## 1 Introduction  
Filamentous fungi are pervasive in the natural environment. They can cause the decomposition of different food stuffs of vegetable or animal origin [1]. They also contribute to the biodegradation of synthetic and natural materials, such as wood, leather, paper, or silk [2, 3]. Often, these microorganisms are harmful to humans. They play a significant role in allergies, mycosis, and pulmonary infections, especially in immunocompromised patients. Mycotoxins--the secondary metabolites of molds--are also dangerous as they can accumulate over the years and contribute to the development of cancers in humans and animals [4, 5]. Filamentous fungi have been used for decades as natural producers of enzymes, acids, and antibiotics [6, 7]. One of the most common and important molds is the genus _Aspergillus_, and especially the species _Aspergillus niger_. Even though citric acid and many enzymes produced by these molds are Generally Recognized as Safe (GRAS status) [8], _A. niger_ often causes food spoilage and biodegradation of building materials [9, 10].  
Various kinds of biocide are used to destroy microorganisms, including molds. Some fungicides have been prohibited by the European Chemical Agency, including carbendazim, which was used previously both as a pesticide [11] and as a preservative film in buildings [12]. New compounds are therefore being sought that are safer for both humans and the environment. Only a few articles have described the mechanisms of action of these new antifungal agents at the cellular and molecular levels, especially in molds. Biocides can act in several places on the cells of these microorganisms, such as the cell wall, cell membrane, and the entire cell interior, affecting the DNA, mitochondria, and cellular reactions [13]. Destruction of molds is difficult, as molds occur in two basic morphological forms: mycelia and spores. Many of the previously -used antifungal agents are becoming less effective due to spontaneous mutations or phenotypic adaptation of molds to harmful environmental conditions.  
Gemini surfactants are a group of cationic and neutral compounds, described as antifungal agents. However, the antifungal activity of these substances has only been investigated in yeast [14; 15]. No studies have been conducted on molds that are common in various places. In previous studies [16] on the effect of these substances on _Aspergillus brasiliensis_ ATCC 16404 (formerly _A. niger_ ATCC 16404), the authors found ionic gemini surfactants were characterized by high antimicrobial efficacy. Dry weight and ergosterol were reduced and the mycelium formed a conglomerated structure. Cationic gemini surfactants also had better affinity to ergosterol than their neutral analogues. The aim of our current research was to further examine the changes in mycelium cells of _A. brasiliensis_ caused by the activity of hexamethylene-1,6-bis-(\(N\),\(N\)-dimethyl-\(N\)-dodecylammonium bromide) (C6) and pentamethylene-1,5-bis-(\(N\),\(N\)-dimethyl-\(N\)-dodecylammonium bromide) (C5), in comparison to their neutral analogues: hexamethylene-1,6-bis-(\(N\)-methyl-\(N\)-dodecylamine) (A6) and pentamethylene-1,5-bis-(\(N\)-methyl-\(N\)-dodecylamine) (A5). We investigated the protein and enzyme profiles as well as the ultrastructure of _Aspergillus brasiliensis_ ATCC 16404. The results of our study provide an analysis of several potential areas of action of gemini surfactants in mycelial cells and prove the high effectiveness of these compounds.