Comparative In Vitro Drug Susceptibility Study of Five Oxazolidinones Against Mycobacterium tuberculosis in Hainan, China

Oxazolidinones, novel synthetic antibacterials, inhibit protein biosynthesis and show potent activity against Gram-positive bacteria, including <i>Mycobacterium tuberculosis</i> (MTB). In this study, we aimed to compare the in vitro activity of linezolid (LZD) and four oxazolidinones, including tedizolid (TZD), contezolid (CZD), sutezolid (SZD), and delpazolid (DZD), against multidrug-resistant tuberculosis (MDR-TB) and pre-extensively drug-resistant tuberculosis (pre-XDR-TB) isolates from Hainan. We established their epidemiological cut-off values (ECOFFs) using ECOFFinder software and analyzed mutations in <i>rrl</i> (23S rRNA), <i>rplC</i>, <i>rplD</i>, <i>mce3R</i>, <i>tsnR</i>, <i>Rv0545c</i>, <i>Rv0930</i>, <i>Rv3331</i>, and <i>Rv0890c</i> genes to uncover potential mechanisms of oxazolidinone resistance. This study included 177 MTB isolates, comprising 67 MDR and 110 pre-XDR-TB isolates. Overall, SZD exhibited the strongest antibacterial activity against clinical MTB isolates, followed by TZD and LZD, with CZD and DZD showing equivalent but weaker activity (SZD_MIC50 = TZD_MIC50 < LZD_MIC50 < CZD_MIC50 = DZD_MIC50; SZD_MIC90 < TZD_MIC90 = LZD_MIC90 < CZD_MIC90 = DZD_MIC90). Significant differences in MIC distribution were observed for TZD (<i>p</i> < 0.0001), CZD (<i>p</i> < 0.01), SZD (<i>p</i> < 0.0001), and DZD (<i>p</i> < 0.0001) compared to LZD but not between MDR-TB and pre-XDR-TB isolates. We propose the following ECOFFs: SZD, 0.5 µg/mL; LZD, TZD, and CZD, 1.0 µg/mL; DZD, 2.0 µg/mL. No statistically significant differences in resistance rates were observed among these five drugs (<i>p</i> > 0.05). We found that eight MTB isolates (4.52% [8/177]) resisted these five oxazolidinones. Among these, only one isolate, M26, showed an amino acid substitution (Arg79His) in the protein encoded by the <i>rplD</i> gene, which conferred cross-resistance to TZD and CZD. Three distinct mutations were identified in the <i>mce3R</i> gene; notably, isolate P604 displayed two insertions that contributed to resistance against all five oxazolidinones. However, no significant correlation was observed between mutations in the <i>rrl</i>, <i>rplC</i>, <i>rplD</i>, <i>mce3R</i>, <i>tsnR</i>, <i>Rv0545c</i>, <i>Rv0930</i>, <i>Rv3331</i>, and <i>Rv0890c</i> genes with oxazolidinone resistance in the clinical MTB isolates tested. In summary, this study provides the first report on the resistance of MTB in Hainan to the five oxazolidinones (LZD, TZD, CZD, SZD, and DZD). In vitro susceptibility testing indicated that SZD exhibited the strongest antibacterial activity, followed by TZD and LZD, while CZD and DZD demonstrated comparable but weaker effectiveness. Mutations in <i>rplD</i> and <i>mce3R</i> were discovered, but further research is needed to clarify their role in conferring oxazolidinone resistance in MTB.