There is credible research on the antimicrobial activity of Colloidal Silver - examples include:
Langmuir. 2009 Oct 6;25(19):11741-7. doi: 10.1021/la901024p.
Intracellular biogenic silver nanoparticles for the generation of carbon supported antiviral and sustained bactericidal agents. http://www.ncbi.nlm.nih.gov/pubmed/19746940
J Phys Chem B. 2006 Aug 24;110(33):16248-53.
Silver colloid nanoparticles: synthesis, characterization, and their antibacterial activity http://www.ncbi.nlm.nih.gov/pubmed/16913750
J Biomater Sci Polym Ed. 2009;20(3):311-24. doi: 10.1163/156856209X412182.
Antimicrobial properties of hydrated cellulose membranes with silver nanoparticles. http://www.ncbi.nlm.nih.gov/pubmed/19192358
Shanghai Kou Qiang Yi Xue. 2003 Oct;12(5):356-8.
[Comparison of the antibacterial activity on oral pathogens among six types of nano-silver base inorganic antibacterial agents]. http://www.ncbi.nlm.nih.gov/pubmed/14966611