Antimicrobial resistance is increasing around the world and the search for effective treatment options, such as new antibiotics and combination therapy is urgently needed. The present study evaluates oregano essential oil (OEO) antibacterial activities against reference and multidrug-resistant clinical isolates of Acinetobacter baumannii (Ab-MDR). Additionally, the combination of the OEO and polymyxin B was evaluated against Ab-MDR. Ten clinical isolates were characterized at the species level through multiplex polymerase chain reaction (PCR) for the gyrB and bla_OXA-51-like genes. The isolates were resistant to at least four different classes of antimicrobial agents, namely, aminoglycosides, cephems, carbapenems, and fluoroquinolones. All isolates were metallo-β-lactamase (MβL) and carbapenemase producers. The major component of OEO was found to be carvacrol (71.0%) followed by β-caryophyllene (4.0%), γ-terpinene (4.5%), p-cymene (3,5%), and thymol (3.0%). OEO showed antibacterial effect against all Ab-MDR tested, with minimum inhibitory concentrations (MIC) ranging from 1.75 to 3.50 mg mL⁻¹. Flow cytometry demonstrated that the OEO causes destabilization and rupture of the bacterial cell membrane resulting in apoptosis of A. baumannii cells (p < 0.05). Synergic interaction between OEO and polymyxin B (FICI: 0.18 to 0.37) was observed, using a checkerboard assay. When combined, OEO presented until 16-fold reduction of the polymyxin B MIC. The results presented here indicate that the OEO used alone or in combination with polymyxin B in the treatment of Ab-MDR infections is promising. To the best of our knowledge, this is the first report of OEO and polymyxin B association against Ab-MDR clinical isolates.

全世界的抗菌素耐药性正在增加，迫切需要寻找有效的治疗选择，例如新抗生素和联合疗法。本研究评估牛至精油（OEO）对鲍曼不动杆菌（Ab-MDR）的参考和多重耐药临床分离株的抗菌活性。另外，针对Ab-MDR评估了OEO和多粘菌素B的组合。通过多重聚合酶链反应（PCR）鉴定了针对gyrB和bla _{OXA-51-like的}十种临床分离株基因。分离物对至少四种不同类型的抗微生物剂具有抗性，即氨基糖苷，头孢类，碳青霉烯类和氟喹诺酮类。所有分离物均为金属-β-内酰胺酶（MβL）和碳青霉烯酶生产者。发现OEO的主要成分是香芹酚（71.0％），其次是β-石竹烯（4.0％），γ-萜品烯（4.5％），对伞花烯（3.5％）和百里酚（3.0％）。OEO对所有测试的Ab-MDR表现出抗菌作用，最小抑菌浓度（MIC）为1.75至3.50 mg mL ^-1。流式细胞术表明，OEO引起细菌细胞膜的失稳和破裂，导致鲍曼不动杆菌细胞凋亡（p <0.05）。使用棋盘试验观察到OEO和多粘菌素B（FICI：0.18至0.37）之间的协同作用。结合使用时，OEO表现出多粘菌素B MIC降低至16倍。此处显示的结果表明，单独或与多粘菌素B结合使用的OEO在治疗Ab-MDR感染方面很有希望。据我们所知，这是OEO和多粘菌素B联合抗Ab-MDR临床分离株的首次报道。