## 4 Discussion
These observations suggest that, apart from the effect of the photocatalyst, the observed susceptibility is also dependent on the group of microorganisms under investigation. Several different studies have shown that TiO\({}_{2}\)-NT surface photocatalysis is capable of neutralizing a wide range of organisms, including Gram-negative [49; 50; 51] and Gram-positive bacteria [52; 53], including their endospore forms [54], fungi [55; 56], algae [57; 58], protozoa [59; 60] viruses, and prions [61; 62]. The complete mechanism of microbial cell death via photocatalysis is still not completely elucidated [63]; however, it was shown that it is a combination of non-specific oxidation-related damage to the cell wall and DNA [64] via hydroxyl, peroxide and superoxide radicals generated during photocatalysis. This oxidative damage leads to the activation of several genes coding for enzymes and proteins [65] that try to minimize the entry of free radicals inside the cell. [PERSON] et al. [66] showed that free radical attack had a significant damaging effect on the cell wall and its components. The disturbance of the cell wall and the cells activates a number of genes and proteins that produce lipids for the cell membrane found immediately below the cell wall as a second line of defence. The results obtained in the study at hand are in line with the literature [65; 66], in which it was shown that Gram-positive bacteria were more resistant than Gram-negative bacteria. This difference is usually attributed to the difference in cell wall structure. Gram-positive bacteria have a thicker peptidoglycan layer that buffers the entry of free radicals inside the cell. In the case of yeast and fungi, the cell wall characteristics are more complex than the ones of bacteria. The results showed that yeast cells tend to be more susceptible to free radical oxidation when compared to fungal cells; in fact, _Cryptococcus albidus_ and _Penicillium chrysogenum_ were the most resistant. The difference between the susceptibility of fungi and yeast can also be explained in terms of their cell wall structure. In the case of yeast cells, the inner cell membrane is protected by a cell wall that is composed of an inner chitin layer followed by a layer of \(\beta\)-1,6-glucans and \(\beta\)-1,3-glucans, which connect the inner and outer cell wall [67; 68]. The outer most layer of the cell wall is packed with mannoproteins that cross-linked to \(\beta\)-1,6-glucans [69]. In the case of fungi, the cell wall composition is more complex than that found in yeast cells. Fungal genera such as _Aspergillus_ have been shown to mannoproteins and are covalently bonded to the glucanchitin core, while, in the case of yeasts such as _S. cerevisiae_ and _C. albicans_, these are linked but not covalently bonded [70; 71]. Furthermore,_Aspergillus_ genera have been shown to have additional \(\beta\)-1,3-1,4-glucan and \(\alpha\)-1,3-glucan apart from of \(\beta\)-1,6-glucan and \(\beta\)-1,3-glucan found in yeast. Furthermore, studies carried out by [PERSON] et al. [71] and [PERSON] et al. [70] showed that the outermost layer is composed of galactosaminogalactan, which is constituted of \(\alpha\)-1-4 linked galactose and \(\alpha\)-1-4 linked N-acetylgalactosamine residues rather than mannoproteins such as those found in yeast. The additional \(\beta\)-1,3-1,4-glucan, \(\alpha\)-1,3-glucan and the presence of a stronger bonding interaction between the chitin layer and mannoproteins could in fact explain the higher percentage survival rate of fungi when compared to yeast.