Silver nanoparticles (AgNPs) were obtained by solar irradiation of cell-free extracts of Bacillus amyloliquefaciens and AgNO3. Light intensity, extract concentration, and NaCl addition influenced the synthesis of AgNPs. Under optimized conditions (solar intensity 70,000 lx, extract concentration 3 mg/mL, and NaCl content 2 mM), 98.23 +/- 0.06% of the Ag+ (1 mM) was reduced to AgNPs within 80 min, and the zeta-potential of AgNPs reached -70.84 +/- 0.66 mV. TEM (Transmission electron microscopy) and XRD (X-ray diffraction) analysis confirmed that circular and triangular crystalline AgNPs with mean diameter of 14.6 nm were synthesized. Since heat-inactivated extracts also mediated the formation of AgNPs, enzymatic reactions are likely not involved in AgNPs formation. A high absolute zeta-potential value of the AgNPs, possibly caused by interaction with proteins likely explains the high stability of AgNPs suspensions. AgNPs showed antimicrobial activity against Bacillus subtilis and Escherichia coli in liquid and solid medium. (C) 2011 Elsevier Ltd. All rights reserved.