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Nanomaterials 2019, 9, 1042 Nanomaterials 2019, 9, x FOR PEER REVIEW Figure 8. Zeta potential of the colloidal nanoparticle solution after purification. Figure 8. Zeta potential of the colloidal nanoparticle solution after purification. bacterium (E. coli) and a Gram-negative bacterium (S. epidermidis) by the MIC method. The minimum (E. coli) and a Gram-negative bacterium (S. epidermidis) by the MIC method. The minimum concentration 3.4. MIC and MBC Studies 3.4. MIC and MBC Studies The anti-microbial activity of the biogenic AgNPs was assessed against a Gram-positive The anti-microbial activity of the biogenic AgNPs was assessed against a Gram-positive bacterium concentration of AgNPs required to cause inhibition as well as cell death was noted and compared of AgNPs required to cause inhibition as well as cell death was noted and compared with that of a with that of a standard broad-spectrum antibiotic, streptomycin (Table 3). The pigment showed no standard broad-spectrum antibiotic, streptomycin (Table 3). The pigment showed no inhibitory activity inhibitory activity at the tested concentrations against either organism. AgNPs seemed to exhibit at the tested concentrations against either organism. AgNPs seemed to exhibit superior activity against superior activity against Gram-positive bacteria than against Gram-negative bacteria, which is Gram-positive bacteria than against Gram-negative bacteria, which is contrary to the theory that owing contrary to the theory that owing to the different cell wall characteristics, AgNPs are less effective to the different cell wall characteristics, AgNPs are less effective towards Gram-positive bacteria than towards Gram-positive bacteria than towards Gram-negative bacteria [39]. Gram-negative cells are towards Gram-negative bacteria [39]. Gram-negative cells are more vulnerable to AgNPs due to the more vulnerable to AgNPs due to the presence of lipopolysaccharides on their surface that carry a presence of lipopolysaccharides on their surface that carry a negative charge; this results in higher negative charge; this results in higher interaction with the positive silver ions, that eventually results interaction with the positive silver ions, that eventually results in the degradation of the cell wall. in the degradation of the cell wall. In this case, the stronger action against a Gram-positive bacterium In this case, the stronger action against a Gram-positive bacterium might be due to the presence of might be due to the presence of bioactive functional groups on the surface of AgNPs. It might also be bioactive functional groups on the surface of AgNPs. It might also be a case of strain specificity, that is, a case of strain specificity, that is, this particular strain of S. epidermidis is more susceptible to the this particular strain of S. epidermidis is more susceptible to the antibacterial action of bio-coated AgNPs. 12 of 20 12 of 20 antibacterial action of bio-coated AgNPs. Several concerted mechanisms are responsible for the cytotoxic action of AgNPs against Table 3. MIC and MBC values for AgNPs and Streptomycin. bacterial cells. These mechanisms include the disintegration of cell walls by physical interaction with AgNPs and Ag+ ions, followed by the leakage of intracellular components; denaturation of proteins Organism Sample MIC (μg/mL) MBC (μg/mL) and enzymes by the ions released from AgNPs, leading to the cessation of adenosine triphosphate AgNPs 32 64 (ATP) production; and the interaction with DNA and consequent production of reactive oxygen E. coli Streptomycin 8 32 AgNPs 4 32 streptomycin inhibits bacterial cell proliferation by attaching to the 16S rRNA of the 30S subunit, species (ROS), which come in contact with the ribosomes and decrease their activity (Figure 9) [40– S. epidermidis 43]. AgNPs might also be able to impede cell respiration by reacting with oxygen and sulfhydryl Streptomycin <0.5 2 groups present atop the cell surface, leading to ATP depletion and cell death [44]. In contrast, Several concerted mechanisms are responsible for the cytotoxic action of AgNPs against bacterial thereby leading to the inhibition of protein synthesis. Other researches have shown that the cells. These mechanisms include the disintegration of cell walls by physical interaction with AgNPs nanoparticle size is an important factor in terms of penetration power and interaction with the cell and Ag+ ions, followed by the leakage of intracellular components; denaturation of proteins and membrane [45]. With increasing cases of antibiotic resistance, AgNPs might be able to provide a enzymes by the ions released from AgNPs, leading to the cessation of adenosine triphosphate (ATP) potential alternative to conventional anti-microbial agents. production; and the interaction with DNA and consequent production of reactive oxygen species (ROS), which come in contact with the ribosomes and decrease their activity (Figure 9) [40–43]. AgNPs might also be able to impede cell respiration by reacting with oxygen and sulfhydryl groups present atop the cell surface, leading to ATP depletion and cell death [44]. In contrast, streptomycin inhibits bacterial cell proliferation by attaching to the 16S rRNA of the 30S subunit, thereby leading to the inhibition of protein synthesis. Other researches have shown that the nanoparticle size is an importantPDF Image | Biosynthesis of Silver Nanoparticles Talaromyces purpurogenus
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