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Nanomaterials 2020, 10, 1763 8 of 24 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 surface modification through coating with aminoglycoside antibiotics, increasing their activity range to Gram-positive bacteria as well [129,130]. Wang et al. discovered that gentamicin enhanced bactericidal toxicity of AgNPs against foodborne bacteria S. Aureus, E. coli, and gentamicin-resistant E. coli because the drug promotes NPs dissolution, increasing Ag ions concentration, and promoting bacterial growth inhibition and death [131]. It is worth to mention that this can be a possibility for agricultural labors that employ antibiotics. Although different research groups are raising awareness of the problem and encouraging to dismiss the use of antibiotics in any other field different than medical, under a controlled environment, and highly regulated application, this method can be highly effective and reduce drug resistance issues caused by this human practice [132]. Many research papers in which AuNPs have shown promising antimicrobial activities have highlighted a majority spherical shape character of the NPs. Nevertheless, rod-shaped, triangular, hexagonal, and cubic NPs have also been found as part of the obtained mixture [133]. Thangamani et al. synthesized AuNPs using Simarouba glauca leaf extract. NPs’ size and shape were sensitive to leaf broth concentration; particles tended to decrease in size with an increase in leaf broth concentration, while different morphologies were obtained, such as a mixture of the prism and spherical-like particles. Aside from that, they assessed the antimicrobial activity by testing them against Gram-positive and Gram-negative organisms. The antimicrobial assay showed better results for S. aureus, S. mutans, B. subtilis, E. coli, Proteus vulgaris, and K.a pneumonia [134]. Euphorbia hirta aqueous leaf extract was used for biosynthesizing AuNPs by Annamalai et al. NPs sizes ranged from 6–71 nm, which greatly contributed to their highly effective antimicrobial activity against bacterial strains of E. coli, P. aeruginosa, and K. pneumonia. The study evaluated concentrations of 1.25–200 ug/mL, where 200 μg/mL completely inhibited the bacterial growth [135]. Additionally, Folorunso et al. investigated the antimicrobial potency of synthesized AuNPs from the leaf extracts of Annona muricata. The resulted NPs showed a spherical structure with an average size of 25.5 nm and high antibacterial activity against Clostridium sporogenes, where the efficacy increased as the NPs’ concentration increased [136]. In another approach, Montes de Oca et al. evaluated the impact of AgNPs’ usual concentrations in nature soils grown with Arabian Coffee in customary and organic operating systems. In this study, biomass, extracellular enzyme activities, and the diversity of the soil microbial community were assessed by a microcosm experiment as a function of time. After 7 days of incubation, the increase in the microbial biomass was independent of NPs concentration [137]. In contrast, after 60 days, there was a decrease in Gram-positive and actinobacterial biomass in soils at all the evaluated AgNPs concentrations. The physicochemical properties of the soil and the enzymatic activities were not affected by AgNPs. Within the composition of the microbial community, only a few differences were observed in abundance relative to the phylum level and gender in the fungal community [137]. The results indicated that the environmental factors of AgNPs affected microbial biomass but had a low impact on microbial diversity and might have a poor effect on soil biogeochemical cycles by extracellular enzyme activities [137,138]. A study by Lediga et al. functionalized AgNPs with the extracts of Sclerocarya birrea and Eucomis autumnalis. Remarkable antimicrobial properties against two Gram-negative and two Gram-positive bacteria were observed. Both the S. birrea and E. autumnalis AgNPs exhibited negligible or low toxicity to plants [139]. Additionally, Shahryari et al. formed AgNPs with an average size of 35 nm using the sumac aqueous extract through a green synthesis method for controlling bacterial canker disease of stone fruit trees caused by Pseudomonas syringae. The results revealed that different NPs concentrations reduced the severity of the disease, reaching the highest decrease at 100 ppm NPs concentration [140].PDF Image | Green Synthesis of Gold and Silver Nanoparticles from Plant Extracts
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