The combined effects of the essential oil (EO) mixtures can significantly enhance their antimicrobial activity by harnessing the synergistic interaction among their diverse bioactive compounds. Additionally, combining EOs with conventional antimicrobial agents can improve or restore the efficacy of these treatments, providing a promising strategy for combating pathogenic and resistant microorganisms. Various methods have been used to evaluate the EO-based combinations, resulting in different types of interactions. This review highlights that EOs from several medicinal and aromatic herbs exhibit synergistic effects when used in mixtures or in combination with antibiotic and antifungal drugs against pathogenic and resistant bacterial and fungal strains. Additive, indifferent and antagonistic effects have also been reported. Recorded synergistic effects suggest the potential therapeutic effects of EOs as natural alternatives or complements to conventional antimicrobial drugs to combat antimicrobial resistance. Despite the demonstrated synergistic effects of many EOs and conventional antimicrobials, there is a notable lack of studies elucidating the mechanisms underlying these synergistic interactions and their specific targets within microbial cells. In this review, the investigations of synergistic studies of EO with conventional antibiotics and the progress in antimicrobial effect of EO mixtures have been reported.

The development of opportunistic pathogenic Candida strains insensitive to several classes of antifungals has emerged as a major health care problem during the last years. Combinational therapy of natural products (e.g., essential oils, EOs) with conventional antifungals has been suggested as a promising alternative to overcome this medical problem. The present study investigates the potential antifungal activity of EOs extracted from some selected medicinal plants, alone and in combination with two common conventional antifungals (fluconazole and amphotericin B) against four clinical Candida isolates. MIC assays indicated that EOs induced strong anticandidal activities with MIC values ranging from 0.162 to 4.950 mg/mL. The combination of amphotericin B with Thymus leptobotrys, Origanum compactum and Artemisia herba alba EOs provided a synergistic effect against C. krusei only, with MIC gain of four-fold, and additive effect against remaining strains (MIC gain = two-fold). Interesting synergistic interactions were observed by combining all studied EOs with fluconazole, with reduction rates of their MICs ranging from 16 to 512-fold. This synergistic effect was very pronounced with the combination of T. leptobotrys EO and fluconazole. These findings indicate that studied EOs can be used as anti-candidals in combination with antifungals, particularly fluconazole, to counteract the emergence of resistant Candida spp.

The application of natural compounds extracted from seaweeds is a promising eco-friendly alternative solution for harmful algae control in aquatic ecosystems. In the present study, the anti-cyanobacterial activity of three Moroccan marine macroalgae essential oils (EOs) was tested and evaluated on unicellular Microcystis aeruginosa cyanobacterium. Additionally, the possible anti-cyanobacterial response mechanisms were investigated by analyzing the antioxidant enzyme activities of M. aeruginosa cells. The results of EOs GC–MS analyses revealed a complex chemical composition, allowing the identification of 91 constituents. Palmitic acid, palmitoleic acid, and eicosapentaenoic acid were the most predominant compounds in Cystoseira tamariscifolia, Sargassum muticum, and Ulva lactuca EOs, respectively. The highest anti-cyanobacterial activity was recorded for Cystoseira tamariscifolia EO (ZI = 46.33 mm, MIC = 7.81 μg mL⁻¹, and MBC = 15.62 μg mL⁻¹). The growth, chlorophyll-a and protein content of the tested cyanobacteria were significantly reduced by C. tamariscifolia EO at both used concentrations (inhibition rate >67% during the 6 days test period in liquid media). Furthermore, oxidative stress caused by C. tamariscifolia EO on cyanobacterium cells showed an increase of the activities of superoxide dismutase (SOD) and catalase (CAT), and malondialdehyde (MDA) concentration was significantly elevated after 2 days of exposure. Overall, these experimental findings can open a promising new natural pathway based on the use of seaweed essential oils to the fight against potent toxic harmful cyanobacterial blooms (HCBs).

Antimicrobial resistance is a growing global challenge, rendering many standard treatments ineffective. Essential oils (EOs) of cinnamon (Cinnamomum aromaticum Nees) and clove (Syzygium aromaticum (L.) Merr. et Perry) may offer an alternative solution due to their high antimicrobial properties and their abilities to fight resistant pathogens. This study evaluates the antimicrobial activity of these two EOs, and their synergistic potential when combined with two antibiotics (ciprofloxacin and vancomycin) and two antifungals (fluconazole and amphotericin B) against various bacterial and yeasts strains. The antimicrobial activities of each EO were evaluated by agar diffusion and broth microdilution assays, while the synergetic effects with antimicrobials were determined by calculating the fractional inhibitory concentration index (FICI) using the checkerboard method. The chemical composition of the EOs was analyzed using Gas Chromatography-Mass Spectrometry (GC-MS). The identification of individual components in the EOs was achieved by comparing their mass spectra with the NIST MS Search database and by correlating their retention times with those of known standards. GC-MS analysis revealed that the main constituents of S. aromaticum EO were eugenol (71.49%) and β-caryophyllene (23.43%), while C. aromaticum EO were dominated by cinnamaldehyde (47,04%) and cinnamyl acetate (18.93%). Antimicrobial activity showed that cinnamon EO exhibits highest effectiveness against all tested strains, with inhibition zones (IZ) ranging from 16.99 mm to 53.16 mm, and minimum inhibitory concentrations (MIC) and minimum microbicidal concentrations (MMC) ranging from 0.039 mg/mL to 0.156 mg/mL. However, for clove EO, the IZ ranged from 9.31 mm to 29.91 mm, with MIC and MMC values from 0.313 mg/mL to 1.25 mg/mL. In combination with antibiotics (ciprofloxacin and vancomycin), the studied EOs showed promising synergistic effects with reduction up to 128-fold. As regards antifungals (amphotericin B, and fluconazole), the synergistic effects were recorded with MIC gains up to 32-fold. Our findings demonstrate that the EOs from C. aromaticum and S. aromaticum exhibit significant broad-spectrum antimicrobial activity against diverse yeast and bacterial strains. This highlights their potential as bases for the development of novel plant-based antimicrobial agents. Importantly, the observed synergistic effects of these EOs with conventional antibiotics support their integration into medical treatments as a strategy to address microbial resistance. Future research should aim to elucidate the mechanisms underlying these synergistic actions, optimize their application, and enhance their therapeutic efficacy.

Background: Lavandula maroccana Murb. is renowned for its valuable essential oil (EO), with a complex chemical composition and potent antimicrobial properties, which can be subjected to environmental influences. The objective of the present study was to assess how seasonal variations affect the yield, chemical composition and antimicrobial activity of Lavandula maroccana Murb. EO. Methods: The plant materials were collected throughout the four seasons and subjected to EO extraction using hydro-distillation method. The chemical composition was analyzed using gas chromatography/mass spectrometry (GC/MS), while the antimicrobial properties were evaluated using disc diffusion and microdilution assays against four yeasts ( Candida albicans, C. glabrata, C. krusei, and C. parapsilosis), and six pathogenic bacteria ( Staphylococcus aureus, Micrococcus luteus, Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa and Klebsiella pneumoniae). Results: The highest EO yield was recorded from the samples collected in autumn (0.19%), while the spring harvest yielded the lowest EO production (0.06%). The GC/MS analysis identified 41 different compounds, with notable variations in the major compound, carvacrol, which ranged from 29.33% to 60.60%. The summer collections contained the highest concentration of carvacrol. Antimicrobial testing, revealed that EOs obtained from all seasons displayed substantial activity against both bacteria and yeast strains. Notably, the summer-harvested EO showed the most potent antimicrobial effects, with inhibition zone and MIC values ranging from 15.32 ± 0.45 mm to 24.09 ± 0.65 mm and 0.16 mg/mL to 2.50 mg/mL, respectively. Conversely, the spring-harvested EO showed the least antimicrobial activity. Conclusion: These findings emphasize the importance of harvesting L. maroccana during the summer and/or autumn to maximize the EO yield and carvacrol content, thereby enhancing its antimicrobial effectiveness.