Chemical composition, cytotoxic effects and antimicrobial activity of combined essential oils from Citrus meyeri, Citrus paradise, and Citrus sinensis leaves

Plants are the most abundant primary source of active phytochemicals, which are needed to treat a wide range of ailments. The current study aimed to investigate the chemical composition, cytotoxic, and antimicrobial activities of combined essential oils (EOs) from Citrus meyeri, Citrus paradise, and Citrus sinensis leaves. Gas chromatography-mass spectrometry identified the chemical composition, while the MTS technique was used to measure the EOs’ cytotoxicity. The antimicrobial activities were assessed by broth microdilution assay against several fungal and bacterial strains. The EO content of C. meyeri revealed the presence of 27 compounds, totaling 97.5% of the EO. In comparison, C. paradise EO has 20 compounds, making up 99.8% of the composition, while C. sinensis EO contains 32 compounds, constituting 99.9% of its content. Limonene (43.2%), β-pinene (44.5%), and sabinene (55.9%) were the most abundant EOs identified in the three analyzed Citrus species namely C. meyeri, C. paradise, and C. sinensis, respectively. Moreover, MIC values of three EOs showed variable antimicrobial activity against the tested strains. On the contrary, the examination of the combined effects of the identified EOs showed synergistic results, as indicated by the Fractional Inhibitory Concentration (FIC) Index. Specifically, C. meyeri and C. paradise EOs exhibited synergistic effects (with FIC Index values of 0.73 and 0.08, respectively) against Escherichia coli and Candida albicans. Similarly, C. meyeri and C. sinensis EOs demonstrated synergy (with FIC Index values of 0.83 and 0.13, respectively) against Proteus vulgaris and C. albicans. Moreover, when C. meyeri, C. paradise, and C. sinensis EOs were combined, they synergistically affected C. albicans, with an FIC Index of 0.88. The cytotoxicity evaluation of the EO isolated from C. meyeri against HeLa, HepG2, and CaCo-2 cancer cell lines revealed its robust anticancer potentialities compared to the other studied Citrus species. Collectively, the achieved results support the possible therapeutic applications of the EOs from C. meyeri, C. paradise, and C. sinensis, and their combinations to be used as antimicrobial or anticancer agents.