## 3 Results and Discussion
### Effect of AuNPs on MIC  
The results in Table 1 indicated that the MICs of _E. coli_ and _S. aureus_ were 6.25 and 12.5 mg/L in BLAuNPs (Table 1). The results were similar to those of AuNPs biosynthesized by _Olav nana_, which had an MIC of 9.14 mg/L [22]. [PERSON] et al. demonstrated that AuNPs caused bacterial membrane damage and cell disruption by inducing the production of intracellular reactive oxygen species (ROS). AuNPs can react with water vapor or oxygen present in the air, and generate ROS. Such active species can subsequently interact with other molecules present on the surface of a bacterial membrane, leading to further antimicrobial reactions. These cause oxidative damage to nucleic acids and bacteria [23]. Additionally, AuNPs can also play a role as an antioxidant (illustrated in Section 3.4). The inhibitory effect of BLAuNPs and chemically synthesized AuNPs on gram-negative bacteria _E. coli_ and _P. aeruginosa_ was stronger than that of BLAuNPs on gram-positive bacteria _S. aureus_, _C. acnes_, and _B. cereus_ (Table 1). However, the inhibitory effects of ECAuNPs on all tested strains were equally strong. Due to their small size and positive charge, NPs can destroy the bacterial cell wall and interact with DNA, thereby resulting in cell death [23]. Based on the results of the release of the cell constituents, the leakage of nucleotides from _E. coli_ and _S. aureus_ treated with BLAuNPs increased with an increase in contact time (data not shown). Higher amounts of nucleotides were released from _E. coli_ than from _S. aureus_, indicating that BLAuNPs had a stronger bactericidal effect on _E. coli_. [PERSON] et al. reported that the numerous peptidoglycan layers on the walls of gram-positive bacteria prevent the NPs from reaching the cytoplasmic membrane. Thus, gram-positive bacteria are less affected by the NPs than gram-negative bacteria are [24]. Both ECAuNPs and BLAuNPs exhibited optimal inhibitory effects on the opportunistic pathogenic yeast _C. albicans_ (MIC: 3.125 mg/L). Furthermore, the antimicrobial activity of chemically synthesized AuNPs (Chemicals-5 nm) was significantly inferior to those of biogenic AuNPs (Table 1). Although the particle size of AuNPs was similar, the antimicrobial activity was obviously different. This result might be attributed to the shape of AuNPs, the uniform dispersion of AuNPs, or the surface capped molecules on AuNPs [25]. AuNPs synthesized by _Sargassum incisionium_ and _Prunus persica_ were ineffective against bacteria [26; 27]. The MIC values of AuNPs synthesized by _Alpinia nigra_ against _E. coli_, synthesized by _Convolvulus fraticosus_ against _E. coli_ and _S. aureus_, and synthesized by _Trichoderma harziannum_ against E. coli_ were 300, 75, and 20 mg/L, respectively [20; 24; 28]. Clearly, ECAuNPs and BLAuNPs were effective broad-spectrum antibiotics. As _E. coli_, _S. aureus_, and _B. cereus_ are foodborne bacteria [29] and _E. coli_, _S. aureus_, _P. aeruginosa_, _C. albicans_, and _C. acnes_ are frequently found in cosmetics or associated with common skin disorders [30], ECAuNPs and BLAuNPs have the potential applications in the food, environmental hygiene, and cosmetics.  
Figure 2: Effect of pH on the particle size of AuNPs synthesized by (**a**) _B. lactis_ after 72 h of incubation and (**b**) _E. coli_ after 24 h of incubation.