Chemical composition, cytotoxicity, antimicrobial and antifungal activity of several essential oils

Essential oils (EOs) are known and used for their biological, antibacterial, antifungal and antioxidant properties. Numerous studies have shown that EOs exhibit a large spectrum of biological activities in vitro. The incidence of drug-resistant pathogens and the toxicity of antibiotics have drawn attention to the antimicrobial activity of natural products, encouraging the development of alternative treatments. The aim of this study was to analyse the phytochemical and the cytotoxic characteristic of 36 EOs; we then evaluated the antimicrobial activity of the less-toxic EOs on Gram-positive, Gram-negative and fungi strains. The results showed low cytotoxicity in seven EOs and good activity against Gram-negative and Candida spp. strains. Based on our results, EOs could be proposed as a novel group of therapeutic agents. Further experiments are necessary to confirm their pharmacological effectiveness, and to determine potential toxic effects and the mechanism of their activity in in vivo models.


Introduction
Medicinal plants have been used for centuries in traditional medicine because of their therapeutic usefulness; they are extensively used in folk medicine, as sources of natural products because they represent an economic alternative, are easily accessible and can be applicable to various pathologies (Prabuseenivasan et al. 2006;Cruz et al. 2007;Sardi et al. 2011). EOs, also called volatile oils, are obtained from flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits and roots. An estimated 3000 EOs are known; 300 are commercially important in the fragrance market (Van de Braak & Leijten 1999) and are known and primarily used for their biological, antibacterial, antifungal and antioxidant properties (Deans & Waterman 1993). In addition, there is currently great interest by the pharmaceutical, food and cosmetics industries in the use of new volatile oils; despite the large progress in medicine and pharmacy in the last few decades, the traditional treatment of bacterial or viral diseases is frequently ineffective and has some side effects. Terpenes and their derivatives are a large class of natural organic components of EOs and are widespread in the plant kingdom. The antimicrobial activity of EOs is due to a number of small terpenoids and phenolic compounds (Cannas et al. 2014); several of these are generally recognised as safe (Sharififar et al. 2007). The spread of drug-resistant pathogens is one of the most serious threats to the successful treatment of microbial diseases. Therefore, researchers have been interested in biologically active compounds isolated from plant species for the elimination of pathogenic microorganisms because of the resistance that microorganisms have built against current antibiotics. In addition to the specific chemical composition of these EOs, it is very important to understand their cytotoxicity before using them in humans.
Further studies are needed to determine any potential toxic effects, their pharmacological effectiveness and the mechanism of their activity in in vitro and in vivo models.
The results of the cytotoxicity test revealed that 29 EOs showed high cytotoxicity and only 7 were slightly toxic: S. sclarea, M. alternifolia, T. hyemalis, T. vulgaris sample 24 (red thyme geraniol bio.), T. vulgaris sample 25 (red thyme geraniol sel.), T. vulgaris sample 26 (red thyme bio. France) and T. vulgaris sample 27 (red thyme sel. France). In particular, it is considered to be the concentration of the EO that allowed the 60% and 50%, respectively, in cell viability WKD, Caco-2 and Hep-2; all the results are summarised in Table 1. The toxicity test carried out on the selected EOs showed an IC 60 for VKD in a range of 0.019 -0.039% (v/v) while the CaCo-2 and Hep 2, IC 50 value ranges were between 0.0039% and 0.062% (v/v). The seven EOs with the lowest cytotoxicity showed a very high concentration of monoterpene alcohols (in particular linalool and/or geraniol); in the case of a low percentage of these alcohols, however, their related esters were present at high levels.
In conclusion, EOs are potential sources of novel antimicrobial compounds especially against bacterial pathogens. In vitro studies in this work showed that the EOs inhibited bacterial growth, but their effectiveness varied. Study of the EOs in their complexity and especially an understanding of the specific activity of each component will increase the appropriate uses of these natural products. Finally, the synergy between the components of EOs still requires more study before these complex matrices can be reliably used in antimicrobial applications.

Conclusion
In conclusion, EOs are potential sources of novel antimicrobial compounds, especially against bacterial pathogens. In vitro studies in this work showed that the EOs inhibited bacterial growth, but their effectiveness varied. Study of the EOs in their complexity and especially an understanding of the specific activity of each component will increase the appropriate uses of these natural products. Finally, the synergy between the components of EOs still requires more study before these complex matrices can be reliably used in antimicrobial applications.

Supplementary material
The underlying research materials for this article can be accessed at http://dx.doi. org/10.1080/14786419.2015.1060592.