###### Abstract  
Copper contamination of natural waters is a global problem that affects ecosystems and public health, yet this metal is an essential micronutrient and has important applications. The efficacy of _Staphylococcus epidermidis_ CECT 4183 as a Cu(II) biosorbent in synthetic solutions and its potential ability to synthesize CuO nanoparticles (CuO-NPs) from its cellular extract was investigated. In addition, the biocidal potential of the nanoparticles was evaluated against five microorganisms. Using response surface methodology, the optimal operating conditions were determined to be biomass dose, 0.2 g/L, and pH 5.5. Equilibrium tests were performed, and biosorption isotherms were obtained for four models with a maximum biosorption capacity of 48.14 mg/g for the Langmuir model. Different spectroscopic and microscopic techniques were used to determine the mechanisms involved in the biosorption process, which was dominated by surface physicochemical interactions with strong involvement of methyl, methylene, carbonyl, amino, and phosphate groups. The techniques also allowed for characterizing the obtained nanoparticles, which had a quasi-spherical morphology and an average size of 14 nm. Finally, biocidal tests showed that the CuO-NPs had a good inhibitory capacity for the microorganisms tested, with minimum inhibitory concentrations (MIC) between 62.5 and 500 \(\upmu\)g/mL for bacteria and between 1000 and 2000 \(\upmu\)g/mL for yeasts. _S. epidermidis_ CECT 4183 showed good potential for Cu(II) bioremediation and for the synthesis of CuO-NPs with biocidal capacity. _S. epidermidis_ CECT 4183 showed good potential for use in Cu(II) biosorption, and its cell extract presented a high capacity for the green synthesis of CuO-NPs, which at the same time turned out to be good biocidal agents. biosorption; copper; CuO-NPs; green chemistry; biocide test; minimum inhibitory concentrations; FT-IR; XRD; FESEM-EDX  
often present in the dust produced by these sources and then enters aquatic environments through runoff [3]. Although it is an essential micronutrient, when it is deposited in the soil it can have devastating effects on the soil microbiota, which is why several authors are working to identify methods that incorporate markers capable of identifying the tolerance of microorganisms to basal concentrations of this metal [4]. Recent studies have shown that in addition to inducing cellular damage through induction mechanisms, copper can stimulate the conjugation and transfer of plasmids in the environment, accelerating the spread and amplification of antibiotic-resistance genes (ARGs). In parallel, the formation of complexes between heavy metals and antibiotics can modify the bioavailability of the metal and affect the toxicity of the mixture, which in many cases has a greater ease in crossing the cell membrane [5]. The case of the antibiotic sulfamethoxazole (SMX) is widely studied. [PERSON] and coworkers studied the interaction between this antibiotic and Cu(II) and its role as a carrier of runoff pollutants in roadway sediments (RDS). The authors demonstrated that when SMX and Cu(II) coexist in the runoff, and an appropriate pH is provided, they form a ternary complex RDS-Cu(II)-SMX to increase the occurrence of both [6]. For all these reasons, the search for technological solutions aimed at mitigating the dispersion of copper in natural environments is highly topical, with biological techniques such as biosorption of heavy metals being one of the most promising, effective, and ecological strategies since in most cases this metal is found in low concentrations in the effluents that carry it, preventing its elimination by conventional physicochemical techniques and causing it to end up biomagrifying in ecosystems. In this work, the bacterium _Staphylococcus epidermidis_ CECT 4183 was used to evaluate its capacity for Cu(II) biosorption in synthetic solutions. At the same time, it is known that copper is a metal that can form CuO nanoparticles (CuO-NPs) through the action of some mechanisms involved in biosorption.