## 3 Discussion  
This paper compared the effectiveness of several photobactericidal protocols against biofilms formed on glass and stainless steel discs by four MRSA strains of _S. aureus_. Methylene blue was applied as a photosensitizer due to its many advantages, such as good solubility in water, a wide spectrum of light absorption, high efficiency of ROS release, and affordable price [15; 16]. Stainless steel and glass surfaces were chosen for biofilm formation as both types of materials are found in many sectors of public life. Initially, photoeradication of biofilms formed by the MRSA strains was carried out on the surfaces of glass and stainless steel discs in the presence of this dye at concentrations of 31.25 and 62.5 mg L\({}^{-1}\). In general, the effectiveness of these photoeradication protocols, regardless of the pathogenic isolate and the type of disc surface, was insignificant. It should be noted that our results did not support the data reported previously, indicating an extremely effective photodestruction of _S. aureus_ biofilm in the presence of MB at relatively low concentrations. [PERSON] et al. [17] demonstrated that the optimal concentration of MB as a photosensitizer against MRSA strains was 0.1 mM (that is 31.98 mg L\({}^{-1}\)). The studies by [PERSON] et al. [18] showed that MB concentrations of MB between 0.025 and 0.044 mM (that is, 8-14 mg L\({}^{-1}\)) were the most effective in combating the studied _cocus_. [PERSON] et al. [19] reported that irradiation of the _S. aureus_ biofilm with a light-emitting diode (625 nm; 18 J cm\({}^{-2}\)) in the presence of MB at a concentration  
Figure 5: Fluorescence microscopy images of biofilm formed on the glass surface by _S. aureus_ 3375 after the first (**a**), second (**b**) and third (**c**) exposures to laser light in the presence of MB alone at a nontoxic concentration of 31.25 mg L\({}^{-1}\). In the presence of the SYTO 9 and propidium iodide mixture, bacteria with intact cell membranes stain fluorescent green, whereas bacteria with damaged membranes stain fluorescent orange/red. of 64 mg L\({}^{-1}\) resulted in a reduction in cell number by 6 log\({}_{10}\) in colony-forming units. On the other hand, [PERSON] et al. [20] presented the results of the combating of the bacterial biofilm of _S. aureus_ from the lumen of the endotracheal tube. These authors used MB at a concentration of 500 mg L\({}^{-1}\) and non-thermal activated light (664 nm; fluidity coefficient 150 mW cm\({}^{-1}\)). After a single light treatment, the bacterial count has been shown to decrease by at least 99.9% (a 3 log\({}_{10}\) reduction in colony-forming units). It is worth noting that MB alone (without light activation) reduced the number of bacteria by approximately 90% (a reduction of 1 log\({}_{10}\) in colony-forming units). It is not possible to explain these contradictory results relating to the MB concentration that causes a satisfactory level of biofilm photomortality. It seems that it would be necessary to standardize the procedure of photoeradication of bacterial biofilms. To enhance the efficiency of photodestruction of biofilms formed by _S. aureus_ isolates, two types of spherical gold nanoparticles of similar sizes, 15 \(\pm\) 3 nm/20 \(\pm\) 3 nm, but differing in the type of stabilizer were used. The biogenic gold nanoparticles were synthesized by the cell-free filtrate from _Trichoderma koninigii_. This mycological filtrate played an essential role not only in the reduction of Au(III) ions but also in the formation and stabilization of gold nanoparticles [21]. The second type of gold nanoparticles were polyethyleneimine-stabilized structures. The results obtained clearly showed that the presence of gold nanoparticles in the photosensitizing mixture improved the inactivation of the biofilms developed on the glass and steel discs, and this effect was highly strain-dependent.