Identification of oxyresveratrol, a food-as-medicine substrate, as a novel EPI that targets the RND efflux pumps of Serratia surfactantfaciens sp. nov. YD25 T

Serratia marcescens is an opportunistic pathogen with increasing drug resistance. Serratia surfactantfaciens sp. nov. YD25T, a novel strain with multidrug resistance, shares similarities with S. marcescens. This study investigated the resistance mechanisms of YD25, with a focus on efflux pumps and their regulators. Bioinformatics analysis revealed numerous resistance genes in YD25, including five RND-type efflux pumps, SdeAB, SdeCDE, SdeGH, SdePQ-OmsA, and SdeXY. Oxyresveratrol, evernic acid, and morin were identified as efflux pump inhibitors, with oxyresveratrol showing the strongest effect targeting SdeXY and sensitizing YD25 to seven antibiotics. Among the RND pumps, SdeXY and SdeCDE are constitutively expressed and are regulated negatively by AcrR and positively by BaeSR, respectively. YD25 harbors four global regulators, including RamA, SoxS, Rob, and a novel Roblike protein. The deletion of these genes downregulated SdeXY and SdeCDE, increasing antibiotic susceptibility and confirming their role as positive regulators. Rob and Roblike exhibited functional overlap. This study elucidates the RND efflux pumps and their regulators in YD25, advancing the understanding of Serratia resistance mechanisms. Oxyresveratrol, a newly identified SdeXY inhibitor, offers potential for combination therapies against drug-resistant Serratia.IMPORTANCEThis study reveals novel insights into Serratia surfactantfaciens YD25's multidrug resistance, identifying RND efflux pumps (e.g., SdeXY and SdeCDE) as key drivers. The key discoveries include the following: (i) oxyresveratrol, a natural compound functioning as an SdeXY efflux pump inhibitor, effectively reverses bacterial resistance, thereby restoring the clinical utility of antibiotics that had been rendered nearly obsolete by drug resistance, and (ii) YD25 uniquely possesses two Rob regulators (Rob and Roblike). Despite their different origins, both positively regulate RND pump function-a finding reported for the first time in bacteria. These findings highlight the potential of natural efflux pump inhibitors against Serratia infections while uncovering an unusual regulatory redundancy in bacterial resistance.