Spherical silver nanoparticles with an average size of ca. 5 nm were synthesized in aqueous medium using a charged silsesquioxane containing a quaternary ammonium group, the bridged 1,4-diazoniabicyclo[2.2.2]octane nitrate, as a stabilizer and size controller. For the first time this system was synthesized and applied as an antibacterial agent and its activity was confirmed with excellent results. The new system shows high stability, which can be confirmed by the unchanged UV-Vis band even one year later. The magnitude of the zeta-potential (ζ) (+24.7 mV) indicated electrostatic contribution for the silver nanoparticles stability and the signal showed that the nanoparticles have a positively charged surface. In vitro antibacterial tests were performed against E. coli, P. aeruginosa and S. aureus bacteria, and the minimum concentrations of silver in the nanoparticle form for complete inhibition of bacteria were 0.60, 1.1 and 2.0 μg mL⁻¹, respectively. These values are very low when compared to the previous reports, making this system very promising. The cytotoxicity assay showed that these silver nanoparticles are safe for mammalian cells at the studied concentrations.