## 2 Results  
The antimicrobial susceptibility to antibiotics of the clinical isolates of _S. aureus_ is shown in Table S1 (see: Supplementary Information). As can be seen in this table, the tested strains were resistant to penicillin (methcillin), but these bacteria were sensitive to gentamicin, erythromycinomycin, and tetracycline. The assessment of the susceptibility of biofilms formed by the studied isolates of _S. aureus_ on the surface of the glass and stainless steel discs to photosensization with MB was preceded by the determination of nontoxic concentrations of this photosensitizer and two types of gold nanoparticles designed as AuBNPs and AuChNPs (dark toxicity). The nontoxic concentration has been defined as the highest concentration of the MB, AuBNPs, and AuChNPs that, under the experimental conditions, caused a decrease in cell viability by no more than 20% (see Tables S2 and S3). As can be seen in these tables, in most cases, MB at a concentration of 31.25 mgL\({}^{-1}\) was found to be nontoxic to the tested _coccus_. It was estimated that AuBNPs and AuChNPs were nontoxic, most often at a concentration of 10 ppm. The effectiveness of photoeradication of biofilms developed on the surface of the glass and stainless discs after 30 min of laser light irradiation (energy fluence = 189 Jcm\({}^{-2}\)) was shown in Figures 2a-d and 3a-d. As can be seen in Figure 2a-d, MB alone at a concentration of 31.25 mg L\({}^{-1}\) was ineffective as a photosensitizer against the _S. aureus_ MRSA isolates. Figure 2: _Cont._  
Figure 3: _Cont_. Figure 2: Effect of MB alone, MB + AuBNPs, and MB+AuChNPs on the viability of biofilm formed on the surface of glass discs by _S. aureus_ 3374 **(a)**; _S. aureus_ 3375 **(b)**; _S. aureus_ 3515 **(c)**; _S. aureus_ 3690 **(d)** after exposure to laser light for 30 min (energy fluence was 189 J cm\({}^{-2}\)). Average \(\pm\)SD of three independent experiments is shown; in all groups, \(p<0.05\). Age of biofilm: (24 h) = biofilm formed on the glass surface within 24 h; (48 h) = biofilm formed on the glass surface within 48 h.  
The most effective photocidal activity of this dye against biofilms formed on the glass within 24 h, resulting in a reduction in the number of viable _S. aureus_ 3374 by no more than 0.19 \(\pm\) 0.02 log\({}_{10}\) units (bacterial mortality was approximately 67%). A similarly ineffective biocidal activity of MB was found for biofilms formed on the glass surface within 48 h with the highest reduction in the number of viable bacteria of 0.2 \(\pm\) 0.02% log\({}_{10}\) units (bacterial mortality was not higher than 63%) (Figure 2a). Figure 2a-d showed that the addition of gold nanoparticles to the MB increased the elimination of the _S. aureus_ biofilm, and this phenomenon depended on the MRSA isolate and the time of biofilm formation (age of biofilm). It should be noted that the results of biofilm destruction efficiency were still insufficient since the highest reduction in cfu of 1.21 \(\pm\) 0.02 log\({}_{10}\) units was observed for 24-h biofilm formed by _S. aureus_ 3515 using MB+AuChNPs as photosensitizer (bacterial mortality rate was about 94 \(\pm\) 1%). Under the same experimental conditions, the viability of biofilms formed on the surface of glass discs for 48 h decreased by 0.51 \(\pm\) 0.03 log\({}_{10}\) units, i.e., cell mortality reduced to 69 \(\pm\) 2%. The cell killing efficiency (regardless of the age of the biofilm formed on stainless steel) in the presence of MB alone was also insignificant and, in most cases, did not exceed 40% (Figure 3a-d). The only exception was the biofilms formed by isolate 3515 after 24 and 48 h of incubation, as bacterial mortality was 68 \(\pm\) 2 (cfu reduction of 0.49 \(\pm\) 0.03 log\({}_{10}\) unit) and 50 \(\pm\) 2% (cfu reduction of 0.38 \(\pm\) 0.03 log\({}_{10}\) unit), respectively (Figure 3c). The low photocidal efficacy of aPDT described above inspired an increase in the MB concentration as a photosensitizer to 62.5 mg L\({}^{-1}\).