Abstract
A trial was carried out to assess the potential use of chemically synthesized silver nanoparticles (AgNPs) with two concentrations (5 and 10 mu g/L) as an antibacterial agent in Oreochromis niloticus and Sparus auratus against Streptococcus agalactiae and Vibrio alginolyticus respectively. All AgNPs were with a mean of 26 nm in diameter. The particles were negatively charged (-30.7 +/- 0.45 mV). In different ways, the in vitro AgNP antibacterial activity was investigated against S. agalactiae and V. alginolyticus in different ways showing clear inhibition zones of the two AgNP concentrations dose-dependent manner. In vitro measurement of extracellular proteins of pathogenic S. agalactiae and V. algenolyticus against AgNPs and the ultrastructural interaction between AgNPs and these bacterial pathogens using transmission electron microscopy (TEM) revealed AgNPs have good antibacterial properties. O. niloticus and S. auratus were exposed to AgNPs for 28 days. Some groups were challenged with S. agalactiae and V. alginolyticus respectively. Haematological, biochemical and immunological parameters and phagocytic activity, phagocytic index, respiratory burst activity of neutrophils and serum lysozyme activity were estimated in addition to the concentration of minerals. Histopathological examinations of all fish groups were explored. From the investigation above, we concluded that AgNPs have significant deleterious effects on O. niloticus exposed to 5 and 10 mu g/L of AgNPs, appearing a significant decrease in leukocytes, red blood corpuscles (RBCs) count and haemoglobin concentration the fish on day 28 of the trial, with more deleterious effect with challenging fish by S. agalactiae and V. algenolyticus. In addition, AgNPs caused a substantial increase in liver enzymes with a reduction in total protein, albumin and globulin in the fish and considerably decreased the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT). Additionally, there was a significant decrease in sodium (Na) and potassium (K) content, nitroblue tetrazolium (NBT) and lysozyme activity (LYZ) in all groups of O. niloticus and S. auratus. In addition, lower survival rates disturb histopathological findings; meanwhile, S. auratus exposed to 5 and 10 mu g/L of AgNPs showed antibacterial potency and immunostimulatory effects, which appeared in their higher survival rates and induced immune parameters and histopathological findings. In low concentration (5 mu g/ml of AgNPs), AgNPs were less cytotoxic, especially in S. auratus, representing an alternative antimicrobial compound against V. algenolyticus. Further studies are still to be explored to clarify the protective role of the AgNPs and the recovery period needed for AgNPs to be entirely safe for usage as an antibacterial for each fish species.