## 3 Results and Discussion
### Characterization of Nanoparticles and Biocidal Assays  
Figure 7a shows the histogram and frequency polygon for the nanoparticles obtained in the synthesis phase. As can be seen from the size distribution, the CuO-NPs presented sizes between 10 and 32 nm with a predominance of sizes in the range of 13 to 15 nm, which is quite consistent with that predicted by the Debye-Scherrer equation (16 nm). Figure 6: Graphical adjustment of the experimental data obtained in the equilibrium tests. The isotherms for the 4 models studied are shown. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Biomass** & **Biomass** & **qm (mg/g) \({}^{*}\)** & **Reference** \\ \hline Plant & _Ageratum convzoid_ & 51.57 & [21] \\ Marine macroalgae & _Sargassum filipendula_ & 40.8 & [28] \\ Yeast & _Saccharomyces cerevisiae_ & 4.73 & [29] \\ Filamentous fungus & _Penicillium citrium_ & 22.7 & [30] \\ Bacteria & _Geobacillus toebi_ & 41.5 & [31] \\ _Geobacillus thermoelocurons_ & 48.5 & [32] \\ Bacterial extract & _Mucliaginousiber_ & 8.ERMR7:07 & 8.36 & [33] \\ _Ochrobactrum cicero_ & 50.56 & & \\ Bacteria & _Stentrorophomonas maltophilia_ & 39.26 & [34] \\ _Pseudomonas putida_ & 38.97 & & \\ Bacteria & _S. epidermidis_ & CECT 4183 & 48.14 & This work \\ \hline \hline \end{tabular} \({}^{*}\)qm: Maximum biosorption capacity expressed in mg of metal per g of biomass.  
\end{table} 
Table 3: Maximum Cu(II) biosorption capacity for different types of biomass. Figure 7: Histogram and frequency polygon (**a**) obtained from an FE-SEM image (**b**) of the CuO-NPs synthesized from the cellular extract of _Staphylococcus epidermidis_ CECT 4183.  
of reactive oxygen species due to the transition metal character of copper. At the same time, stable and spherical morphologies facilitate the biocidal action of nanoparticles due to their greater specific surface area [10]. The average size of the CuO-NPs obtained in this work is smaller than those obtained by other authors [34] and, in general, smaller than those synthesized by bacteria [10], which, in principle, would support their potential as biocidal agents. Table 4 shows the Minimum Inhibitory Concentration (MIC) results of CuO-NPs for the microorganisms studied before and after the use of PVA. The use of PVA in the biocidal tests showed a significant improvement in the dispersion and stabilization of the nanoparticles, which resulted in a lower MIC that was between 50 and 75% of the initial value for most of the bacteria tested, with a 16-fold better effect in the case of _B. cereus_. Likewise, the results of the positive controls that included UPW with 10% PVA (liquid base of the CuO-NPs suspension), MH nutrient medium, and the respective bacterial suspensions (at 0.5 on the McFarland scale) confirmed that PVA does not have biocidal properties at that concentration and therefore the effect was exclusively due to the CuO-NPs. In parallel, Table 5 summarizes the results obtained by other authors, including the microorganisms tested, the sizes of the nanoparticles, their origin, and their biocidal effect. There are not many works that obtain CuO-NPs from bacteria, nor are there studies that evaluate their biocidal effect based on the MIC expressed in \(\mu\)g/mL, and there is also a wide variety of methods used to determine the biocidal effect. In many cases, the method seeks to obtain an approximation to the real values for practical purposes. In this work, the growth curves of the microorganisms tested were obtained, and it was confirmed that the MIC values were in line with reality. Despite all of the above, the results shown in Table 5 confirm that the CuO-NPs synthesized in this work have a good biocidal potential that, in some cases, is higher than that reported in other works. For example, as shown in the table, [PERSON] and [PERSON] [35] reported similar results for bacteria of the same genus, with MICs of 103.5 and 120.4 \(\mu\)g/mL for _E. coli_ and _S. aureus_, respectively. Similarly, [PERSON] et al. [36] obtained a MIC of 128 \(\mu\)g/mL for _E. coli_ and _S.