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Background  Daptomycin (DAP) is a lipopeptide antibiotic targeting membrane anionic phospholipids (APLs) at the division septum, and resistance (DAP-R) has been associated with activation of the  E. faecalis (Efs ) LiaFSR response and redistribution of APL microdomains (predicted to contain cardiolipin) away from the septum.  Efs  encodes two putative cardiolipin synthase genes,  cls1  and  cls2 . While changes in Cls1 are associated with DAP-R, the exact roles of each enzyme in resistance are unknown. This work aims to establish the contributions for both enzymes in the development of DAP-R.    Methods    cls1  and  cls2  were deleted individually and in tandem from  Efs  OG117∆ liaX  (a DAP-R strain with an activated LiaFSR response). Mutants were characterized by DAP minimum inhibitory concentration (MIC) using E-test and localization of APL microdomains with 10-N-nonyl-acridine orange staining. Quantitative PCR (qRT-PCR) was used to study gene expression profiles of  cls1  and  cls2  in  Efs  OG117∆ liaX  relative to  Efs  OG117. Membrane lipid content was analyzed using hydrophilic interaction chromatography-mass spectrometry (HILIC-MS).Results    cls1  was highly upregulated in stationary phase concurrent with a decrease in  cls2  expression. However, independent deletion of  cls1  or  cls2  in the DAP-R background resulted in no significant phenotypic changes from the parent strain. Interestingly, qRT-PCR showed that  cls2  expression was upregulated upon deletion of  cls1  (and vice-versa), suggesting a compensatory role for one enzyme upon deletion of the other (Fig 1). When comparing membrane lipid content between  Efs  OG117∆ liaX ∆ cls1  and  Efs  OG117∆ liaX ∆ cls2,  there were no significant differences in both the overall amount or species of cardiolipin generated, further supporting a potential redundancy between the cardiolipin synthases (Fig 2). Ultimately, double deletion of both  cls  genes lowered the DAP MIC relative to the parent strain and restored septal localization of APL microdomains.Conclusion  Overall, Cls1 has a predominant role in the development of DAP-R in  E. faecalis . However, here, we describe a novel compensatory role for Cls2 under conditions in which there is no functional Cls1 to maintain the DAP-R phenotype.Disclosures    Truc T. Tran, PharmD ,  Merck  (Grant/Research Support)  Cesar A. Arias, M.D., MSc, Ph.D., FIDSA ,  Entasis Therapeutics  (Grant/Research Support) MeMed Diagnostics  (Grant/Research Support) Merk  (Grant/Research Support)

1275. Dynamics of Enterococcus faecalis Cardiolipin Synthase Gene Expression Reveal Compensatory Roles in Daptomycin Resistance