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

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 <i>Capparis spinosa</i> crude extracts and five essential oils (EOs) derived from <i>Salvia officinalis</i>, <i>Eucalyptus globulus</i>, <i>Micromeria barbata</i>, <i>Origanum vulgare</i>, and <i>Juniperus excelsa</i>. The EOs were extracted using hydro-distillation, and <i>C. spinosa</i> extracts were obtained using ethanol and acetone solvents. Microdilution assays revealed that <i>O. vulgare</i> EO exhibited the strongest activity against <i>Listeria monocytogenes</i>, <i>Escherichia coli</i>, <i>Salmonella</i> spp., and <i>Brucella melitensis</i>, while <i>C. spinosa</i> demonstrated significant antibacterial effects against <i>L. monocytogenes</i> and notable inhibition of <i>Pseudomonas aeruginosa</i>. The combination of EOs with antibiotics, including <i>M. barbata</i>, <i>J. excelsa</i>, <i>S. officinalis</i>, and <i>E. globulus</i>, 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.