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Uniform spherical silver nanoparticles with about 26 nm and good stability were synthesized by facile gallic acid reduction method. The antimicrobial activity of silver nanoparticles was investigated using several representative microbial species including gram-negative bacterium (Escherichia coli), gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis and Streptomyces kanamyceticus) and fungus (saccharomyces cerevisiae and Penicillium notatum). The antimicrobial effect of silver nanoparticles was evaluated by determining the inhibition zone of disk diffusion, the minimum inhibitory concentrations and the microbial inhibition ratio. The results consistently showed that the antimicrobial activity of silver nanoparticles was obviously affected by microbial species. Fungus was the most sensitive to silver nanoparticles and followed by gram-negative bacteria and gram-positive bacteria. For all of these microbes, the microbial inhibition ratio gradually increased with the increase of silver nanoparticles and reached more than 95% for silver nanoparticles of 60 µg•mL-1. For silver nanoparticles of 100 µg•mL-1, the inhibition ratios against E.coli, S. kanamyceticus, S. cerevisiae and P. notatum reached 100%, while that against S. aureus and B. subtilis were 98.9% and 97.6%, respectively. Therefore, as-prepared silver nanoparticles have high efficient and broad-spectrum antimicrobial activity, making them be promising antimicrobial agents. [ABSTRACT FROM PUBLISHER]