Study of Antibacterial and Synergistic Activities of Cinnamomum verum, Eucalyptus camaldulensis and Zataria multiflora Boiss. in Persian Medicine Against Some Gram-Negative and Gram-positive Pathogenic Bacteria

Antibacterial activities of essential oils (EOs) of three widely used herbal elements of cinnamon (Cinnamomum verum J.Presl), eucalyptus (Eucalyptus camaldulensis Dehnh.), and thyme (Zataria multiflora Boiss.), whose antimicrobial effects are indirectly described in the old literature of Persian medicine as antiseptic agents and treatments of diseases caused by Havaye Vabaee (e.g., air-borne infectious diseases), were investigated. Antibacterial activities of the EOs were evaluated against some important Zoonosis and medicine pathogenic gram-negative (Escherichia coli, Klebsiella pneumonia, Listeria monocytogenes, and Pseudomonas aeruginosa) and gram-positive (Staphylococcus aureus, S. epidermidis, and Streptococcus pyogenes) bacteria. These pathogens cause disease in animals and humans.To this end, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and double-diffusion assay (DDA) tests were utilized using a microdilution method. While MIC values for cinnamon, eucalyptus, and thyme ranged between 0.25-1, 8-64, and 0.5-16 μg mL-1, respectively, the range of MBC values were 1-4, 32-256, and 2-64 μg mL-1. In DDA tests, a concentration of 0.5 μg mL-1 of the three EOs induced inhibition zones ranging between 20-35, 9-25, and 0-35 mm in diameter in the culture media of the above bacteria. Antibacterial effects of double combinations of the EOs against E. coli and S. aureus were also investigated by checkerboard test and fractional inhibitory concentration (FIC) index. Combinations of the EOs resulted in FIC index values of 1.06-2, indicating no interaction between most of the compounds combined. Our study also suggested the use of thyme oil with caution in combination with cinnamon or eucalyptus oils against E. coli because of their possible antagonistic effects (FIC index = 4).