## 1 Introduction  
_Candida albicans_ is a common opportunistic fungal pathogen that inhabits the bodies of healthy individuals. Although other species of non-_albicans Candida_, such as _C. tropicalis_, _C. parapsilosis_, _C. krusei_, and _C. lusitaniae_, have shown an increased incidence of nosocomial infections, _C. glabrata_ is still considered the most common non-_albicans_ isolated _Candida_ species [1; 2; 3]. In immunocompromised patients, both _C. albicans_ and _C. glabrata_ can cause superficial mucosal infection, such as oral thrush and vaginitis, as well as potentially life-threatening systemic disorders [4]. High-risk groups include organ transplant recipients, cancer patients receiving chemotherapy, and people with HIV/AIDS. In the last few years, _Candida_ infections have occurred more frequently, with high mortality rates, and have been recognized as one of the most significant causes of hospital-acquired infections [5]. Despite the high incidence and the severity of _Candida_ infections, treatments are still limited and insufficient. In the treatment of fungal infections there are only a few drug classes available: polyenes, triazole derivatives,echinocandins, allylamines, and flucytosine [6]. Nevertheless, none of them meets all the expectations (especially regarding low toxicity to patients, convenience of administration, and a low possibility of resistance acquiring).  
Therefore, new therapeutic alternatives based on exploitation of natural resources have been intensively investigated recently [7]. Essential oils (EOs) have gained increased interest due to their antiseptic and antimicrobial activity. Many researchers have reported on their antibacterial [8], antifungal [9], anti-parasitic [10], and antiviral activity [11]. EOs are rich mixtures of chemical compounds belonging to different chemical families, including terpen, aldehydes, alcohols, esters, phenols, ethers, and ketones. Most essential oils are composed of terpen, terpenoids, and other aromatic and aliphatic constituents with low molecular weights. Terpenes are a class of natural substances of vegetable origin formed by the condensation of isoprene units (C\({}_{5}\)H\({}_{8}\)) and are classified as monoterpenes (C\({}_{10}\)), the most representative molecules, and sesquiterpenenes (C\({}_{15}\)) [12]. Their derivatives containing oxygen are called terpenoids. Usually, the chemical characterization of many essential oils reveals the presence of only 2-3 major components at a fairly high concentration (20-70%) compared to other components present in trace amounts [13].  
The mechanism of antimicrobial action of EOs is complex and depends on their chemical composition and the quantity of the major single compounds. The antifungal mechanism of action of EOs is similar for antibacterial. Many reports revealed that constituents of EOs mixture cause cell membrane damage; moreover, they influence many other cellular activities including energy production [8]. The antimicrobial effect may be linked to reduced membrane potentials, the disruption of proton pumps, and the depletion of the ATP [14]. The effect of EOs activity is also the coagulation of cell content, cytoplasm leakage, and finally cell apoptosis or necrosis, leading to cell death [8].  
The present study aimed to select the most potent EOs and to evaluate their activity on a large group of _C. albicans_ and _C. glabrata_ clinical isolates. Furthermore, a synergistic effect of the EOs with commonly used antimycotics was investigated, as well as the mechanism of action of the selected EOs.