## 3 Results and Discussion
The study indicated that the percentage composition of zinc was 65.93% and that of oxygen was 34.07%. This result agrees with the findings of [PERSON] and [PERSON] [42], who found the percentage composition of zinc was 66.16% and that of oxygen was 33.84% for ZnO NPs synthesized by _Agaricus bisporus_ mushroom extract. The results of ZnO mapping shown in Figure 2A,B proved that zinc and oxygen were present in the synthesized ZnO NPs, which further verifies that the mushroom-synthesized ZnO NPs had no impurities. Figure 1: Characterization of _P. ostreatus_-synthesized ZNO NPs: (**A**) UV-Vis spectrum of the synthesized ZnO NPs; (**B**) FTIR spectrum of ZnO NPs; and (**C**) XRD pattern of ZnO NPs. Figure 2D shows the transmission electron microscopy (TEM) images of the amorphous ZnO NPs, which had dark spots that are probably due to the stacking of a large number of layers of nanosheets. This indicates that the synthesized ZnO NPs were probably agglomerated, since ZnO nanoparticles are also prone to forming agglomerations that are either hard or soft, where hard agglomeration is caused by the chemical reaction of the surface groups and soft agglomeration is caused by other physical effects [43]. Furthermore, the selected area electron diffraction (SAED) pattern in Figure 2F shows bright rings, which confirm the preferential orientation of mushroom-synthesized ZnO crystals. The SEM images in Figure 2E indicate that the synthesized ZnO NPs have a spherical morphology and are agglomerated together. Furthermore, as expected, the green-synthesis method is known to usually cause agglomeration in NPs because the green-synthesized NPs mostly have a high surface area, allowing NPs to stick together [44]. However, polarity and the electrostatic attraction of ZnO NPs may also have contributed to the agglomeration of ZnO NPs [39]. The surface charge and stability of _Pleurotus ostreatus_-mediated ZnO NPs were examined using the zeta potential (Figure 3). In this study, the _Pleurotus ostreatus_-synthesized ZnO NPs were observed to be negatively charged with good stability; thus, the zeta potential was found to be \(-23.3\) mV. These results suggest that the synthesized ZnO NPs had negatively charged capping molecules, which are responsible for the stability of the synthesized nanoparticles [45]. These findings corroborate well with the results reported by [PERSON] and [PERSON] [42], who found the zeta potential of _Agaricus bisporus_-mediated ZnO NPs to be \(-20.5\) mV, with good stability and a negative charge. Other researchers also confirmed that particles with charges of or above \(-25\) mV and \(+25\) mV tend to be more stable [46]. Figure 2: Structural characterization of ZnO NPs by electron microscopy prepared from _P. ostreatus_: (**A**,**B**) are EDS elemental maps (20 kV) of oxygen (Ka1) and zinc (Ka1) emission lines, respectively; (**C**) EDX analysis (20 kV) of O Ka1 and Zn Ka1; (**D**) TEM images of synthesized ZnO NPs; (**E**) SEM micrograph image of synthesized ZnO NPs; and (**F**) SAED pattern of ZnO NPs. Most pathogenic microorganisms, such as those of clinical isolates, have been reported to exhibit a higher level of resistance towards antibiotics that are particularly used in medical care facilities [47]. In addition to the clinical isolates, the bacterial strains associated with foodborne outbreak infections also proved to have higher levels of resistance towards antibiotics [48]. The literature states that zinc oxide nanoparticles can be used as antimicrobial agents against pathogenic microorganisms [49]. In the present study, the antibacterial activity of mushroom-synthesized ZnO NPs was investigated for both Gram-positive (_C. albicans, B. cereus, S. aureus_, and _E. faecalis_) and Gram-negative (_K. pneumoniae, P. aeruginosa, P. vulgaris, E. coli_, and _M.