## 3 Results and Discussion
### Antimicrobial Activity of AuNPs  
The activity of AuNPs to control or inhibit the growth of potentially pathogenic microbes was investigated against _Bacillus subtilis_, _Staphylococcus aureus_, _Escherichia coli_, _Pseudomonas aeruginosa_, _Candida albicans_, and _Aspergillus brasiliensis_, as summarized in Figure 5A,B. Analysis of variance showed that the antimicrobial activity of synthesized AuNPs was dose-dependent. These results are compatible with those reported by [PERSON] et al. [26] and [PERSON] et al. [85], who showed that the antimicrobial activity of biosynthesized nanoparticles was dependent on various factors, such as size, shape, and concentration. Herein, the maximum zone of inhibition (ZOI) was reached at high AuNP concentrations (200 \(\mu\)g mL\({}^{-1}\)): 18.7 \(\pm\) 0.6, 19.0 \(\pm\) 1.0, 21.3 \(\pm\) 0.6, 20.7 \(\pm\) 0.6, 17.7 \(\pm\) 0.6, and 16.7 \(\pm\) 0.6 mm towards _B. subtilis_, _S. aureus_, _P. aeruginosa_, _E. coli_, _C. albicans_, and _A. brasiliensis_, respectively (Figure 5A,B). These inhibition zones decreased when the AuNP concentration was reduced to 100 \(\mu\)g mL\({}^{-1}\) (16.3 \(\pm\) 1.2, 17.3 \(\pm\) 0.6, 19.3 \(\pm\) 0.6, 17.7 \(\pm\) 1.2, 15.7 \(\pm\) 0.6, and 14.3 \(\pm\) 0.6 mm, respectively). With a dose of 50 \(\mu\)g mL\({}^{-1}\), the ZOIs were reduced to 13.6 \(\pm\) 0.6, 14.7 \(\pm\) 0.6, 17.3 \(\pm\) 0.6, 16.3 \(\pm\) 0.6, 13.3 \(\pm\) 0.5, and 12.7 \(\pm\) 0.6, respectively. In a similar study, the antimicrobial and antifungal activity of AuNPs fabricated by the reaction of gold tetrachloride with the aqueous extract of _Amona muricata_ increased with the dose of the antimicrobial agent [86]. Hence, [PERSON] et al. reported that bacterial strains _Clostridium sporogenes_, _Enterococcus faecalis_, and _Klebsiella pneumoniae_ and fungal strains _A. flavus_, _F. oxysporum_, _P. camemeri_, and _C. albicans_ were inhibited with percentages of 54, 46, 52, 31, 50, 66, and 42%, respectively, at a concentration of 4 mg L\({}^{-1}\). These values were slightly reduced to 50, 40, 49, 30, 45, 60, and 40%, respectively, when the concentration decreased to 2 mg L\({}^{-1}\). The maximum antibacterial activity of AuNPs formed by harnessing the metabolites of fresh leaves of _Pergularia daemia_ was attained at 300 \(\mu\)L mL\({}^{-1}\) against _S. aureus_, _E. coli_, and _P. aeruginosa_, with ZOIs of 16, 19, and 17 mm, respectively [87]. The incorporation of NPs into polymers or natural substances enhances their antibacterial activity. For instance, the coating of an amorphous carbon matrix with silver nanoparticles improves its antibacterial activity against _E. coli_ and _S. aureus_[88]. In the current study, AuNPs exhibited promising antimicrobial activity with minimal cytotoxic effect against normal cells, supporting interest in their application in biomedicine. Consistently with this study, AgNPs synthesized by an aqueous extract of _Acanthosperm australe_ exhibited antimicrobial activity toward various skin and soft tissue infection microbes with low cytotoxic activity against human normal cells [89]. [PERSON] et al. reported that this high antimicrobial activity is due to AgNPs rather than to the plant metabolites that coat the NP surfaces. The effectiveness of infection control depends on the treatment strategy chosen; this also affects the evaluation of MIC (minimum inhibitory concentration) values [90]. In the current study, MIC values were evaluated based for the lowest concentration of AuNPs to inhibit microbial growth. Data analysis revealed that the MIC value was 25 \(\mu\)g mL\({}^{-1}\) for _B. subtilis_ and _A. brasiliensis_, with ZOIs of 11.3 \(\pm\) 0.6 and 10.3 \(\pm\) 0.6 mm, respectively. The MIC value was 6.25 \(\mu\)g mL\({}^{-1}\) for _P. aeruginosa_ and _E. coli_ and 12.5 \(\mu\)g mL\({}^{-1}\) for _S. aureus_ and _C. albicans_, with ZOIs in the range of 9-11 mm (Figure 5A,B). Therefore, the antimicrobial activity of _Z. officinal_-based AuNPs against selected microorganisms can be ranked as follows: _P. aeruginosa_>_E. coli_>_S. aureus_>_B.