## 3 Results and Discussion  
### Maximum Tolerable Concentration (MTC) of L. plantarum TA4 against Ag\({}^{+}\)  
The research on the use of bacteria for the biosynthesis of NPs has gained considerable attention in the last decade as an alternative to the conventional method. In general, not all bacteria can synthesize NPs. The synthesis of NPs depends on the ability of bacteria to resist the metal ion, where the bacteria tend to reduce the ion into respective NPs metal [25] under the metal stress. Some bacteria have been documented to tolerate metal ion and have the potential for metal NPs synthesis [10; 14; 22; 26; 27]. In our study, the cell biomass of _L. plantarum_ TA4 demonstrated a MTC against Ag\({}^{+}\) at 4 mM (based on cell growth). Moreover, no growth was observed beyond the MTC value, indicating the toxic effect of Ag\({}^{+}\) to the cell (Figure 2a). Further, the formation of AgNPs was confirmed using UV-Visible spectroscopy. The reduction of Ag\({}^{+}\) into AgNPs was evaluated by recording the absorption spectrum of the reaction. UV-Visible spectra show no evidence of absorption in the range of 400 to 500 nm for the control suggests the absence of AgNPs. As presented in Figure 2b, a narrow absorption spectrum with a main band centered at 429 nm was obtained at a concentration of 2 mM, which corresponds to surface plasmon resonance (SPR) of AgNPs. Moreover, a broad absorption spectrum was observed at concentrations of 1, 4, 6, 8, and 10 mM with main bands centered at 441, 410, 335, 406, and 407 nm, respectively, which implies the formation of AgNPs. The results show that the SPR band intensity increased as the AgNO\({}_{3}\) concentration increased from 1 mM to 2 mM and the band intensity began to decrease above this concentration (at concentration of 4, 6, 8, and 10 mM). The possible reason for this may be due to the toxic effect of high Ag\({}^{+}\) concentration on cells that restricting the formation of NPs, resulting in only a limited amount of AgNPs produced (based on gradually changes of color from dark brown to transparent color). Furthermore, this behavior correlates with no viable cells (Figure 2a) obtained beyond 4 mM. Meanwhile, a broad absorption spectrum was obtained at Ag\({}^{+}\) concentration of 1 mM compared to 2 mM which may be attributed to lower availability of Ag\({}^{+}\) to react with the cells, resulting in less AgNPs production and consequently generating wider spectrum. However, the SPR band intensity was found to be higher at a concentration of 2 mM probably because the cells were able to tolerate with Ag\({}^{+}\) and more AgNPs were produced resulting in increased band intensity. It is clear that _L. plantarum_ TA4's capability to produce AgNPs depends on its ability to resist Ag\({}^{+}\). Likewise, _Bacillus_ strain CS11, which was isolated from heavy metal contamination areas, can form AgNPs when treated with 1 mM Ag\({}^{+}\)[21]. [PERSON] et al. [22] also reported _Rhodococcus aetheriorans_  
Figure 2: (**a**) Maximum tolerable concentration of _L. plantarum_ TA4 exposed to increasing concentration of Ag\({}^{+}\). The bacterial cell growth was determined by the spot planting technique. (**b**) UV–Visible absorption spectra of biosynthesized AgNPs by _L. plantarum_ TA4 exposed with different concentrations of Ag\({}^{+}\). (Control) AgNO\({}_{3}\), (i) 1 mM, (ii) 2 mM, (iii) 4 mM, (iv) 6 mM, (v) 8 mM and (vi) 10 mM. BCP1 as having the ability to detoxify selenium ion at higher concentration and simultaneously produced selenium NPs. Nevertheless, _L. plantarum_ TA4 exhibited lower resistance to Ag\({}^{+}\) compared to other reported microorganisms [28]. Several studies have been done in the isolation of metal tolerance-microorganisms from ecological niches rich in metal for the synthesis of NPs [14; 15]. In our study, _L. plantarum_ TA4 was isolated from local fermented food, which explained its low resistance to Ag\({}^{+}\).