Bactericidal and Synergistic Effects of <i>Lippia origanoides</i> Essential Oil and Its Main Constituents against Multidrug-Resistant Strains of <i>Acinetobacter baumannii</i>

Bacterial resistance in <i>Acinetobacter baumannii</i> is a significant public health challenge, as these bacteria can evade multiple antibiotics, leading to difficult-to-treat infections with high mortality rates. As part of the search for alternatives, essential oils from medicinal plants have shown promising antibacterial potential due to their diverse chemical constituents. This study evaluated the antibacterial, antibiofilm, and synergistic activities of the essential oil of <i>Lippia origanoides</i> (EOLo) and its main constituents against multidrug-resistant clinical isolates of <i>A. baumannii</i>. Additionally, the antibacterial and antibiofilm potential of a nanoemulsion containing carvacrol (NE-CAR) was assessed. EOLo was extracted through hydrodistillation, and its components were identified via gas chromatography coupled with mass spectrometry. The <i>A. baumannii</i> isolates (<i>n</i> = 9) were identified and tested for antimicrobial susceptibility using standard disk diffusion methods. Antibacterial activity was determined by broth microdilution, while antibiofilm activity was measured using colorimetric methods with crystal violet and scanning electron microscopy. Synergism tests with antibiotics (meropenem, ciprofloxacin, gentamicin, and ampicillin+sulbactam) were performed using the checkerboard method. The primary constituents of EOLo included carvacrol (48.44%), <i>p</i>-cymene (14.58%), and thymol (10.16%). EOLo, carvacrol, and thymol demonstrated significant antibacterial activity, with carvacrol showing the strongest effect. They were also effective in reducing biofilm formation, as was NE-CAR. The combinations with antibiotics revealed significant synergistic effects, lowering the minimum inhibitory concentration of the tested antibiotics. Therefore, this study confirms the notable antibacterial activity of the essential oil of <i>L. origanoides</i> and its constituents, especially carvacrol, suggesting its potential as a therapeutic alternative for <i>A. baumannii</i> infections.