## 3 Results and Discussion
### Antibacterial Study  
#### 3.3.1 Antibacterial Activity (MIC/MBC)  
The antibacterial activity of Mn\({}_{0.5}\)Zn\({}_{0.5}\)Sm\({}_{x}\)Eu\({}_{x}\)Fe\({}_{1.8-2\times}\)O\({}_{4}\) (0.01 \(\leq\) x \(\leq\) 0.05) NPs was evaluated by determining the MIC/MBC values. The nanoparticles were tested at the concentration ranging from of 16 to 0.5 mg/mL. For _E. coli_, the MIC/MBC values obtained were in the range of 8/16, 8/16, 4/8, 2/4, and 2/4 mg/mL for x = 0.01, 0.02, 0.03, 0.04, and 0.05, respectively (Figure 5A). The MIC/MBC values for _S. aureus_, were also found in the range of 8/16, 8/16, 8/16, 4/8, and 4/8 for x = 0.01, 0.02, 0.03, 0.04, and 0.05 mg/mL, respectively (Figure 5B). The activity of the broth culture was determined as the effectiveness of the content of element Fe (x = n) in the test material. The obtained results demonstrated that the effectiveness of the test material showed little improvement with the increasing ratio of the Fe content, i.e., the minimum MIC/ MBC was obtained by Fe x = 0.05 and 0.06. However, an enhanced activity was found against _E. coli_ as compared to _S. aureus_; this slight difference could be attributed to the varying cell wall composition among these bacterial strains [41]. In some earlier studies, various metal-substituted NPs, like copper, zinc, nickel, and manganese, were recorded as possessing antibacterial activities [42; 43], although the antibacterial activity of the current combination of Mn\({}_{0.5}\)Zn\({}_{0.5}\)Sm\({}_{x}\)Eu\({}_{x}\)Fe\({}_{1.8-2\times}\)O\({}_{4}\) (0.01 \(\leq\) x \(\leq\) 0.05) NPs is the first of its kind to the best of the author's knowledge.  
3.2 Effects of Mn\({}_{0.5}\)Zn\({}_{0.5}\)Sm\({}_{x}\)Eu\({}_{x}\)Fe\({}_{1.8-2x}\)O\({}_{4}\) (\(0.01\leq\) x \(\leq\) 0.05) NPs on the Morphology of Bacteria  
The morphological alteration in _E coli_ and _S. aureus_ cells caused by Mn\({}_{0.5}\)Zn\({}_{0.5}\)Sm\({}_{x}\)Eu\({}_{x}\)Fe\({}_{1.8-2x}\)O\({}_{4}\) (\(0.01\leq\) x \(\leq\) 0.05) NPs was further studied by SEM. The untreated (control) cells of both organisms were normal in shape, and intact with a regular and smooth cell surface (Figure 6I). However, cells treated with Mn\({}_{0.5}\)Zn\({}_{0.5}\)Sm\({}_{x}\)Eu\({}_{x}\)Fe\({}_{1.8-2x}\)O\({}_{4}\) (x \(\leq\) 0.05) NPs were not found intact, i.e., the cells started appearing abnormal in shape with irregularities at the cell surfaces (Figure 6(Ib-If)). During the examination, untreated cells had no obvious visible damage, but the treated cells were observed as moderately damaged to severely damaged, with the increasing ratio of Fe\({}_{(x=n)}\), i.e., the maximum damage was caused by x = 0.04 and x = 0.05. Furthermore, it was observed that the effect of Mn\({}_{0.5}\)Zn\({}_{0.5}\)Sm\({}_{x}\)Eu\({}_{x}\)Fe\({}_{1.8-2x}\)O\({}_{4}\) (x \(\leq\) 0.05) NPs on both gram positive and gram negative were somehow similar, although a slightly enhanced activity was observed against _E. coli_ (Figure 6I,II). This difference in activity can be attributed to the varying cell wall composition between the two organisms, which in turn may play an important role in the attachment of Mn\({}_{0.5}\)Zn\({}_{0.5}\)Sm\({}_{x}\)Eu\({}_{x}\)Fe\({}_{1.8-2x}\)O\({}_{4}\) (x \(\leq\) 0.05) NPs to the bacterial cell surface, and is therefore essential in obtaining enhanced antibacterial activity [44].