Synergistic Antibacterial Effects of Plant Extracts and Essential Oils Against Drug-Resistant Bacteria of Clinical Interest

Abstract

Infectious diseases, the second leading cause of death worldwide, have traditionally been treated with antimicrobials. However, the emergence of drug-resistant microorganisms has driven the need for alternative therapies. This study aimed to assess the antibacterial efficacy of Capparis spinosa crude extracts and five essential oils (EOs) derived from Salvia officinalis, Eucalyptus globulus, Micromeria barbata, Origanum vulgare, and Juniperus excelsa. The EOs were extracted using hydro-distillation, and C. spinosa extracts were obtained using ethanol and acetone solvents. Microdilution assays revealed that O. vulgare EO exhibited the strongest activity against Listeria monocytogenes, Escherichia coli, Salmonella spp., and Brucella melitensis, while C. spinosa demonstrated significant antibacterial effects against L. monocytogenes and notable inhibition of Pseudomonas aeruginosa. The combination of EOs with antibiotics, including M. barbata, J. excelsa, S. officinalis, and E. globulus, enhanced the efficacy of the antibiotics against recalcitrant bacterial strains. The synergistic effects were evaluated through Fractional Inhibitory Concentration Index (FICI) analysis. These findings confirm that the antibacterial efficacy observed in the tested EOs, especially when used in synergy with antibiotics, offers a promising therapeutic strategy to combat antimicrobial resistance.

Keywords: antibacterial activity; antimicrobial resistance; bacterial resistance; crude extract; essential oil; synergism.

Figure 1

Extraction methods of essential oils, Capparis spinosa crude extracts, and evaluation of the antibacterial activity by microdilution.

Figure 2

Minimum inhibitory concentration (%) of Capparis spinosa crude extracts, showing the highest antibacterial activity against Listeria monocytogenes, as indicated by the lowest MIC value. Error bars represent the mean ± standard deviation (%) for the susceptibility of bacterial strains to the crude extracts.