The Combined Effect of Carum copticum (Ajwain) and Foeniculum vulgare (Fennel) Essential Oils on Escherichia coli and Clostridium sporogenes Using Checkerboard Assay

Increasing awareness of the adverse effects of chemical preservatives on health has led to an increased tendency to use natural preservatives, especially herbal essential oils in pharmaceutical and food products. Carum copticum (Ajwain) and Foeniculum vulgare (Fennel) are among the aromatic medicinal plants that can be used in this regard. In this study, the agar diffusion method was used to evaluate and compare the antimicrobial effects of the herbal essential oils in question and the dilution method in the liquid medium was also used to determine the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The checkerboard assay was used to evaluate the interaction between the herbal essential oils of Ajwain and Fennel and to determine the fractional inhibitory concentration. The results showed that the MIC of Ajwain and Fennel herbal essential oils against Escherichia coli was 1562.5 μg/ml and 12500 μg/ml, respectively, and in the case of Clostridium sporogenes this value was 1562.5 μg/ml and 50000 μg /ml, respectively. Calculation of fractional inhibitory concentration (FIC index) indicated that there was no interaction between the herbal essential oils of two plants against both bacteria in question. In general, the absence of synergistic and antagonistic interactions between the Ajwain and Fennel herbal essential oils can be related to the identically-affected sites of these essential oils in Escherichia coli and Clostridium sporogenes. Also, the results obtained through the identification of herbal essential oils composition by GC/MS showed that the main components of Ajwain were Thymol (49.96%), p-cymane (22.73%) and γ-Terpinene (15.32%) and in case of Fennel these components were Trans-anethole (63.88%), Estragole (9.97%), limonene (7.65%) and fenchone (5.72%).