Chemical Composition of Essential Oilsof Thymus and Mentha Speciesand Their Antifungal Activities

1Moulds have various health effects. Excessive mould growth in the human environment needs to be taken care of, regardless of the species, as it may lead to increased allergies, toxicity, and house/building structural problems. Some Aspergillus species are able to produce mycotoxins such as aflatoxins that are potent hepatocarcinogens in animals and humans. Therefore, the presence of toxigenic fungi in foods and grains presents a potential hazard to human and animal health. Superficial fungal infections, dermatomycoses, are probably the most common communicable fungal disease affecting humans. They have become a serious problem in immunocompromised patients. Fungal infections remain a therapeutic problem despite the availability of a number of treatments. Being largely synthetic and non-biodegradable, these agents can cause adverse effects and may have residual toxicity. As the number of reported cases of food-associated infections continues to increase, food safety is a fundamental concern of both consumers and the food industry. As aromatic plants, herbs and spices have been used for ages both as flavouring agents and as preservatives of food, they may be effective sources of biodegradable fungitoxicants without harmful side effects. In the present study, a search was made to find essential oils that could safely be used as natural alternatives for chemical fungicides/fungistatics.

2Various Mentha and Thymus species have been credited with a long list of pharmacological properties. They have been used for their flavours in cooking, in folk medicine as antiseptic and as antimicrobial agents. We chose these species for investigation of the antifungal activities of their oil. The essential oil of T. vulgaris is a known antiseptic, antiviral, antimicrobial agent. The essential oil of T. tosevii contains a high amount of thymol and carvacrol and is also used in ethnobotany. Its pharmacological activities are unknown. The present study reports the antifungal activities of the essential oils of two Thymus and two Mentha species and their main components against soil-borne pathogens, food storage fungi, mycotoxicogenic species, phytopathogens and opportunistic human pathogens.

3We have identified 27 components, accounting for total 97.2% of the oil, in the essential oil of T. vulgaris. The main components were thymol (48.9%) and p-cymene (19.0%) (Table 1). The essential oil of T. vulgaris, tested by macrodilution method, showed very strong antifungal activity. A concentration of 0.25 μL/mL of oil inhibited Alternaria alternata, Fusarium tricinctum, all Aspergillus species and dermatomycetes. Phomopsis helianthi and Cladosporium cladosporioides were inhibited at lower concentrations (0.125 μL/mL). The essential oil dissolved in Tween® showed better antifungal potential, with MIC 0.05 μL/mL. Penicillium species and Trichoderma viride were the most resistant fungi towards this oil; MIC in ethanol dissolved oil was 0.5 μL/mL, and 0.25 μL/mL when the oil was dissolved in Tween®. MICs of T. vulgaris oil in ethanol were 0.125-0.5 μL/mL, and 0.05-0.25 μL/mL in Tween®. Thymol showed the same antifungal potential as the oil, while carvacrol exhibited a slightly better effect, with MICs of 0.05-0.25 μL/mL in ethanol and 0.02-0.125 μL/mL in Tween®, (Figure 1). In agreement with earlier publications the essential oil of T. vulgaris showed very strong antifungal activity at low concentrations, 0.05-1.0 μL/mL. Strong antifungal activity of thymol and carvacrol were also reported in the literature. Previous investigations of Thyme oil by our group showed very high activity against the three major pathogens of the button mushroom, Agaricus bisporus, i.e. the fungi Verticillium fungicola and Trichoderma harzianum and the bacterium Pseudomonas tolaasii. This oil also showed very strong antibacterial activity against food spoilage bacteria. From the essential oil of T. tosevii we identified 42 components, 97.3% of the total amount. Carvacrol (12.8%), α-terpinyl acetate (12.3%), cis-myrtanol (11.2%) and thymol (10.4%) were the dominant components (Table 1). Earlier analysis has shown great variability in the presence of dominant components of T. tosevii oil; geraniol (0.15-32.66%), thymol (19.43-48.19%), geranyl acetate (0.04-11.60%), carvacrol (1.36-10.58%), linalool (0.25-20.92%), p-cymene (3.79-10.98%), depending on where the plants had been collected.

4The essential oil of T. tosevii possessed slightly lower antifungal potential than T. vulgaris oil. MICs and MFCs of this oil dissolved in ethanol were 0.25-1.0 μL/mL, and 0.125-0.5 μL/mL in Tween. Phomopsis helianthi was the most susceptible fungus, while Trichoderma viride was the most resistant species for this oil (Figure 1). There is no published data about the antifungal activity of T. tosevii oil.

5MICs of these oils were the same in macro- and microdilution methods, and the same as MFCs obtained by microdilution method, which means that concentrations that stopped fungal growth also killed the fungi. The commercial fungicide bifonazole, which was used as a control, possessed much higher MICs (8.0-15.0 μL/mL) than the essential oils and components investigated (Figure 1). The lower antifungal effect of T. tosevii oil in comparison with T. vulgaris oil can be explained by its lower amount of thymol and its precursors (p-cymene and γ-terpinene) and by the higher percentage of acetates (α-terpinyl acetate and geranyl acetate) which may lead to lower antifungal potential.

6Comparing the previous data with the chemical composition of the oils, it becomes evident that there is a relationship between the high activity of the Thymus type oils and the presence of phenol components, such as thymol and carvacrol. The high antifungal activity of these essential oils could be explained by the high percentage of phenol components. It seems possible that phenol components may interfere with cell wall enzymes like chitin synthase/chitinase as well as with the α- and β- glucanases of the fungus. Consequently, the high content of phenol components may account for the high antifungal activity of oils. From our results it can be seen that essential oils of Thymus species and carvacrol and thymol have very high antifungal activities, even higher than the commercial fungicide bifonazole.

7In M. piperita essential oil 26 components were detected and identified (97.7%). Menthol (37.4%), menthyl acetate (17.4%) and menthone (12.7%) were the main components in this oil (Table 1). In essential oil of M. spicata 27 components were identified (92.4%). The main components were carvone (49.5%) and menthone (21.9%) (Table 1). The results of antifungal activities of both essential oils and components are presented in Figure 2. The essential oil of M. spicata showed the same fungistatic activity in macro- and microdilution method with MICs of 1.0-2.5 μL/mL in ethanol and 0.5-1.5 μL/mL in Tween®. MICs of M. piperita essential oil were higher, 1.5-3.0 μL/mL in ethanol and 1.0-2.5 μL/mL in Tween®. Figure 2 shows that that essential oil of M. spicata possesses greater fungistatic activity than M. piperita oil. Minimum fungicidal concentrations (MFC) of M. spicata essential oil obtained by microdilution method, diluted in ethanol and Tween® were 1.0-2.5 μL/mL and 0.5-2.5 μL/mL, respectively. These values (MFC) for M. piperita essential oil were 1.5-3.0 μL/mL and 1.0-2.5 μL/mL. It can be seen that MFCs of essential oil of M. spicata are higher than MFCs of essential oil of M. piperita (Figure 2). 1,8-cineole diluted in ethanol inhibited fungal growth in concentrations of 3.0-8.0 μL/mL, while in Tween®, MICs were 2.0-7.0 μL/mL.

8Menthol showed the same fungistatic and fungicidal activities in both methods, with MICs of 0.25-1.5 μL/mL in ethanol and 0.05-1.0 μL/mL in Tween® (Figure 2). Limonene possessed moderate fungistatic activity with MICs 8.0-12.0 μL/mL in ethanol and 5.0-9.0 μL/mL in Tween®. MICs of limonene obtained in microdilution method were 6.0-11.0 μL/mL in ethanol and 5.0-9.0 μL/mL in Tween®. MFCs of this component were 7.0-11.0 μL/mL in ethanol and 5.0-10.0 μL/mL in Tween®. Carvone showed higher antifungal activities than previous components. MICs and MFCs of this component were 0.25-1.0 μL/mL in ethanol and 0.05-0.5 μL/mL in Tween®. There is no effect of ethanol and Tween against tested micromycetes. The commercial fungicide (bifonazole), which was used as a control showed lower antifungal potential than the essential oils investigated (Figure 2).

9The differences between antifungal activities of these two essential oils could be due to different chemical composition of essential oils. The greater antifungal potential of M. spicata essential oil could be explained by the presence of carvone, which possesses very strong antifungal activity. The essential oil of M. piperita possesses menthol and 1,8-cineole as main components, which also exhibited very good antifungal properties but lower than carvone. Carvone has better antifungal properties because of its high water solubility. One of the reasons for lower antifungal activity of M. piperita essential oil could be the large amount of menthyl acetate, which causes a decrease of antifungal properties.

10Earlier investigations of the antimicrobial activity of Mentha spicata and M. piperita oils showed strong activity against the three major pathogens of the button mushroom, Agaricus bisporus, i.e. the fungi Verticillium fungicola and Trichoderma harzianum and the bacterium Pseudomonas tolaasii. Both Mentha oils tested inhere showed strong antibacterial activity against a variety of bacteria. Our results are in accordance with previous investigations of antifungal activities of Mentha essential oils. Previous results showed a fungicidal effect of M. spicata essential oil against human pathogens in concentration of 0.25 -2.0 μL/mL, while this oil possessed lower antifungal potential against phytopathogenic species. The results obtained by both methods suggested that carvone and menthol possessed greater antifungal activities than other compounds investigated, while limonene showed the lowest antifungal activity. Previous results of investigation of antifungal properties had shown that hydrocarbon monoterpenes had the lowest antifungal activity, larger antifungal potential could be due to the presence of oxygenated terpenes or of those with phenolic structures. The hydrocarbons tend to be relatively inactive regardless of their structural type, and this inactivity is closely related to their limited hydrogen binding capacity and water solubility. Ketones, aldehydes and alcohols showed activity but with differing specificity and levels of activity, which is in connection with the functional groups present but also associated with hydrogen-binding parameters in all cases.

11From our results it can be seen that MICs and MFCs are generally lower for all the essential oils and components investigated in microdilution method. The low water solubility of the oil and its components limit their diffusion through the agar medium. Only the more water-soluble components, such as carvone or 1,8-cineole diffuse into the agar. The hydrocarbon components either remain on the surface of the medium or evaporate. That could be the reason for better results obtained by the microdilution method. Broth method, carried out in microtitre trays, has the advantage of lower workloads for a larger number of replicates and the use of small volumes of the test substance and growth medium. In this method dilution of the oil is better and there is no agar in the medium, which both enable better diffusion through the liquid medium.

12Also, it can be seen that the essential oils and components investigated showed greater antifungal activities when they are diluted in Tween®. Non-ionic emulsifiers, such as Tween® 20 or 80, are relatively inactive when tested alone and have been widely reported in the literature for their use as emulsifying agents. It was achieved that dispersion of the oil and components is better than when ethanol was used as a solvent.

13However, essential oils and all the components investigated showed inhibitory effects against all 17 fungi tested. The mycelial growth of test fungi responded differently to the investigated essential oil, which indicated that oils may have different modes of action or that the metabolism of some fungi was able to better overcome the effect of the oil or adapt to it. Phytopathogen Phomopsis helianthi was the most susceptible to the investigated essential oils, while Trichoderma viride and Penicillium species were the most resistant species (Figure 1 and Figure 2).

14In an earlier study of one of us, it was found that the LC50 value of orally applied Thyme oil in rat was 9543.5 μL kg-1 bodyweight. The LD50 values of pyrethrin (350-500 mg kg-1) and carvone (1640 mg kg-1) are well below that of Thyme oil. Mentha piperita and Thymus vulgaris as well as their oils and the major oil components are generally regarded as safe and full toxicology has been obtained.

15Taken together, these results suggest that the essential oils investigated here could find practical application in the prevention and protection of fungal infections of plants, animals and humans. Essential oils could safely be used as preservative materials on foods for protection to fungal infection, since they are natural, and mostly non-toxic to humans. The selected plant species are popular culinary herbs, and their essential oils have been used extensively for many years in food products, perfumery, and dental and oral products.

16The essential oils of T. vulgaris, T. tosevii and of M. piperita and M. spicata are highly active as fungitoxicants and could safely be used as natural preservatives to replace synthetic fungicides in the prevention and cure of some plant, human and animal fungal disease.