The combined effect of the combined Fennel and Clove essential oils on Staphylococcus epidermidis, Bacillus cereus, Salmonella typhi and Enterobacter aerogenes using Checkerboard assay (fractional inhibitory concentration index)

Clove and Fennel have been widely used in traditional medicine for treatment of diseases. Regarding the existence of the active biological compounds in these plants, it seems that these plants have considerable antibacterial effects.In this experimental study, the Kirby–Bauer, the dilution method in the liquid medium and minimum bactericidal concentration (MBC) were used to evaluate and compare the antibacterial effects of the Clove and Fennel essential oils. The interactions were determined by calculating fractional inhibitory concentration (FIC), according to the European Committee for antimicrobial susceptibility testing protocol. The checkerboard method was performed using 96-well microtiter plates. The results showed that Staphylococcus epidermidis was the most sensitive strain to Clove and Fennel essential oils. The effects of interaction of Clove and Fennel essential oils with chloramphenicol antibiotic on all bacteria showed synergistic status (except for the fennel essential oil on Bacillus cereus). Calculation of fractional inhibitory concentration indicated that there was no interaction between the Clove and Fennel essential oils on Salmonella typhi, Enterobacter aerogenes and Staphylococcus epidermidis. Regarding the checkerboard data, one FICIs (>0.5 to 1) indicated the additive effect (Bacillus cereus). Bacillus cereus is one of the most important bacteria that causing food poisoning, it is possible to use a combination of Fennel and Clove essential oils as a natural preservative in the food industry.