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Nanomaterials 2020, 10, 1763 7 of 24 Table 1. Plant extract-mediated green synthesis of AuNPs and AgNPs with antimicrobial activity. Plant Extract Citrus limetta peel Luffa acutangula leaf Parkia speciosa leaf A. indica leaf Gomphrena globosa leaf Pedalium murex leaf Musa acuminate peel Caulerpa racemosa Eclipta alba Nepenthes khasiana leaf Abelmoschus esculentus pulp Synthesized NPs AgNPs AgNPs AgNPs AgNPs AgNPs AgNPs AgNPs AuNPs AuNPs AuNPs AuNPs Target Pathogen Micrococcus luteus, Streptococcus mutans, Staphylococcus epidermidis, S. aureus, E. coli, Candida spp. E. coli, Saccharomyces cerevisiae E. coli, S. aureus, Pseudomonas aeruginosa, Bacillus subtilis. S. aureus, E. coli. S. aureus, B. subtilis, M. luteus, E. coli, P. aeruginosa, Klebsiella pneumoniae. E. coli, K. pneumonia, Micrococcus flavus, P. aeruginosa, B. subtilis, Bacillus pumilus, S. aureus. B. subtilis, S. aureus, P. aeruginosa, E. coli. Aeromonas veronii, Streptococcus agalactiae E. coli, P. aeruginosa, B. subtilis, S. aureus. E. coli, Bacillus spp., Aspergillus niger, Candida albicans. B. subtilis, Bacillus cereus, E. coli, M. luteus, P. aeruginosa. Ref [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] A. indica: Azadirachta indica; M. luteus: Micrococcus luteus; S. aureus: Staphylococcus aureus; E. coli: Escherichia coli; P. aeruginosa: Pseudomonas aeruginosa; B. subtilis: Bacillus subtilis; K. pneumonia: Klebsiella pneumonia. However, under conventional synthesis methods, these NPs exhibit nearly no antimicrobial activity [117]. The difference may lie in a synergistic effect due to the combination of AuNPs or AgNPs and the plant extracts, which provide high concentrations of steroids, sapogenins, carbohydrates, and flavonoids that act as reducing agents of ions and cover agents, contributing to NPs high stability [118]. Antimicrobial application is primarily due to their ultra-small size and shape (250 times smaller than bacteria), enabling an electrostatic interaction between the Au or Ag from the NPs, and the negative charge on the cell wall or surface of microorganisms, resulting in a distortion of membrane’s power functions, such as permeability, osmoregulation, electron transport, and respiration, leading them towards cellular death. The previous depends also on surface availability; thus, smaller NPs will present greater bactericidal effect by binding with the large surface area of the bacteria’s cell membrane [119–121]. Nevertheless, the exact antimicrobial mechanism for AuNPs and AgNPs is still poorly understood, for which several theories have been proposed. In the first place, NPs have a strong preference for reacting with sulfhydryl and phosphorus groups on the cell wall, causing great damage that results in the release of bacterial cell components [122]. Another hypothesis states that NPs can penetrate the bacterial cell membrane and attach to NADH dehydrogenases, generating a high amount of reactive oxygen species (ROS), which cause the depletion of ATP and interrupts the respiratory chain. These radicals have the capacity to interact with proteins, sulfur, phosphorus-containing cell constituents, and DNA, destroying them (Figure 3) [122,123]. Finally, NPs’ release of Au or Ag ions may contribute to their antimicrobial activity since DNA loses its replication activity, and proteins get inactive after interacting with these ions. This biocidal effect is considered to be size and dose-dependent; a higher concentration of NPs have demonstrated to interact more with cytoplasmatic organelles and bacterial nucleic acids [124,125]. Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 25 Figure 3. AgNPs antimicrobial mechanism through bacterial cell membrane penetration, interruption Figure 3. AgNPs antimicrobial mechanism through bacterial cell membrane penetration, interruption of the respiratory chain, and DNA damage by reactive oxygen species (ROS) [126]. of the respiratory chain, and DNA damage by reactive oxygen species (ROS) [126]. Moreover, the antimicrobial activity shows differences related to bacterial cell wall composition. AuNPs and AgNPs exhibit higher activity against Gram-negative bacteria than to Gram-positive. The latter possesses a thick layer of peptidoglycan, consisting of linear polysaccharide chains cross-linked by short peptides, which creates a hard rigid structure for NPs to penetrate, while Gram-negative has a thinner that represents a feasible composition to pierce [127,128]. Nevertheless, works from Grace et al. and Padalia et al. reported that NPs’ antimicrobial activity could be enhanced by surfacePDF Image | Green Synthesis of Gold and Silver Nanoparticles from Plant Extracts
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