Differential Antimicrobial Effect of Essential Oils and Their Main Components: Insights Based on the Cell Membrane and External Structure

Abstract

The biological activity of essential oils and their major components is well documented. Essential oils such as oregano and cinnamon are known for their effect against bacteria, fungi, and even viruses. The mechanism of action is proposed to be related to membrane and external cell structures, including cell walls. This study aimed to evaluate the biological activity of seven essential oils and eight of their major components against Gram-negative and Gram-positive bacteria, filamentous fungi, and protozoans. The antimicrobial activity was evaluated by determination of the Minimal Inhibitory Concentration for Bacillus cereus, Staphylococcus aureus, Listeria monocytogenes, Escherichia coli, Salmonella Typhimurium, Shigella sonnei, Aspergillus niger, Aspergillus ochraceus, Alternaria alternata, and Fusarium oxysporium, the half-maximal inhibitory concentration (IC₅₀) for Trypanosoma cruzi and Leishmania mexicana, and the median lethal dose (LD₅₀) for Giardia lamblia. Results showed that oregano essential oil showed the best antibacterial activity (66-100 µg/mL), while cinnamon essential oil had the best fungicidal activity (66-116 µg/mL), and both showed excellent antiprotozoal activity (22-108 µg/mL). Regarding the major components, thymol and carvacrol were also good antimicrobials (23-200 µg/mL), and cinnamaldehyde was an antifungal compound (41-75 µg/mL). The major components were grouped according to their chemical structure as phenylpropanoids, terpenoids, and terpinenes. The statistical analysis of the grouped data demonstrated that protozoans were more susceptible to the essential oils, followed by fungi, Gram-positive bacteria, and Gram-negative bacteria. The analysis for the major components showed that the most resistant microbial group was fungi, which was followed by bacteria, and protozoans were also more susceptible. Principal Component Analysis for the essential oils demonstrated the relationship between the biological activity and the microbial group tested, with the first three components explaining 94.3% of the data variability. The chemical structure of the major components was also related to the biological activity presented against the microbial groups tested, where the three first principal components accounted for 91.9% of the variability. The external structures and the characteristics of the cell membranes in the different microbial groups are determinant for their susceptibility to essential oils and their major components.

Keywords: antimicrobial; cell membranes; essential oils; mechanism of action; phenylpropanoids; terpenoids.

Figure 1

Overall antimicrobial activity of the essential oils (EO) on bacteria, fungi, and protozoans. The essential oils analyzed included anise (Pimpinella anisum), cinnamon (Cinnamomum verum), clove (Syzygium aromaticum), cumin (Cuminum cyminum), laurel (Laurus nobilis), and Mexican oregano (Lippia berlandieri). The biological activity for Gram-positive bacteria (B. cereus, S. aureus, L. monocytogenes), Gram-negative bacteria (E. coli, S. typhimurium, S. sonnei), and molds (A. niger, A. ochraceus, A. alternata, F. oxysporium) was CMI. IC50 was used for T. cruzi and L. mexicana, and LD50 was used for G. lamblia. The data represent the mean and standard deviation of 39 analysis. Differences among groups were established based on Tukey means analysis, following an ANOVA test.

Figure 2

Chemical structures of the major components evaluated. (a) Anethole, (b) cinnamaldehyde, (c) eugenol, (d) cuminaldehyde, (e) eucalyptol, (f) limonene, (g) carvacrol, and (h) thymol.

Figure 3

Interaction on the antimicrobial activity of major components of essential oils and bacteria, fungi, and protozoans. The chemical components were grouped according to their chemical structure in phenylpropanoids (cinnamaldehyde, eugenol, anethole), terpenoids (carvacrol, cuminaldehyde, eucalyptol, thymol) and terpinenes (limonene). The biological activity for Gram-positive bacteria (B. cereus, S. aureus, L. monocytogenes), Gram-negative bacteria (E. coli, S. typhimurium, S. sonnei), and molds (A. niger, A. ochraceus, A. alternata, F. oxysporium) was CMI. IC50 was used for T. cruzi and L. mexicana, and LD50 was used for G. lamblia. The points are the average of the measures for terpinenes (n = 12 for fungi, n = 9 for bacteria and protozoans), terpenoids (n = 48 for fungi, n = 36 for bacteria and protozoans), and phenylpropanoids (n = 36 for fungi, n = 27 for bacteria and protozoans).

Figure 4

Principal Component Analysis (PCA) for the biological activity of essential oils against the different microorganisms analyzed. Scores for each microorganism for the first three PCA are shown.

Figure 5

Principal Component Analysis (PCA) for the biological activity of major components of EOs against the different microorganisms analyzed. Scores for each chemical compound identified by their chemical structure for the first three PCA are shown.