## 3 Discussion
Interestingly, a relatively high efflux of potassium ions was caused by cinnamon oil, compared to a very low one caused by lemon oil, which is not in agreement with the results of the ergosterol assay. In our previous research we evaluated _CDR1_ and _CDR2_ genes expression level coding for drug efflux transporters in _C. glabrata_ clinical isolates [37]. Taking into consideration isolates with significantly elevated gene expression levels according to the isolate susceptible to fluconazole, the MIC values for the tested oils were in the range: thyme < 0.005-0.31, lemon < 0.005-0.08, geranium 0.04-1.25, cinnamon < 0.002-0.125, basil < 0.005-0.63 and clove 0.04-1.25. If most constituents of EOs were substrates for these transporters, the assumed values of MIC concentrations would rather reach the highest range. In our experiment we observe that the MIC values of all tested EOs were not significantly different in the group of strains that upregulate drug pumps. Thus, we suggest that components of EOs do not necessarily have to be the substrates for CDR1p and CDR2p transporters. We also considered the possibility of synergism between common antifungals and essential oils. [PERSON] et al. estimated that thymol and its isomer carvacrol possess synergistic action with fluconazole toward some isolates of the genus _Candidia_[38]; however, the calculated FIC index was in most cases 0.5, indicating small synergistic action. [PERSON] showed a synergistic interaction between thymol and fluconazole or amphotericin B [39]. In our study the interaction between fluconazole and thyme oil was assigned as indifferent; however, thymol was not the predominant compound of the tested thyme oil. We found the possibility of interaction of amphotericin B with cinnamon or geranium oil. Amphotericin B is still a very powerful antifungal drug and resistance is rarely seen. However, at the same time it is a toxic compound used only in advanced fungemia, so any dosage reduction relating to the simultaneous usage of plant extract could be useful in antifungal chemotherapy. Molecular switching between yeast, pseudohyphae, and hyphae phenotype is considered one of the most important virulence factors of _C. albicans_ as it enables the evasion of the host immune system and rapid infection establishment. Therefore, factors inhibiting hyppha formation are considered interesting leads that could help in the prevention of invasive fungemia. In our study we showed that most of the tested EOs have the potential for _C. albicans_ hyppha formation inhibition. In this matter our results are convergent with those obtained by Braga (inhibition of hyppha formation by thymol) [40], Pozzatti (inhibition of germ tube formation by basil, cinnamon, and thyme oils) [41], and Zone (inhibition of germ tube formation by geranium oil) [42]. The findings reported in this paper indicate the high antifungal potential of some essential oils against _C. albicans_ and _C. glabrata_ clinical isolates. This result is very interesting from the point of view of their potential use as an alternative for conventional treatment. Additionally, experiments confirming the possibility of a synergistic effect with amphotericin B allow us to conclude that EOs can be used as a supplement for traditional chemotherapy. The range of effective concentrations would allow for their use in treatment, e.g., in topical applications. Modes of EOs action undoubtedly require further clarification but it is commonly known that a rich mixture of compounds may cause many simultaneous cellular effects. Therefore, future experiments will focus on investigations of the mode of action of single compounds. Additionally, it is of great importance to establish which particular components of a mixture exert a synergistic effect with common antifungal drugs.