Antibacterial action of silver nanoparticles (AgNP) on Gram-negative bacteria (planctonic cells and biofilms) is reported in this study. AgNP of 8.3 nm in diameter stabilized by hydrolyzed casein peptides strongly inhibited biofilms formation of Escherichia coil AB1157, Pseudomonas aeruginosa PAO1 and Serratia proteamaculans 94 in concentrations of 4-5 mu g/ml, 10 mu g/ml and 10-20 mu g/ml, respectively. The viability of E. coil AB1157 cells in biofilms was considerably reduced by AgNP concentrations above 100 to -150 mu g/ml. E. coil strains with mutations in genes responsible for the repair of DNA containing oxidative lesions (mutY, mutS, mutM, mutT, nth) were less resistant to AgNP than wild type strains. This suggests that these genes may be involved in the repair of DNA damage caused by AgNP. E. coli mutants deficient in excision repair, SOS-response and in the synthesis of global regulators RpoS, CRP protein and Lon protease present similar resistance to AgNP as wild type cells. LuxI/LuxR Quorum Sensing systems did not participate in the control of sensitivity to AgNP of Pseudomonas and Serratia. E. colt mutant strains deficient in OmpF or OmpC porins were 4-8 times more resistant to AgNP as compared to the wild type strain. This suggests that porins have an important function related AgNP antibacterial effects. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved.