Biologically synthesized nanoparticles are gaining importance as they offer several advantages, such as the ease with which they can be scaled up, the cost-effectiveness of the process and the green route of production. In this study, silver (Ag) nanoparticles were biosynthesized using the cellular extract of Penicillium oxalicum GRS-1 and then characterized by ultraviolet visible spectroscopy, X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy. The biosynthesis of nanoparticles was optimized by following the one factor at a time approach, wherein the temperature of 60°C, pH 7.0 and 1.5 mm silver nitrate (AgNO3) concentration were found to be most favorable factors for the production of Ag nanoparticles. Upon statistical optimization, the maximum production of Ag nanoparticles with a concentration of 136 ppm was achieved at pH 7.2, AgNO3 concentration 1.975 mm and 86 h using the crude cellular extract of P. oxalicum GRS-1 having nitrate reductase activity. TEM analysis showed that the Ag nanoparticles were spherical in shape with sizes ranging from 10 to 40 nm. The biosynthesized nanoparticles showed strong antimicrobial activity against the common food-borne, pathogens including Staphylococcus aureus, Escherichia coli and Salmonella typhimurium with respective minimum bactericidal concentrations of 32, 16 and 32 μg/ml.

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