Antibacterial effects of silver ions on growth of gram-negative bacteria and biofilm formation

The average minimal inhibiting concentrations (MICs) of AgNO₃ for the gram-negative bacteria Escherichia coli K12, Serratia proteamaculans 94, and Serratia liquefaciens MG1 were found to be 0.075–0.3 μg/ml and, for Pseudomonas aeruginosa PAO1 and P. chlororaphis 449, this concentration was 0.15–0.3 μg/ml. The formation of a biofilm by Escherichia coli AB1157 and S. proteamaculans 94 was completely inhibited by 0.3 μg/ml of AgNO₃ and, in the case of Pseudomonas aeruginosa PAO1, biofilm formation was inhibited by 0.6 μg/ml of AgNO₃. Mutations in E. coli genes that encode global regulators of gene expression, such as sigma S and sigma N subunits of RNA polymerase, the catabolite repression protein CRP, and the Lon protease, had no marked effect on the sensitivity of cells to silver. Wild-type E. coli strains and strains deficient in excision repair (uvrA and uvrB), SOS repair, and recombination (recA, lexA, recBC, and recF mutants) did not differ in their sensitivity to silver. This suggests that the sensitivity of the bacteria to the silver was not associated with DNA lesions that could be repaired by these repair and recombination systems. E. coli mutant strains deficient in OmpF or OmpC porins were three to four times more resistant to silver ions than the wild-type strain. Experiments with the pME6863 plasmid, which bears the gene of N-acyl-homoserine lactonase AiiA, demonstrated that the quorum sensing (QS) regulation did not participate in controlling the sensitivity of S. proteamaculans 94 and P. chlororaphis 449 to silver. The same conclusion was drawn from a comparison of AgNO₃ MICs of the S. liquefaciens wild-type strain and a mutant strain deficient in QS.