**1. Introduction**

Nowadays, when antimicrobial resistance becomes a worldwide problem [1], the alternative ways of treatment, such as the use of natural products, are getting the foreground [2]. EOs are synthetized via the secondary metabolism from different parts of plants, such as buds, leaves, flowers, twigs, seeds, fruits, roots, bark or wood, located in secretory tissues [3] (p. 95). From a chemical point of view, EOs are a complex mixture of volatile elements, mainly terpenoids, phenol-derived aromatic compounds and aliphatic compounds, with high antibacterial, antifungal, antiviral, antiparasitical and insecticidal activities [4].

The use of spices and herbs in food preservation or in medicine for their health benefits has a long history [5]. The botanical family *Lamiaceae* has significant representatives for the production of EOs, such as *Origanum vulgare* L., *Mentha piperita* L. and *Thymus vulgaris* L. with a large-scale of antioxidant and antimicrobial characteristics [6]. Tea tree EOs from *Melaleuca alternifolia* Maiden

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and Betche (*Myrtaceae*) have shown antiseptic and anti-inflammatory e ffects [7] and medical plant *Eugenia caryophyllata* Thunb., which belongs also to the family *Myrtaceae,* is known for its antifungal, antiviral, antioxidant, anti-inflammatory and anticancer attributes [8]. Previous studies have shown diverse e ffects of cinnamon plants (*Cinnamomum cassia* L.) of the *Lauraceae* family: antidiabetic, antioxidant, anti-inflammatory, antimicrobial, anticancer and other important activities, such as lipid or cardiovascular disease-lowering compounds [9]. *Cymbopogon flexuosus* Nees ex Steud., worldwide known as lemongrass (from East Indian), belongs to the *Poaceae* family, which is important in EO productions [10,11], possessing antifungal and antimicrobial properties [12,13]. EOs obtained from *Thuja plicata* Donn. (*Cupressaceae*) tree leaves were assigned a broad spectrum of antimicrobial abilities, as well as potential uses in the reduction of sick building syndrome (SBS) [14].

Airborne microorganisms, such as fungi and their spores, are widespread in outdoor and indoor habitats, too [15]. From a health point of view, fungi are representatives of serious health hazards, including respiratory problems, fungal infections (mycoses), irritant e ffects, allergy reactions and other nonspecific medical troubles [16,17], and could also be a potential problem linked with sick building syndrome—SBS [17,18]. Members of the genus *Penicillium* are considered to be among these colonizers, with diverse roles: decomposition of organic materials by its enzymatic properties, producing toxic secondary metabolites (SM; mycotoxins) and even indoor air quality contaminators [19]. Filamentous fungus *Penicillium rubens* was, according to the study of Wilson and Straus [20], the most detected species associated with SBS.

The fungal contamination in an indoor environment can be solved using fungicides able to kill or inhibit the fungal growth [16]. Synthetic antifungal agents such as bleach, alcohol (100%), quaternary ammonium compounds, formaldehyde and multipurpose industrial disinfectants such as Cavicide ® and Virkon ® are detergents frequently used for contamination removal [16], for their unpleasant e ffects on human health are considered as hazardous, which can cause respiratory tract irritation and allergic respiratory and skin reactions. The demand of natural fungicides instead of synthetic chemicals is rising [21] due to concerns regarding human health [22,23]. Replacing synthetic antifungal agents with plant extracts could be an alternative that can reduce, remove or control fungi and molds in a specific environment [23].

Many studies are present in the literature about the e ffects of EOs against microorganisms [24], but there is little information, especially about fungi, regarding the genetic and metabolism mechanisms that cause these antimicrobial e ffects. Several authors have described the fungicide activity of EOs as that their components cause the disintegration of fungal hyphae, amplifying membrane permeability [25,26]. An antifungal e ffect is associated with the lipophilicity of EOs that allows penetration through cell walls and the manipulation with enzymes implicated in the synthesis of cell walls, thus making changes in the morphological characteristics of the fungus [26,27]. In order to extend our knowledge, a key research is to understand the mechanisms of action of EOs to molds. A microarray hybridization approach is a useful method that enables the simultaneous detection of the expression levels of thousands of genes [28]. The expression of an organism's genes is a ffected by environmental growth conditions [29] and serves to provide information about gene regulation [30]. In this study, we have investigated the fungicide e ffects of the vapor of eight EOs on the gene expression of *P. rubens* using a microarray strategy.
