129. LiaX, a Member of the LiaFSR System, is Essential for Cell Envelope Adaptation System in <i>Enterococcus faecium</i>

Abstract Background Daptomycin (DAP) is an important antibiotic for enterococci. Our previous studies identified LiaX as a surface-exposed protein that mediator of cell envelope homeostasis in Enterococcus faecalis (Efs) upon exposure to DAP via activation the LiaFSR system. encodes soluble with N-terminal α-helices, and C-terminus which β-pleated sheets. Its role E. faecium, species clinical relevance, remains unknown. In this work, we aim elucidate function faecium (Efm). Methods We used commensal strain Efm TX1330RF (DAP minimum inhibitory concentration [MIC] 3 μg/ml), its ΔliaR derivative (TX1330RFΔliaR [DAP MIC 0.125 μg/ml]) targeted liaX gene mutagenesis. Using PheS* counterselection system, aimed create truncation or in-frame deletion liaX. Mutants were characterized by determination visualization anionic phospholipid domains 10-N-nonyl-acridine orange (NAO). The nisin-controlled expression vector, pMSP3535, was express from (LiaXTX1330RF) Efs host OG1RFΔliaX, lacks Expression (LiaXOG1RF) control. Results presence liaR, unable after many attempts. TX1330RFΔliaR, successfully created liaX, TX1330RFΔliaRliaX μg/ml). NAO staining all strains TX1330RF, TX1330RFΔliaR ΔliaX mutant showed localization at septa cells. Interestingly, multiple attempts complement liaR also futile. OG1RFΔliaX demonstrated microdomains redistributed away septa. LiaXOG1RF restored septal polar regions while LiaXTX1330RF did not show any significant difference fluorescence compared parental (Efs OG1RFΔliaX). Conclusion findings suggest has divergent functions enterococci may be essential redponse DAP. Efforts resistance are ongoing. Disclosures Cesar A. Arias, MD, PhD, Entasis Phramceuticals: Grant/Research Support|MeMed Diagnostics: Support|Merck: Support.