Study examines the antibacterial properties of cinnamon and eucalyptus essential oils compared with their isolated major compounds

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.Jul 20 2023

A recent study published in PLoS One reviews the antibacterial properties of Cinnamonum cassia and Eucalyptus globulus as compared to isolated formulations of their most abundant bioactives metabolites.

Study: Short communication: Antibacterial effects of essential oils from Cinnamomum cassia bark and Eucalyptus globulus leaves–The involvements of major constituents. Image Credit: Pixel-Shot / Shutterstock.com

About the study

The primary metabolite found within the C. cassia essential oil is (E)-cinnamaldehyde, whereas E. globulus primarily consists of 1,8 cineole, both of which are associated with medicinal properties. Moreover, (E)-cinnamaldehyde comprises between 70-90% of cinnamon essential oil, while the eucalyptus essential oil contains 50-84% of 1,8 cineole.

In the current study, the researchers assessed the antibacterial effects of these two oils and compared these properties with those of (E)-cinnamaldehyde and 1,8 cineole. Broth dilution, agar well diffusion, and disc volatilization were some of the techniques used to analyze the liquid- and vapor-phase effects.

Study findings

For both whole cinnamon and eucalyptus essential oils, the primary active ingredients were present at a lower concentration as compared to concentrations of 99% in the purified form. More specifically, (E)-cinnamaldehyde comprised about 90%, whereas 1,8 cineole comprised only 61% of the whole oil, respectively.

Liquid- and vapor-phase effects of both essential oils and their active ingredients were examined using both gram-positive and gram-negative bacteria. In all three assays, the antibacterial effects were stronger with the whole essential oils than with the major metabolites alone.

Moreover, cinnamon essential oil had greater antibacterial potency than E. globulus essential oil, with a broader spectrum of action that corroborates earlier studies. This could be because cinnamon oil permeabilizes the cell membrane, irrespective of the nature of the membrane.

In the vapor phase, both cinnamon oil and (E)-cinnamaldehyde inhibited gram-positive and gram-negative bacteria in a dose-dependent manner. In contrast, there were no observable effects with either E. globulus essential oil or 1.8 cineole. At identical concentrations, (E)-cinnamaldehyde produced less inhibition than cinnamon essential oil on several bacterial species.

In the liquid phase, cinnamon essential oil produced greater effects on gram-positive and gram-negative bacteria, whereas the E. globulus essential oil only inhibited gram-positive bacteria growth. The whole essential oil, or crude extract of either plant, produced greater antibacterial effects than the isolated compounds, as demonstrated by their lower minimum inhibitory concentrations (MIC).

In fact, the MIC for the essential oils was approximately half of the MIC for either of the major bioactives. The inhibitory zones were larger with cinnamon essential oil by four millimeters (mm) for all but two bacteria.

With E. globulus essential oil or 1,8 cineole, the inhibitory zones were observed only for gram-positive bacteria. In fact, these inhibitory zones were almost double in size following treatment with the essential oil as compared to that following treatment with 1,8 cineole. 

What are the implications?

This is the first study to directly compare the antibacterial effects of major active ingredients in C. cassia and E. globulus with the whole essential oils. The stronger effects of the crude extracts as compared to the active ingredients indicate that the bioactive properties of cinnamon oil are only partly attributed to the activity of (E)-cinnamaldehyde.

Interestingly, previous studies on E. radiata containing 84% 1,8 cineole reported that this essential oil exhibited greater antibacterial properties than E. globulus essential oil containing 91% 1,8 cineole. Thus, various components of the same plant may act differently on various biological processes. Different therapeutic applications may be envisaged for the crude extract accordingly.

Other reports indicate that the purified major ingredient of cinnamon essential oil has more anti-tyrosinase, anti-melanogenic, and anti-xanthine oxidase activity but lower anti-inflammatory activity than the crude oil. Similar differences have been observed with the anxiolytic property of 1,8, cineole as compared to 1,8 cineole.

These results partly explain the advantages of using whole essential oils over isolated ingredients, and therefore support the application of traditional dosage forms for bacterial infections in ethnomedicine.”

The increased antibacterial potency of the whole oils could be due to synergism between various components, as with Glycyrrhiza uralensis or Gingko biloba.

Further research is needed to validate these results in vivo and provide evidence for determining a standardized formulation for essential oils in traditional medicine.