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 coli AB1157, Pseudomonas aeruginosa PAO1 and Serratia 
proteamaculans 94 in concentrations of 4-5 μg/ml, 10 μg/ml and 10-20 μg/ml, 
respectively. The viability of E. coli AB1157 cells in biofilms was considerably 
reduced by AgNP concentrations above 100 to -150 μg/ml. E. coli 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. coli 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.