الفهرس 
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Abstract
The fundamental worldwide aquaculture production is implemented in developing countries. With the lack of regulations, preventing the prophylactic administration of antibiotics, the emergence of antibiotic-resistant bacteria has been raged in the aquaculture sector. Silver nanoparticles (Ag NPs) have recently gained a drastic interest in defeating this confounding antibiotic resistance. However, there is a lengthy unpaved path between the antibacterial effects and safety assessment of such fascinating Ag NPs. Furthermore, the impact of the size of Ag NPs and the silver ions (Ag+) on the antibacterial effects and in vivo toxicities has not yet been concurrently studied nor fully understood. The present thesis, therefore, demonstrates the synthesis of two different sizes of Ag NPs (9 nm, nAg1) and (30 nm, nAg2). The antibacterial effects of nAg1 and nAg2 had been studied, in comparison with Ag+, against Aeromonas hydrophila, Pseudomonas putida, Escherichia coli, Staphylococcus aureus, and Bacillus subtilis. To pave the path between the antibacterial effects and safety assessment, the in vivo toxicities of nAg1, nAg2, and Ag+ had also been studied in zebrafish embryos, a meritorious model to assess the toxicity of nanomaterials.
Ag NPs demonstrated antibacterial effects against only Staphylococcus aureus in a size-dependent fashion, the smaller size (nAg1) demonstrated higher antibacterial effects. AgNO3 demonstrated antibacterial effects against all the bacterial species tested, except B. subtilis. Ag NPs and Ag+ demonstrated utterly different antibacterial mechanisms of action. The small nAg1 demonstrated the destruction of bacterial cell walls and leakage of intracellular substances from the bacterial cells. AgNO3 demonstrated central bacterial DNA condensation in the bacterial electron light region. On the question of the in vivo toxicity, Ag NPs increased the mortality rates and decrease the hatchability rates of the zebrafish embryos in a size-dependent fashion, the smaller nAg1 were more toxic. The embryos exposed to AgNO3 demonstrated fewer mortalities than those exposed to nAg1. Ag NPs and Ag+ demonstrated utterly different phenotypic defects to the exposed embryos and larvae. The small nAg1 mainly demonstrated axial deformity. Ag+ mainly deposited on the chorions of embryos. Altogether, this thesis provides means of understanding, on prokaryotic and eukaryotic levels, that the toxicity induced by Ag NPs is size-dependent and different from that of Ag+. The administration of such Ag NPs as antibacterial armors in the aquaculture sector remains controversial due to the associated toxicities in zebrafish embryos.