## 3 Results and Discussion
### Biosynthesis of AgNPs Using L. plantarum TA4 Biomass  
For the biosynthesis of AgNPs, the aforesaid method was performed. Similarly, the reduction of Ag\({}^{+}\) to AgNPs was determined by visual observation correlated with color change. Figure 3a displays the reaction mixture of _L. plantarum_ TA4 cell biomass in the Ag\({}^{+}\) suspension (white) that turned dark brown after 24 h of incubation, suggesting the formation of AgNPs. It is well known that the color change is due to the excitation of SPR of the AgNPs. The UV-Visible absorption spectra analysis presented in Figure 3b revealed the maximum SPR excitation band of biosynthesized AgNPs centered at 429 nm, which is the typical peak obtained for AgNPs. The maximum absorption band obtained in this study is similar to the range reported in other studies [31; 32]. While several studies have been conducted on the biosynthesis of AgNPs using a microbial source, a critical aspect to the development of a safe and non-pathogenic microbial platform is still lacking. A previous study by [PERSON] et al. [33] utilized the supernatant of _E. coli_ for the synthesis of AgNPs. Likewise, [PERSON] and [PERSON] [34] produced AgNPs with the culture supernatant of _S. aureus_. Both bacteria are recognized as common opportunistic pathogens that may pose safety issues during the AgNPs synthesis process. Therefore, this study exploited beneficial bacteria, _L. plantarum_ TA4, in the production of AgNPs to overcome the problems that may arise when using pathogenic organisms. _Lactobacillus_ is of particular interest due to its non-pathogenic feature and food-grade status, i.e., the \"generally recognized as safe\" (GRAS) bacteria confirmed not to exhibit any adverse effects on the individual handling it [25]. Microorganisms exhibit an inherent potential to synthesize AgNPs through either the culture supernatant or the cell biomass route. In contrast, several studies documented the biosynthesis process using culture supernatant, and fewer studies used the cell biomass of _Lactobacillus_. Microorganism-mediated synthesis of AgNPs employing _Lactobacillus_ through  
Figure 3: (**a**) Visual observation of AgNPs formation by cell biomass of _L. plantarum_ TA4. (**b**) UV–Visible absorption spectra of biosynthesized AgNPs. The inset image shows the visual observation of biosynthesized AgNPs. various routes is presented in Table 1. It should be noted that biosynthesis using culture supernatant involved a nutrient in the culture medium that might facilitate the formation of AgNPs; hence, the exact identification of biomolecules involved in the synthesis process using culture supernatant is still uncertain. Nevertheless, in this study, we solely use cell biomass of _L. plantarum_ TA4 for the biosynthesis process, which denotes the capability of the microorganism to synthesize AgNPs. Moreover, the AgNPs generated in this study are smaller than the other reported studies (Table 1) using the _Lactobacillus_ species and on par with the route using cell culture supernatant, which underlines the significance of our finding. Earlier studies indicated that the fundamental mechanisms for the formation of AgNPs were facilitated by the enzyme nitrate reductase [7; 39], which involved the transfer of electrons from nitrate to the metal group [40]. Likewise, [PERSON] et al. [41] reported that the reduction of selenium nanoparticles by _Proteus mirabilis_ YC801 occurred by cytoplasmic enzymatic reduction involving thioredoxin reductase, glutathione reductase, or NADH-related reductase. On the other hand, [PERSON] et al. [11] discovered the biosynthesis of AgNPs through the interaction of Ag\({}^{+}\) with the functional groups present on the cell membrane of several _Lactobacillus_ species. The AgNPs were observed to be distributed on the outer layer of the bacterial cell, as visualized by TEM [11]. As a Gram-positive bacteria, _Lactobacillus_ is composed of a thick layer of peptidoglycan, polysaccharides, teichoic acids, and proteins that provide many anionic surface groups on the cell membrane [10; 42].