The biosynthesis of silver nanoparticles extracellularly using baker’s yeast, Saccharomyces cerevisiae and its antibacterial activity was investigated in this study. Biosynthesised silver nanoparticles were chaterized by using UV-Visible spectroscopy, which showed a distinct observed absorption peak at 429.00 nm that is attributed to the plasmon resonance of silver nanoparticles; X-ray diffraction, which determined the average size of the silver nanoparticles to be approximately 16.07 nm; presence of oval shaped silver nanoparticles determined by scanning electron microscopy; and Fourier-transform infrared, which revealed notable peaks at 3332.2, 2903.6 and 1636.3 cm corresponding to the binding of the silver nanoparticles to active biomolecules, alcohols and phenols, carboxylic acids and aromatic amines respectively. The silver nanoparticles were also found to be stable for ninety days. Antibacterial activity of the silver nanoparticles was also studied. The silver nanoparticles was significantly active (P>0.05) against the test organisms at an extract concentration of 75 µg ml. Concentrations less than or equal to 50 µg ml were not as effective as the colony forming units at this concentration, 1.61×106 for methicillin-resistant Staphylococcus aureus and 1.45×106 for Pseudomonas aeruginosa respectively were about the same range a small the colony forming units of the controls. The silver nanoparticles inhibited methicillin-resistant S. aureusmore than they inhibited P. aeruginosa. The LD50 of the synthesized silver nanoparticles after oral administration was seen to be greater than 5000 mg kg body weight and is therefore thought to be safe. This study supports the use of silver nanoparticles as therapeutic agents.

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