Terpenes as natural disinfectants

Rodrigo Valladares Linares

Wayakit

21 May 2020

What are natural terpenes?

Essential oils are mixtures of lipophilic, liquid, volatile hydrocarbons (most of them in forms of terpenes) present in higher plants. Terpenes, primarily extracted from medicinal plants, are volatile compounds with molecular components that are composed of only carbon, hydrogen and oxygen atoms. The basic chemical structure of terpenes consists of a number of repeated isoprene (C5H8) units which are used to classify terpenes [1]. Due to their high enhancement effect and low skin irritation, terpenes of natural origin are now receiving much attention in pharmaceutical and cosmetic formulations [2,3].

There is a broad range of biological properties of terpenes described in literature, including cancer chemo-preventive effects, antimicrobial, antifungal, antiviral, antihyperglycemic, anti-inflammatory, and antiparasitic activities [4].

Natural terpenes as disinfectants: antibacterial properties

Various studies have performed tests to assess the antibacterial and antiviral properties of natural terpenes, either as individual substances, or constituents of mixtures such as essential oils, plant extracts, and other synthetic compounds.

Wang et al. 2019 [5] studied the antibacterial activity of seven terpenes. These activities were tested against typical foodborne pathogenic bacteria (Escherichia coli, Salmonella enterica, and Staphylococcus aureus). Table 1 shows the minimum bactericidal concentration (MCB) in mg/mL for the 7 compounds studied for each bacterial strain. At a minimum bactericidal concentration range of 0.673 - 1.682 mg/mL, limonene showed the strongest antibacterial activity among the three terpenes without a hydroxyl group – α-pinene, limonene, and myrcene.

As observed in Figure 1, limonene was found to have the highest antibacterial activity against Gram-positive/Gram-negative foodborne pathogenic bacteria. This effect corresponds to the use of limonene at a minimum inhibitory concentration (MIC) of 0.421 mg/mL.

Table 1. Minimum bactericidal concentration (MBC) of seven terpenes against foodborne pathogenic bacteria. Adapted from Wang et al. 2019 [5].

Figure 1 – Time-kill curve of limonene against foodborne pathogenic bacteria (Escherichia coli, Salmonella enterica and Staphylococcus aureus) at concentrations of minimum inhibitory concentration (MIC). Adapted from Wang et al. 2019 [5].

In another study, the evaluation of the activity of 33 terpenes, including minority compounds, was performed to understand the antimicrobial activity of essential oils [6]. They found that out of these 33 terpenes, 16 (m-Cymene, R-(+)-Limonene, (−)-Borneol, (+)-Borneol, (±)-Camphor, l-Carveol, l-Carvone, Citral, (±)-Citronellal, β-Citronellol, trans-Geraniol, (±)-Linalool, Terpineol, Carvacrol, Eugenol and Thymol) have antibacterial activity after being tested in 4 different bacteria strains (B. cereus, S. Typhimurium, E. coli and S. aureus).

Figure 2 shows the bactericidal effect of eugenol and terpineol against S. Typhimurium. After a short contact time, the bacterial assay was found to be dead using the minimum inhibitory concentration times 2 and 4 (MIC x2 and x4) for both terpenes [6].

Figure 2 – Time-kill curves for terpineol and eugenol at different MIC for the bacteria S. Typhimurium. Control: bacteria untreated. Adapted from Guimarães et al. 2019 [6].

Mechanisms of action of terpenes

The antimicrobial effect on a selected terpene, thymol, was tested in a study on S. Typhimurium bacteria [7]. Scanning electron microscopy images confirmed that the main mechanism of action of thymol is the membrane dysfunction and suggests that thymol can be used as a naturally occurring drug against this bacterial strain in place of synthetic drugs. This same result was verified for eugenol and terpineol for S. Typhimurium [6]. Hydroxyl groups (known to be highly reactive), such as those found in thymol, eugenol and terpineol form hydrogen bonds with active sites of target enzymes on the membrane cell, inactivating them, and consequently, creating a dysfunction on the membrane, killing the bacterial cells.

Cristani et al. 2007 [8] propose that for selected monoterpenes, its antimicrobial effect may result from a gross perturbation of the lipidic fraction of the plasmic membrane of microorganisms.

Studies by Di Pasqua et al. 2007 [9] evaluated the mechanism of action of essential oil antimicrobial compounds (thymol, carvacrol, limonene, eugenol, and cinnamaldehyde). They found that the compounds analyzed quickly exerted their antimicrobial activities on the microbial cell envelope, determining structural alterations of the cell envelope, which are in accordance with results presented by Guimarães et al. 2019 [6] and by Chauhan et al. 2014 [7].

Terpenes against viruses

Currently, the antiviral activity of terpenes has been reported, but is not fully understood yet. Therefore, there is a lot of research aimed at discovering agents, also from natural sources, which could have potent antiviral activity [4].

Isoborneol, a natural monoterpene, has been shown to exhibit relatively strong anti-herpes simplex virus-1 (anti-HSV-1) action. The mechanism of isoborneol activity relies on interactions of its hydroxyl groups with virus envelope lipids. As a result, HSV-1 loses its effectivity [10]. Potent anti-HSV-1 activities have also been reported for monoterpenes such as cineol and borneol, a stereoisomer of isoborneol.

In the present study, the inhibitory effects of several essential oils against herpes simplex virus infection were compared with the antiviral potential of their major monoterpenic compounds. All tested essential oils and most monoterpenes exhibited high levels of antiviral activity against HSV-1 in viral suspension tests.

Researchers have also studied the inhibitory effects of several essential oils against herpes simplex virus infection, comparing the antiviral potential of their major monoterpenic compounds. All tested essential oils and most monoterpenes exhibited high levels of antiviral activity against HSV-1 in viral suspension tests [11].

Sesquiterpenes, e.g. triptofordin C-2 and sesquiterpene coumarins inhibit cytomegalovirus, (Hayashi et al., 1996), severe acute respiratory syndrome coronavirus (Wen et al., 2007) and rhinovirus (Rollinger et al., 2008). A study by Wen et al. 2007 [12] found that sesquiterpenes, i.e. triptofordin C-2 and sesquiterpene coumarins (abietane-type diterpenoids), inhibit the severe acute respiratory syndrome coronavirus (SARS-CoV).

Concluding remarks

There are numerous studies that explore the antibacterial properties of natural terpenes, with a broad and consented positive conclusion for its use to inhibit bacterial activity. In terms of antiviral activity, the number of studies found in literature is still reduced, but the scientific community seems to agree on the urgent need to deepen this research filed.

At present, there are different mechanisms of antimicrobial and antiviral activity for different terpenes and essential oils studied. This series of mechanisms involved supports the idea proposed by many researchers that a combination of natural terpenes might be a better choice when used alone.

Numerous terpenes appear to possess beneficial healthcare effects. It was not discussed in this text, but there are plenty of studies even on antifungal and antiparasitic activities of terpenes and essential oils, which have given new insights on the development of disinfectants targeting fungus and parasites. Altogether, this group of substances (natural terpenes) should be further employed in several healthcare areas, giving special attention to introducing them into modern natural disinfectants.

References

[1] Chen, J.; Jiang, Q.; Chai, Y.; Zhang, H.; Peng, P.; Yang, X. Natural Terpenes as Penetration Enhancers for Transdermal Drug Delivery. Molecules. 2016, 21(12),1709; DOI:10.3390/molecules21121709

[2] Aqil, M.; Ahad, A.; Sultana, Y.; Ali, A. Status of terpenes as skin penetration enhancers. Drug Discov. Today. 2007, 12, 1061–1067. DOI:10.1016/j.drudis.2007.09.001

[3] Sapra, B.; Jain, S.; Tiwary, A.K. Percutaneous permeation enhancement of terpenes: Mechanistic view. AAPS J. 2008, 10, 120–132. DOI:10.1208/s12248-008-9012-0

[4] Paduch, R.; Kandefer-Szerszeń, M.; Trytek, M.; Fiedurek, J. Terpenes: substances useful in human healthcare. Arch Immunol Ther Exp (Warsz). 2007, 55 (5) : 315‐327. DOI:10.1007/s00005-007-0039-1

[5] Wang C.Y.; Chen Y.; Hou C. Antioxidant and antibacterial activity of seven predominant terpenoids, International Journal of Food Properties. 2019, 22:1, 230-238. DOI:10.1080/10942912.2019.1582541

[6] Guimarães, A.C.; Meireles, L.M.; Lemos, M.F.; Guimarães, M.C.C.; Endringer, D.C.; Fronza, M.; Scherer, R. Antibacterial Activity of Terpenes and Terpenoids Present in Essential Oils. Molecules. 2019, 24, 2471. DOI: 10.3390/molecules24132471

[7] Chauhan, A.K.; Kang, S.C. Thymol disrupts the membrane integrity of Salmonella ser. Typhimurium in vitro and recovers infected macrophages from oxidative stress in an ex vivo model. Res. Microbiol. 2014, 165, 559–565. DOI: 10.1016/j.resmic.2014.07.001

[8] Cristani, M.; D’Arrigo, M.; Mandalari, G.; Castelli, F.; Sarpietro, M. G.; Micieli, D.; Venuti, V.; Bisignano, G.; Saija, A.; Trombetta, D. Interaction of Four Monoterpenes Contained in Essential Oils with Model Membranes: Implications for Their Antibacterial Activity. J. Agric. Food Chem. 2007, 55(15), 6300–6308. DOI:10.1021/jf070094x

[9] Di Pasqua, R.; Betts, G.; Hoskins, N.; Edwards, M.; Ercolini, D.; Mauriello, G. Membrane Toxicity of Antimicrobial Compounds from Essential Oils. J. Agric. Food Chem. 2007, 55 (12), 4863-4870. DOI: 10.1021/jf0636465

[10] Armaka, M.; Papanikolaou, E.; Sivropoulou, A.; Arsenakis, M. Antiviral properties of isoborneol, a potent inhibitor of herpes simplex virus type 1. Antiviral Res. 1999, 43 (2) : 79‐92. DOI:10.1016/s0166-3542(99)00036-4

[11] Astani, A.; Reichling, J.; Schnitzler, P. Comparative study on the antiviral activity of selected monoterpenes derived from essential oils. Phytother Res. 2010, 24 (5) : 673‐679. DOI:10.1002/ptr.2955

[12] Wen C.C.; Kuo Y.H.; Jan J.T. Speciﬁc plant terpenoids and lignoids possess potent antiviral activities against severe acute respiratory syndrome coronavirus. J Med Chem. 2007, 50 : 4087–4095. DOI:10.1021/jm070295s