Nanoparticles are been synthesized by different chemical and physical methods. The biological approaches to the synthesis of nanoparticles are better than chemical and physical procedures because this method requires no toxic solvents and no dangerous material for the environment. In this research biosynthesis of silver nanoparticles has been explored by Shewanella ME1 isolated from sediments of the Caspian Sea. After samples collected and identification of isolates based on morphology, physiology, and molecular characteristics, one was chosen and its ability to synthesis silver nanoparticles was evaluated. The isolate was inoculated in LB medium and incubated at 28ºC for 24 hours. The supernatant of the broth was examined for the ability to produce silver nanoparticles. The supernatant was added to the silver nitrate solution at a concentration of 50mM and incubated for 24 hours at 28°C. Addition of ME1 isolate supernatant to aqueous AgNO3 solution led to the appearance of brown color in solution after 24h of reaction, indicating the formation of silver nanoparticles. The UV-Vis spectrum exhibits an absorption band at around 414 nm suggesting the formation of silver nanoparticles. SEM photographs showed that the silver nanoparticles formed were fairly uniform in size with a spherical shape and an average diameter of 47 nm. Antibacterial properties of nanoparticles were examined against human pathogenic bacteria by Kirby- Bauer method. The formation of a clear zone around the disk depicts antimicrobial activity. The diameter of zones of inhibition was measured )Escherichia coli 23±1 mm, Bacillus subtilis 22/3±0/5 mm, Pseudomonas aeruginosa 20±1 mm, Klebsiella pneumoniae 14±1 mm).
