## 1 Introduction  
Nanotechnology is a major emerging research area for the manufacturing of new materials at the nanoscale level, which involves specialized structures, design, devices, and production at the nanometer scale [1]. The synthesis of nanoparticles (NPs) from gold has emerged as a vital application of nanotechnology [2]. Among metal NPs, AuNPs are very useful for efficient drug delivery owing to their simple, well-studied synthesis process and low toxicity [3]. AuNPs with a diameter of \(\leq\)5 nm can be used as a catalyst in biomedical reactions [4]. The major biomedical applications of AuNPs include targeted drug delivery, anticancer therapy, molecular imaging in living cells, antimicrobial activities, biosensors, and biocatalysts [4]. Furthermore, AuNPs have been used for cosmetic purposes, such as skin wound disinfection, anti-inflammation, and antiaging creams [5]. However, few studies have explored the antioxidant, antityrosinease, and antimalenogenesis properties of AuNPs.  
In recent years, NPs have been synthesized using physical and chemical techniques. However, these synthesis techniques are expensive and involve the use of toxic chemicals [6]. NP synthesis using physical techniques requires high energy utilization, leading to high operational costs. Chemical synthesis techniques are low cost but involve the use of toxic solvents, which may result in contamination due to toxic byproducts. To overcome these challenges and to increase the range of application of AuNPs, a green and nontoxic synthesis method is urgently required. Biological synthesis methods are green alternatives for NP production. Many bacteria, fungi, yeast, plants, and algae can be used for high-yield, low-cost, and environment-friendly synthesis of NPs [7; 8]. AuNPs synthesized by microorganisms are very useful in the production of effective antibacterial agents because of their nontoxic nature, polyvalent effects, and photothermal activity [9]. Among microorganisms, the use of prokaryotes for AuNP synthesis has received the most attention [10]. The surface energy of the NPs increases due to the surface reactive sites available on the NPs' surfaces. This increases the contaminant removal efficiency of AuNPs even at low concentrations, making them suitable for use in water purification [11]. However, the exact mechanism by which microorganisms produce AuNPs remains unclear. Based on synthesis location, extracellular and intracellular synthesis mechanisms have been speculated on [8]. Different bacteria adopt different synthesis mechanisms and produce AuNPs with different sizes (1.9-100 nm) and shapes (cubic, spherical, triangular, hexagonal, and octahedral) [10]. Consequently, AuNPs produced by microorganisms exhibit different physiological activities or reactivities. However, studies have often focused more on determining the physical, chemical, and crystalline characteristics of AuNPs by using various precision instruments [9], and few studies have explored the multifunctional activities of AuNPs.  
In this study, six different strains of bacteria--_Pseudomonas aeruginosa_, _Bacillus cereus_, _Bacillus subtilis_, _Escherichia coli_, _Bifidobacterium lactis_, and _Lactobacillus acidophilus_--were isolated from an abandoned gold mining area to evaluate their suitability for AuNPs production. Optimal strains were adopted to produce AuNPs and explore the antimicrobial, tyrosinase inhibitory, antioxidant, photocatalytic, and antimelanogenesis activities of the biosynthesized AuNPs. Furthermore, the cytotoxicity of the biosynthesized AuNPs to human epidermal melanocytes (HEMn) and the normal human skin fibroblast cell line (CCD-966 SK) were examined. To our knowledge, this is the first study to evaluate multifunctional activities of biosynthesized AuNPs and their potential for use in food, environmental hygiene, medicine, and cosmetics.