones and phenicols obviously does not involve active efflux and the OptrA protein does not confer resistance via efflux like other ABC transporters.