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Abstract：
Combining antibiotics with resistance reversing agents is a key strategy to overcome bacterial resistance. Upon screening antimicrobial activities of plants used in traditional medicine, we found that a leaf dichloromethane extract from the shea butter tree (Vitellaria paradoxa) had antimicrobial activity against methicillin-resistant Staphylococcus aureus (MRSA) with further evidence of synergy when combined with β-lactams. Using HPLC-MS, we identified ursolic (UA) and oleanolic acids (OA) in leaves and twigs of this species, and quantified them by HPLC-UV as the major constituents in leaf extracts (21% and 6% respectively). Both pure triterpenic acids showed antimicrobial activity against reference and clinical strains of MRSA, with MICs ranging from 8-16 mg/L for UA to 32-128 mg/L for OA. They were highly synergistic with β-lactams (ampicillin and oxacillin) at subMIC concentrations. Reversion of MRSA phenotype was attributed to their capacity to delocalize PBP2 from the septal division site, as observed by fluorescence microscopy, and to disturb thereby peptidoglycan synthesis. Moreover, both compounds also inhibited β-lactamases activity of living bacteria (as assessed by inhibition of nitrocefin hydrolysis), but not in bacterial lysates, suggesting an indirect mechanism for this inhibition. In a murine model of subcutaneous MRSA infection, local administration of UA was synergistic with nafcillin to reduce lesion size and inflammatory cytokine (IL-1β) production. Thus, these data highlight the potential interest of triterpenic acids as resistance reversing agents in combination with β-lactams against MRSA.
摘要：
将抗生素与耐药性逆转剂结合是克服细菌耐药性的关键策略。在筛选用于传统医学的植物的抗菌活性时，我们发现，来自乳木果树（Vitellariaparadoxa）的叶二氯甲烷提取物对耐甲氧西林金黄色葡萄球菌（MRSA）具有抗菌活性，并进一步证明与β-内酰胺类联用时具有协同作用。使用HPLC-MS，我们在该物种的叶子和树枝中鉴定了熊果酸（UA）和齐墩果酸（OA），并通过HPLC-UV定量它们作为叶提取物中的主要成分（分别为21％和6％）。两种纯三萜酸对MRSA的参考和临床菌株均具有抗菌活性，UA的MIC范围为8-16mg/L，OA为32-128mg/L。在亚MIC浓度下，它们与β-内酰胺（氨苄青霉素和苯唑西林）具有高度协同作用。MRSA表型的逆转归因于它们能够将PBP2从间隔分裂部位离域，如通过荧光显微镜观察到的，从而干扰肽聚糖的合成。此外，这两种化合物还抑制活细菌的β-内酰胺酶活性（通过抑制硝化氮水解评估），但不是在细菌裂解物中，暗示了这种抑制的间接机制。在皮下MRSA感染的鼠模型中，UA的局部给药与纳夫西林具有协同作用,可减少病变大小和炎性细胞因子(IL-1β)的产生.因此，这些数据凸显了三萜酸作为抗MRSA抗性逆转剂与β-内酰胺类联用的潜在价值.
