Silver ions and silver nanoparticles also have inhibitory and lethal effects on bacterial species such as E. coli,9,23-25 S. aureus9 and even yeast.9 In the last paper, the authors have prepared silver nanoparticles by mixing silver nitrate with sodium borohydride to obtain particles that were highly monodispersed with an average diameter of 13.5 ± 2.6 nm. They have observed that yeast and E. coli were inhibited at low concentrations of nanoparticles, 6.6 and 3.3 nmol L-1, respectively, whereas the growth-inhibitory activity on S. aureus was mild, with the minimal inhibitory concentration (MIC) estimated to be higher than 33 nmol L-1. This study also suggested that the generation of free-radicals is involved in some way with the antimicrobial activity of silver nanoparticles. In fact, oxidative stress was observed in cells after silver nanoparticles interaction.26 However, results described by Lok et al.27 differ in some way with those obtained by Kim et al.9 regarding the involvement of free-radicals in the antimicrobial activity of silver nanoparticles. Further studies are therefore required in order to clarify the exact role of free-radicals.